Urinary System

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27.1 General Structure and Functions of the Urinary System 818 27.2 Kidneys 820 27 27.2a Gross and Sectional Anatomy of the Kidney 820 27.2b Blood Supply to the Kidney 821 27.2c Nephrons 824 27.2d How Tubular Fluid Becomes Urine 828 27.2e Juxtaglomerular Apparatus 828 Urinary 27.2f Innervation of the Kidney 828 27.3 Urinary Tract 829 27.3a Ureters 829 27.3b Urinary Bladder 830 System 27.3c Urethra 833 27.4 Aging and the Urinary System 834 27.5 Development of the Urinary System 835 27.5a Kidney and Ureter Development 835 27.5b Urinary Bladder and Urethra Development 835

MODULE 13: URINARY SYSTEM

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n the course of carrying out their specific functions, the cells Besides removing waste products from the bloodstream, the uri- I of all body systems produce waste products, and these waste nary system performs many other functions, including the following: products end up in the bloodstream. In this case, the bloodstream is ■ Storage of urine. Urine is produced continuously, but analogous to a river that supplies drinking water to a nearby town. it would be quite inconvenient if we were constantly The river water may become polluted with sediment, animal waste, excreting urine. The urinary bladder is an expandable, and motorboat fuel—but the town has a water treatment plant that muscular sac that can store as much as 1 liter of urine. removes these waste products and makes the water safe to drink. ■ Excretion of urine. The urethra transports urine from the The urinary system is the body’s “water treatment plant.” Without urinary bladder and expels it outside the body. Later in this it, waste products could accumulate in the blood and kill us. This chapter, we discuss the expulsion of urine from the bladder, chapter focuses on the organs of the urinary system and how they which is called micturition or urination. work together to remove waste products from the blood and help ■ Regulation of blood volume. The kidneys control the volume maintain homeostasis. of interstitial fluid and blood under the direction of certain hormones. Also, changes in blood volume affect blood pressure, so the kidneys indirectly affect blood pressure. ■ Regulation of erythrocyte production. As the kidneys filter 27.1 General Structure and Functions the blood, they are also indirectly measuring the oxygen level in the blood. If blood oxygen levels are reduced, cells in of the Urinary System the kidney secrete a hormone called erythropoietin (ĕ -rith- Learning Objectives: rō -poy -́ ĕ-tin) (EPO; described in chapter 20). Erythropoietin acts on stem cells in the bone marrow to increase 1. List and describe the primary organs of the urinary system. erythrocyte production. Having more erythrocytes allows the 2. Explain the functions performed by the urinary system. blood to transport more oxygen. The organs of the urinary (ū r ́i-nā r-ē) system are the kidneys, ■ Regulation of ion balance/acid-base balance. The kidneys ureters, urinary bladder, and urethra (figure 27.1). The kidneys help control the blood’s inorganic ion balance, such as filter waste products from the bloodstream and convert the filtrate sodium ions, potassium ions, and phosphate ions. The into urine (ū r ́in). The ureters, urinary bladder, and urethra are col- kidneys also aid in maintaining acid-base balance by lectively known as the urinary tract because they transport the urine altering rates of hydrogen ion and ammonium secretion as out of the body. well as bicarbonate reabsorption.

Diaphragm Adrenal gland Kidney Renal artery Renal vein Inferior vena cava Descending abdominal aorta Ureter

Iliac crest

Psoas major muscle

Uterus Rectum Urinary bladder

Urethra

(a) Anterior view Figure 27.1 Urinary System. The urinary system is composed of two kidneys, two ureters, a single urinary bladder, and a single urethra, shown here in (a) anterior and (b) posterior views. Dotted lines in (b) show the position of the ureters.

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Table 27.1 Urinary System Organs and Their Functions Organ Location Description/Characteristics Function Kidneys Posterior abdominal wall; right Paired, bean-shaped organs; composed of outer cortex Filters blood and processes fi ltrate kidney is inferior to left kidney and inner medulla into tubular fl uid, then urine Ureters Extend from kidneys to trigone Paired thin, fi bromuscular tubes composed of inner Transport urine from kidney to of bladder, along posterior mucosa, middle muscularis of smooth muscle, and an urinary bladder via peristalsis abdominopelvic wall outer adventitia Urinary bladder Pelvic cavity, posterior to pubic Muscular distensible sac composed of inner mucosa, Reservoir for urine until symphysis (when full, it extends a submucosa, a muscularis, and an outer adventitia or micturition (urination) occurs into inferior part of abdominal serosa cavity) The neck of the bladder is the inferior constricted region where bladder and urethra meet; contains internal urethral sphincter Urethra Inferior to neck of urinary Single muscular tube; 3–5 cm long in females; 18–20 Transports urine from urinary bladder; extends through muscles cm long in males bladder to outside of body of pelvic fl oor and opens into perineum

Table 27.1 summarizes the organs of the urinary system and their functions.

WHATW DID YOU LEARN?

●1 What organs make up the urinary tract, and what is the main function of the urinary tract? ●2 Describe the mechanisms by which the kidneys regulate blood volume and erythrocyte production.

Latissimus dorsi muscle (cut) Lung

11th rib Left kidney 12th rib Psoas major muscle Right kidney Ureter L2 vertebra

Quadratus lumborum muscle (cut) Iliac crest

Urinary bladder

Urethra

(b) Posterior view

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called superior extremity) of the left kidney is at about the level of 27.2 Kidneys the T12 vertebra, and its inferior pole (also called inferior extremity) is at about the level of the L3 vertebra. The superior pole of the right Learning Objectives: kidney is positioned about 2 cm inferior to the superior pole of the 1. Describe the anatomy of the kidneys. left kidney to accommodate the large size of the liver. An adrenal 2. Outline the fundamentals of filtration, tubular gland rests on the superior pole of each kidney. reabsorption, and tubular secretion. The kidney has a concave medial border called the hilum 3. List the components of a nephron, and explain their roles (hı̄ ́lŭ m; hilum = a small bit), where vessels, nerves, and the ureter in urine formation. connect to the kidney. The hilum is continuous with an internal space within each kidney called the renal (rē ń ă l; ren = kidney) The kidneys are two symmetrical, bean-shaped, reddish- sinus. The renal sinus houses renal arteries, renal veins, lymph ves- brown organs located along the posterior abdominal wall, lateral sels, nerves, the renal pelvis, renal calyces, and a variable amount to the vertebral column (figure 27.1). Each kidney weighs approxi- of adipose connective tissue. The kidney’s lateral border is convex. mately 100 grams and measures about 12 centimeters (cm) in Each kidney is surrounded and supported by several tissue length, 6.5 cm in width, and 2.5 cm in thickness. layers. From innermost (closest to the kidney) to outermost, these layers are the fibrous capsule, perinephric fat, renal fascia, and WHAT DO YOU THINK? paranephric fat (figure 27.2): ●1 A person must have at least one functioning kidney to survive. ■ The fibrous capsule (kap sool;́ capsa = box) or renal capsule, Why do you think a lack of kidneys is deadly? is composed of dense irregular connective tissue that covers the outer surface of the kidney. The fibrous capsule 27.2a Gross and Sectional Anatomy of the Kidney maintains the kidney’s shape, protects it from trauma, and The kidneys are retroperitoneal, since only their anterior surface helps prevent infectious pathogens from entering the kidney. is covered with peritoneum and the posterior aspect lies directly ■ The perinephric fat, or adipose capsule, is external to against the posterior abdominal wall. The superior pole (also the fibrous capsule and contains adipose connective

Stomach

Liver Pancreas Large intestine Descending abdominal aorta Inferior Renal vein vena cava Renal artery Peritoneum Renal hilum Body of Right kidney vertebra L2 Fibrous capsule Perinephric fat Spleen Left kidney Renal fascia Paranephric fat Rib Psoas major Quadratus lumborum muscle muscle

Posterior

Figure 27.2 Position and Stabilization of the Kidneys. The kidneys lie along the posterior abdominal wall. A cross-sectional view shows that the kidneys are surrounded by four concentric tissue layers: fibrous capsule, perinephric fat, renal fascia, and paranephric fat.

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tissue that varies in thickness. This layer, which is cup of a flower). There are between 8 and 15 minor calyces—in also called the perirenal fat, completely surrounds the other words, one minor calyx for each renal papilla. Several kidney and offers cushioning and insulation. minor calyces merge to form larger spaces called major calyces— ■ The renal fascia (fash ́ē -ă; a band) is external to the each kidney typically contains two or three major calyces. Urine perinephric fat and is composed of dense irregular from the renal pyramids is collected by the minor calyces and connective tissue. It anchors the kidney to the posterior then drained into the major calyces. The major calyces merge to abdominal wall and peritoneum. form a large, funnel-shaped renal pelvis, which collects urine ■ The paranephric fat is the outermost layer surrounding the and transports it into the ureter. kidney. It is composed of adipose connective tissue and lies A human kidney is divided into 8 to 15 renal lobes. A renal between the renal fascia and the peritoneum. lobe consists of a medullary pyramid and some cortical substance from the renal columns adjacent to it on either side, as well as the When a kidney is sectioned along a coronal plane, an cortex external to the pyramid base. outer renal cortex and an inner renal medulla can be seen (figure 27.3). The medulla tends to be a darker shade than the cortex. Extensions of the cortex, called renal columns, WHATW DID YOU LEARN? project into the medulla and subdivide the medulla into renal ●3 Where are the renal cortex and the renal medulla located? pyramids (or medullary pyramids). An adult kidney typically contains 8 to 15 renal pyramids. The wide base of a renal pyra- ●4 Why is the right kidney more inferiorly placed than the left kidney? mid lies at the external edge of the medulla where the cortex and medulla meet, called the corticomedullary junction. The apex (tip) of the renal pyramid (called the renal papilla) projects toward the renal sinus. 27.2b Blood Supply to the Kidney Each renal papilla projects into a funnel-shaped space Since the kidneys’ primary function is to filter the blood, at called the minor calyx (kā ́liks; pl., calyces, or calices, kā l ́i-sē z; least 20%–25% of the resting cardiac output (see chapter 22)

Superior pole Fibrous capsule Renal cortex

Renal medulla Renal column Minor calyx Corticomedullary junction Minor calyx Renal papilla Major calyx Adipose connective Renal artery tissue in renal sinus Renal pelvis Renal sinus Renal pelvis Renal vein Major calyx

Renal pyramid Renal pyramid Renal lobe Renal column

Ureter

Inferior pole Right kidney, coronal section

Figure 27.3 Gross Anatomy of the Kidney. The bean-shaped kidney is covered by a fibrous capsule and contains an external cortex and an internal medulla.

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CLINICAL VIEW Intravenous Pyelogram

Sometimes a physician needs to visualize the kidneys, ureters, and Minor calyx urinary bladder, especially when the flow of urine from one of the Major calyx kidneys into the bladder becomes blocked. A prime example is the Renal pelvis Kidney obstruction of a ureter by a kidney stone, a painful condition that requires prompt treatment. To learn the location of the blockage and to better understand the condition, a physician may ask for an x-ray study known as an intravenous pyelogram (pı¯ el-ó ¯-gram; Ureter gram = recording). An intravenous pyelogram is produced by injecting a small amount of radiopaque dye into a vein. The dye is formulated to be water-soluble and quickly cleared by the kidneys. Following injection of the dye, a series of abdominal x-rays are taken over a period of about an hour and a half. As the dye passes through the kidneys and is cleared into the urine, the sequential x-rays provide a “time-lapse” view of urinary system function. The dye allows the kidneys, ureters, and urinary bladder to be shown on x-ray. A problem with filling or movement of Urinary bladder urine through one of the kidneys or ureters appears as a narrowing or blockage on the x-ray image.

A pyelogram enables physicians to visualize the urinary system organs and identify the location of any blockages.

normally flows through the kidneys via the pathway shown ball of yarn, ulus = small). Some blood plasma is filtered through in figure 27.4. Blood is carried to a kidney in a renal artery, the fenestrated epithelium of the glomerulus into the capsular which generally arises and extends from the lateral region of the space within the renal corpuscle. Once some of the blood plasma descending abdominal aorta at the level of the first or second has been filtered, the remaining blood leaves the glomerulus and lumbar vertebra. Up to five segmental (seg-men t́ a ̆l) arteries enters an efferent (ef ́er-ent; efferens = to bring out) arteriole branch from the renal artery within the renal sinus. While (or efferent glomerular arteriole). Note that the efferent arteri- still in the renal sinus, the segmental arteries further branch ole is still carrying oxygenated blood because gas and nutrient to form the interlobar (in-ter-lō ́bar; inter = between, lobos = exchange with cells of the kidney has not yet occurred. lobe) arteries. Interlobar arteries travel through the renal col- The efferent arterioles branch into one of two types umns toward the corticomedullary junction, where they branch of capillary networks: peritubular capillaries or vasa recta to form arcuate (ar ́kū -ā t; arcuatus = bowed) arteries. These (figure 27.4). These capillary networks are responsible for arcuate arteries project parallel to the base of the medullary the actual exchange of gases, nutrients, and waste materials pyramid at the corticomedullary junction. The arcuate arteries within the kidney. Peritubular capillaries are associated with give off branches called interlobular (in-ter-lob ́ū -lar) ̆ arteries the convoluted tubules and primarily reside in the cortex of that project peripherally into the cortex. the kidney. Vasa recta (vā ś a ̆ rek t́ a; ̆ vasculum = small vessel, As the interlobular arteries enter the cortex, they extend rectus = straight) are associated with the nephron loop and small branches called afferent (af ́er-ent; ad = toward, ferre = to primarily reside in the medulla of the kidney. lead) arterioles (or afferent glomerular arteriole). An afferent arte- The peritubular capillaries and vasa recta then drain into a riole then enters a structure called a renal corpuscle and forms a network of veins. The smallest of these veins are the interlobular capillary network called the glomerulus (glō -mā r ́ū -lŭs; glomus = veins, which travel alongside the interlobular arteries. Interlobular

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Interlobar artery Arcuate artery Renal Interlobular artery cortex Segmental Cortical nephron artery Afferent arteriole

Juxtamedullary nephron Glomerulus Renal artery

Efferent arteriole

Peritubular capillaries Cortex (associated with convoluted Arcuate tubules) vessels Medulla Interlobular vein Renal vein Vasa recta Nephron (associated with loop nephron loop) Collecting duct

Renal pyramid Fibrous capsule

Interlobar vein Arcuate vein Interlobular vein Figure 27.4 Blood Supply to the Kidneys. (Left) A coronal view depicts kidney circulation. (Right) An expanded view shows circulation to some nephrons. Pink boxes indicate vessels with arterial blood; lavender boxes indicate vessels where reabsorbed materials reenter the blood; blue boxes indicate vessels returning blood to the general circulation.

veins merge to form arcuate veins that project parallel to the base of each medullary pyramid near the corticomedullary junction. Study Tip! Arcuate veins merge to form interlobar veins that travel through The names of the blood vessels in the kidney can give you a clue the renal columns from the corticomedullary junction toward the as to their location or appearance: renal sinus. Interlobar veins merge in the renal sinus to form the renal vein. (Although many veins are associated with arteries of ■ Interlobar vessels are located between (’inter”) the lobes of the the same name, there are no segmental veins; rather, the inter- kidney. lobar veins directly form the renal vein.) The renal veins leave ■ Arcuate vessels form vessel “arcs” at the corticomedullary the kidney at its hilum and traverse horizontally to drain into the junction. inferior vena cava. ■ Interlobular vessels are located between the smaller lobules of Blood filtration occurs at the glomerulus, and the blood the kidney cortex. remains highly oxygenated until it reaches the peritubular capil- ■ Afferent arterioles carry blood to the glomerulus (remember, laries and vasa recta, where exchange of gases occurs. “afferent” means “toward”).

■ Efferent arterioles take blood away from the glomerulus (remember, “efferent” means to take away, or “exit”).

■ Peritubular capillaries are around (“peri”) the tubules (proximal and distal convoluted tubules).

■ Vasa recta means “straight vessels,” and these vessels run parallel to the long, straight tubules of the nephron loop.

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Distal convoluted tubule Nephron loop Juxtamedullary nephron Proximal convoluted tubule Renal corpuscle

Proximal convoluted tubule Renal corpuscle (cut): Glomerulus Glomerular capsule Distal convoluted tubule Cortical nephron

Cortex Nephron loop

Descending limb

Arcuate vessels Nephron loop Ascending limb

Medulla Collecting tubules

Collecting ducts

Papillary duct

Renal papilla

Figure 27.5 Nephron Structure. The bulk of cortical nephrons are located in the cortex, and their nephron loops barely penetrate the medulla. The juxtamedullary nephrons lie close to the corticomedullary junction, and their relatively long nephron loops extend deep into the medulla.

27.2c Nephrons Urine Formation The functional filtration unit in the kidney is the nephron (nef ron),́ The nephrons form urine through three interrelated processes: which consists of the following components: a renal corpuscle filtration, tubular reabsorption, and tubular secretion. (composed of a glomerulus and a glomerular capsule), a proximal convoluted tubule, a nephron loop, and a distal convoluted tubule 1. Filtration is the process by which water and some dissolved (figure 27.5). The proximal convoluted tubule, nephron loop, and solutes in the blood plasma passively move out of the distal convoluted tubule are collectively known as the renal tubule. glomerulus into the capsular space of the renal corpuscle due Together, both kidneys house approximately 2.5 million neph- to pressure differences across the filtration membrane. This rons. These microscopic structures measure less than 5 centimeters water and its dissolved solutes are called filtrate. in total length. There are two types of nephrons: cortical nephrons 2. Tubular reabsorption occurs when substances in the and juxtamedullary nephrons (see figure 27.4, right). Approximately filtrate move by diffusion or active transport across the wall 85% of the nephrons are termed cortical nephrons because the bulk of the renal tubules to return to the blood. Once filtrate of the nephron resides solely in the cortex, and their relatively short begins to be modified, it is called tubular fluid. Usually, nephron loops just barely penetrate the medulla. The remaining all needed solutes and most water that formed the filtrate 15% of nephrons are called juxtamedullary (jŭks t́ ă-med ́ŭ-lăr-ē; are reabsorbed by the blood. As reabsorption occurs, some juxta = close to) nephrons, since their renal corpuscles lie close excess solutes, water, and waste products remain within the to the corticomedullary junction, and their long nephron loops tubular fluid. extend deep into the medulla. The vasa recta are associated with 3. Tubular secretion is the active transport of solutes out of juxtamedullary nephrons. the blood into the tubular fluid.

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Flow of blood Parietal layer of Flow of filtrate glomerular capsule Capsular space Afferent arteriole Vascular pole Tubular pole Proximal convoluted tubule Juxtaglomerular Proximal apparatus: Capsular convoluted space tubule Juxtaglomerular cell Macula densa Glomerulus Glomerulus Podocyte of visceral Distal convoluted Afferent layer of glomerular arteriole capsule tubule Macula densa Efferent arteriole Distal Endothelium convoluted Pedicel of glomerulus tubule LM 300x

(a) Renal corpuscle (b) Histology of renal corpuscle

Pedicels Podocyte cell body

Podocyte Capillary lumen

Pedicels Filtration membrane: Basement membrane Fenestrated Glomerular capillary endothelium capillary covered by podocytes with pedicels Filtration slits SEM 800x

(c) Glomerular capillaries and podocyte (d) Figure 27.6 Renal Corpuscle. (a) The renal corpuscle is composed of a capillary meshwork called the glomerulus housed within the glomerular capsule. The afferent arteriole supplies blood to the glomerular capillaries; blood leaves the glomerulus in the efferent arteriole. (b) Photomicrograph of a renal corpuscle. (c) Filtrate is produced in the renal corpuscle when blood plasma is forced across the glomerular capillary wall under pressure. (d) SEM shows the podocytes covering the glomerular capillaries.

Once the tubular fluid exits the collecting duct, it is called called the glomerulus, and an epithelial capsule surrounding the urine. Urine consists of solutes and water that have been filtered glomerulus called the glomerular capsule (Bowman capsule). The as filtrate and secreted into the tubular fluid. glomerular capsule has two layers: a visceral layer that directly Next we examine how the specific components of the neph- overlies the glomerulus and a parietal layer. The visceral layer is ron carry out these processes. composed of specialized cells called podocytes (to be discussed later), while the parietal layer is formed from a simple squamous Renal Corpuscle A renal corpuscle (kō r pus-l;́ corpus = body, cle = epithelium. Between these two layers is a capsular space. As blood tiny) is an enlarged, bulbous region of a nephron (figure 27.6a). flows through the glomerulus, the solutes and water within the It is composed of two structures: the thick tangle of capillaries blood are filtered from the glomerulus into the capsular space.

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The renal corpuscle has two opposing poles: a vascular pole, The cells of the proximal convoluted tubule actively reabsorb where the glomerular afferent and efferent arterioles are found, almost all nutrients (e.g., glucose and amino acids), ions (espe- and a tubular pole, where the proximal convoluted tubule attaches cially Na+), vitamins, and any plasma proteins from the tubular to the renal corpuscle. fluid. Approximately 60–65% of the water in the tubular fluid is The visceral layer of the glomerular capsule is composed of reabsorbed by osmosis. The reabsorbed solutes and water enter the specialized cells called podocytes (pod ́ō -sı̄t; podos = foot), which blood in the peritubular capillaries and are returned to the general have long processes called pedicels (ped ı̆́ -sel; pedicellus = little circulation. foot) or feet, that wrap around the glomerular capillaries to support the capillary wall but not completely ensheathe it. The pedicels are Nephron Loop The nephron loop (or loop of Henle) originates at separated by thin spaces called filtration slits, which allow materi- a sharp bend in the proximal convoluted tubule and projects inter- als from the blood plasma to pass into the capsular space (figure nally toward and into the medulla (see figure 27.5). Each loop has 27.6c). Finally, the glomerular capillaries are fenestrated capil- two limbs: a descending limb extending from the cortex toward laries, which means they have fenestrations or pores that allow and into the medulla, and an ascending limb that returns back to materials to be filtered. The filtration slits of the podocytes and the renal cortex. The descending limb is first lined with a simple the fenestrated glomerulus endothelium collectively make up the cuboidal epithelium (this segment is called the thick descending filtration membrane of the glomerulus. limb) and then a simple squamous epithelium (this segment is Several factors cause materials to be filtered from the glom- called the thin descending limb). The ascending limb is first lined erulus. First, the afferent arteriole is wider in diameter than the by a simple squamous epithelium (this segment is called the thin efferent arteriole, so the blood in the afferent arteriole enters the ascending limb) and then by a simple cuboidal epithelium (this glomerulus under high pressure, which helps filter the plasma and segment is called the thick ascending limb). The ascending limb solutes out of the glomerulus. Second, the glomerulus is designed of the nephron loop returns to the renal cortex and terminates at to be “leaky” to allow some plasma and solutes to be filtered. the distal convoluted tubule. Third, the filtration slits between pedicles permit the ready passage The primary function of the nephron loop is to facilitate of filtered material into the capsular space. reabsorption of water and solutes from the tubular fluid. From The filtration membrane of the glomerulus, which includes the ascending limb, sodium ions (Na+) and chloride ions (Cl−) are the thick basement membrane of the glomerular endothelium, the primary solutes reabsorbed, although other solutes may be can only crudely filter the blood. The thick basement membrane reabsorbed as well. When these materials are reabsorbed from the also is designed to work with the fenestrations of the capillary nephron loop, they are returned to the blood via the vasa recta in selectively filtering materials from the blood plasma. Thus, capillary network. too much water and some vital nutrients, ions, plasma proteins, and vitamins escape into the filtrate as well. Therefore, the Distal Convoluted Tubule The distal convoluted tubule (DCT) renal tubule (proximal convoluted tubule, nephron loop, and originates in the renal cortex at the end of the thick ascending limb distal convoluted tubule) becomes responsible for refining and of the nephron loop and contacts the afferent arteriole wall at the modifying this filtrate. The filtrate first enters the proximal vascular pole (figure 27.7). Like the proximal convoluted tubule, convoluted tubule, where it begins to be modified and becomes the distal convoluted tubule is also lined with simple cuboidal tubular fluid. epithelium. However, the distal convoluted tubule cells are smaller and contain only sparse, short microvilli (figure 27.7c). Thus, the WHATW DID YOU LEARN? distal convoluted tubule lumen does not appear fuzzy. In addition, the distal convoluted tubule cytoplasm stains more lightly than ●5 Compare and contrast the juxtamedullary nephron and cortical nephron. that of the proximal convoluted tubule, because the distal convoluted tubule has fewer mitochondria. ●6 What is the difference between filtration and reabsorption? The primary function of the distal convoluted tubule is to secrete ions such as potassium (K+) and hydrogen (H+) into the Proximal Convoluted Tubule The proximal convoluted tubule tubular fluid. However, reabsorption of water also occurs here, (PCT) originates at the tubular pole of the renal corpuscle (fig- primarily under the influence of antidiuretic hormone (ADH) ure 27.7; see figure 27.6a). It is lined by a simple cuboidal epithe- and aldosterone (see chapter 20). ADH (posterior pituitary) and lium with tall microvilli that markedly increase its reabsorption aldosterone (adrenal cortex) are secreted in response to low blood capacity. When viewed under a microscope, the lumen of the volume or low solute concentration in the tubular fluid within the proximal convoluted tubule looks fuzzy due to the brush border kidney. These hormones cause the distal convoluted tubule cells to formed by the tall microvilli. In addition, the cytoplasm of these increase water and sodium reabsorption from tubular fluid. Once cells stains brightly, due to an abundance of mitochondria in the the tubular fluid leaves the distal convoluted tubule, it enters the cytoplasm. collecting tubules.

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Proximal Efferent convoluted arteriole Renal tubule Renal corpuscle corpuscle Distal convoluted Distal tubule convoluted Afferent tubule arteriole Collecting Proximal duct convoluted tubule

LM 100 x

(b) Histology of renal cortex

Nephron loop Tall microvilli Short, sparse microvilli

Nucleus Mitochondria Basement Proximal convoluted tubule membrane Distal convoluted tubule

(a) Nephron components (c) Convoluted tubule epithelia

Thick limbs of nephron loops

Collecting ducts

Thin limbs of nephron loops

Vasa recta LM 100x

(d) Histology of renal medulla

Figure 27.7 The Convoluted Tubules and Nephron Loop. The convoluted tubules and nephron loop carry tubular fluid that is being modified to form urine. (a) In this drawing, each of the components of the nephron is distinguished by a different color. (b) A photomicrograph of a section through the cortex compares transverse sections of the proximal and distal convoluted tubules. (c) Comparisons of the simple cuboidal epithelium lining the proximal and distal convoluted tubules show differences in the sizes and numbers of microvilli. (d) A photomicrograph of a transverse section through the medulla compares nephron loops and collecting ducts.

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Table 27.2 Parts of a Nephron and Their Functions Structure Description Function Renal corpuscle Capillary ball or tuft covered by podocytes and surrounded Produces a fi ltrate of blood that must be modifi ed as by an epithelial capsule; capsular space is between the two it passes through the convoluted tubules and nephron layers of the capsule loop Proximal convoluted tubule Tubule lined with simple cuboidal epithelium; has a Reabsorbs ions (especially Na+), nutrients (glucose and prominent brush border (microvilli); cytoplasm tends to amino acids), plasma proteins, vitamins, and water; stain more brightly than DCT cells secretes some H+ ions Nephron loop Tubule that forms a loop; has thick and thin ascending and Reabsorbs water in tubular fl uid; also reabsorbs Na+ descending portions; the most distal part of the loop often and Cl−; secretes some H+ ions extends into the medulla Thick limbs are lined with simple cuboidal epithelium Thin limbs are lined with simple squamous epithelium Distal convoluted tubule Tubule lined with simple cuboidal epithelium with only a Secretes H+ and K+ into tubular fl uid; reabsorbs Na+ and sparse brush border; cytoplasm of cells tends to be paler water from tubular fl uid (in presence of aldosterone than that of PCT cells and ADH)

Table 27.2 summarizes the parts of a nephron and their 27.2e Juxtaglomerular Apparatus functions. Associated with the nephron are some structures collectively referred to as the juxtaglomerular (ju˘ks t́ a-gl ̆ ō -mer ́ū -lar; ̆ juxta 27.2d How Tubular Fluid Becomes Urine = near) apparatus (see figure 27.6a, b). Components of the jux- When the tubular fluid leaves the distal convoluted tubules, it must taglomerular apparatus are juxtaglomerular cells and a macula travel through a series of small collecting tubules that empty into densa. Juxtaglomerular cells are modified smooth muscle cells collecting ducts. Collecting tubules and collecting ducts project of the afferent arteriole located near the entrance to the renal through the renal medulla toward the renal papilla (see figures corpuscle. The macula (mak ́ū -la; ̆ spot) densa (den ś a; ̆ dense) 27.5 and 27.7d). Both collecting tubules and collecting ducts are is a group of modified epithelial cells in a distal convoluted lined by a simple epithelium. The epithelial cells are cuboidal in tubule that touch the juxtaglomerular cells. These cells, which the tubules, but very tall columnar cells in the ducts near the renal are located only on the tubule side next to the afferent arteri- papilla. ole, are narrower and taller than other distal convoluted tubule The collecting ducts are the last structures that have the epithelial cells. capacity to modify the tubular fluid further, and can do so under The structures of the juxtaglomerular apparatus work together the influence of ADH and aldosterone. Secretion of these hormones to regulate blood pressure. The macula densa cells continuously results in increased water and sodium absorption from the tubular monitor ion concentration in tubular fluid. Thus, if either blood fluid in the collecting ducts, thereby reducing water and sodium volume or solute concentration is reduced, tubular fluid reflects this loss from the kidneys. ADH is secreted in response to either a rise reduction, and the macula densa cells detect this change and stimu- in the concentration of ions in the blood or a fall in blood volume, late the juxtaglomerular cells to release renin. Recall from chapter as when the body is dehydrated and needs to conserve water. If an 20 that renin activates the renin-angiotensin pathway, resulting individual is well hydrated, the collecting ducts merely transport the in aldosterone production, which causes increases in blood ion tubular fluid and do not modify it. However, if an individual is dehy- concentrations and blood volume. This arrangement is important drated, water conservation must occur, and more-concentrated urine in maintaining blood volume and blood pressure homeostasis in is produced. ADH may act on the collecting duct epithelium, mak- the body. ing it more able to absorb water from the tubular fluid. Once the tubular fluid leaves the collecting duct, it may 27.2f Innervation of the Kidney be called urine. The remaining structures in the kidney simply Each kidney is innervated by a mass of sensory and autonomic transport the urine and cannot modify this fluid further. Several nervous system axons collectively called the renal plexus. The collecting ducts merge to empty into a papillary duct that opens renal plexus accompanies each renal artery and enters the renal at the edge of the renal papilla into minor calyx. Urine leaves sinus of the kidney through the hilum. The renal plexus contains the renal papilla and enters the minor calyces, which then sympathetic innervation from the T10–T12 segments of the spinal transport the urine to major calyces, which in turn transport the cord and parasympathetic innervation from CN X (vagus nerve). urine to the renal pelvis. The renal pelvis conducts urine into The sympathetic innervation is responsible for renal blood vessel the ureter. vasoconstriction that results in decreased glomerular blood flow and filtrate formation. Increased sympathetic stimulation may also stimulate juxtaglomerular cells to release renin. Pain from WHAT DO YOU THINK? the kidneys is typically referred via the sympathetic pathway to ●2 Is ADH secreted when the body is dehydrated or well hydrated? the T10–T12 dermatomes. The parasympathetic innervation to the kidney has no known effect.

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CLINICAL VIEW: In Depth Renal Failure, Dialysis, artery and vein connect the donor kidney to the patient’s blood supply, and Kidney Transplants having the kidney near the bladder means only a short segment of ureter is needed for the bladder connection. There is no need to remove the Renal failure refers to greatly diminished or absent renal function damaged kidney. The transplanted kidney is a foreign tissue, and so the caused by the destruction of about 90% of the tissue in the kidney. immune system will attack and tr y to reject this foreign tissue. Therefore, Renal failure often results from a chronic disease that affects the immunosuppressant drugs are administered to suppress the activity of glomerulus or the small blood vessels of the kidney. Autoimmune the transplant recipient’s immune system. It may be necessary to take conditions, high blood pressure, and diabetes are the three chronic these drugs for the remainder of the patient’s life. problems accounting for the majority of progressive renal failure. Donor kidneys may come through an organ procurement program Once the kidneys have been destroyed, there is no chance they will or from a living person. Since most people have two kidneys, and regenerate or begin functioning again. Thus, the two main treatments normal renal function actually requires only one, some people have are dialysis or a kidney transplant. donated a kidney (while still alive) to a relative or even to a total The term dialysis (dı¯-al ́i-sis; dialyo = to separate) comes from a Greek stranger. If successfully matched to the genetic requirements of the word meaning “to separate agents or particles on the basis of their recipient, a transplanted kidney assumes all the jobs of the patient’s size.” Two forms of dialysis are commonly used today: peritoneal dialysis functionless kidneys. and hemodialysis. In peritoneal dialysis, a catheter is permanently placed in the peritoneal cavity, with a bag of dialysis fluid attached to the external end. As the fluid enters the peritoneal cavity and sloshes around, the harmful waste products in the blood are transferred, or dialyzed, into the fluid. After several hours, the patient connects a collection bag to the external end of the catheter, positions his or Diseased kidneys her body so the fluid will flow out of the peritoneal cavity, and opens Descending the catheter valve; the dialysis fluid is then removed. abdominal aorta In hemodialysis, the patient’s blood is cycled through a machine that Inferior vena cava filters the waste products across a specially designed membrane. To Transplanted kidney facilitate this type of dialysis, a vascular connection (an arteriovenous fistula) is made between a conveniently located superficial artery and Transplanted ureter vein. The patient must remain stationary for the time it takes to cycle the blood through the dialysis unit while the metabolic waste products are removed. Hemodialysis is needed several times a week. Urinary bladder A kidney transplant from a genetically similar person may successfully restore renal function. The replacement kidney is attached to an artery and vein in the inferior abdominopelvic region, where it is relatively easy to establish a vascular connection. The new kidney rests on the superior surface of or immediately lateral to the urinary bladder. Because a pelvic Location of a transplanted kidney in the abdominopelvic cavity.

WHATW DID YOU LEARN? The urinary tract consists of the ureters, urinary bladder, and ●7 What are the components of the nephron, and how do they urethra. modify the filtrate? 27.3a Ureters ●8 Where is ADH produced, and how specifically does it affect water concentration in the urine? The ureters (ū -rē ter,́ ū ŕ ē -ter) are long, fibromuscular tubes that conduct urine from the kidneys to the urinary bladder. Each tube ●9 What is the structure and function of the juxtaglomerular apparatus? averages 25 centimeters in length and is retroperitoneal. The ureters originate at the renal pelvis as it exits the hilum of the kidney, and then extend inferiorly to enter the posterolateral wall of the 27.3 Urinary Tract base of the urinary bladder. The wall of the ureter is composed of three concentric tunics. From innermost to outermost, these tunics Learning Objectives: are the mucosa, muscularis, and adventitia (figure 27.8). (Note 1. Explain the anatomy and location of the ureters, urinary that the ureter does not have a submucosa.) bladder, and the male and female urethras. The mucosa is formed from transitional epithelium, which 2. Outline the blood vessels and nerves that supply the organs is both distensible (stretchy) and impermeable to the passage of of the urinary tract. urine. External to the transitional epithelium of the mucosa is the

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Mucosa Mucosa Muscularis Adventitia

Lamina Transitional propria epithelium

Mucosa Muscularis

Lumen Adventitia

LM 18x

(a) Ureter cross section (b) Histology of ureter Figure 27.8 Ureters. The ureters conduct urine from the kidneys to the urinary bladder for storage prior to voiding from the body. (a) Features of a ureter in cross-sectional view. (b) A photomicrograph of a ureter in cross section shows its mucosal folds and thick muscularis.

lamina propria, composed of a fairly thick layer of dense irregular Multiple blood vessels supply blood to the ureters. In general, connective tissue. The continuous production of urine ensures that a segment of ureter receives its blood supply from a branch of the the ureters are rarely completely empty, but as peristaltic waves nearest artery. Thus, the superior aspects of the ureter are supplied propel urine through the ureter, the ureter may be temporarily by the renal arteries, while the more inferior aspects may receive empty at specific places along its length. At these locations, the their arterial supply from the aorta, common iliac, and/or internal mucosa folds to fill the lumen. Thereafter, when the ureter is dis- iliac arteries. Venous drainage is through the companion veins. tended, the mucosa can be stretched; this folding of the mucosa The ureters are innervated by the autonomic nervous sys- allows for considerable increase in the luminal diameter when tem. Parasympathetic axons come from CN X (which supplies needed. the superior region of the ureter) and from the pelvic splanchnic The middle muscularis consists of two smooth muscle lay- nerves (which supply the inferior region of the ureter). There are ers: an inner longitudinal layer and an outer circular layer. (Note no known effects of this innervation. Sympathetic axons come that the arrangement of muscle layers is opposite that in the GI from the T11–L2 segments of the spinal cord. Pain from the ureter tract, where the circular layer is the inner layer.) The presence of (e.g., due to a kidney stone lodged in the ureter) is referred to the urine within the renal pelvis causes these muscle layers to produce T11–L2 dermatomes. These dermatomes are along a “loin-to-groin” peristaltic waves that propel the urine through the ureters into the region, so “loin-to-groin” pain typically means ureter and/or kid- urinary bladder. ney discomfort.

WHAT DO YOU THINK? WHATW DID YOU LEARN?

●3 Why do the ureters use peristalsis to actively pump urine to the ●10 Describe the structure and function of the middle tunic of the urinary bladder? Why don’t they rely on gravity to move the urine ureter. to the inferiorly located bladder? ●11 What nerves innervate the ureters? The external layer of the ureter wall is the adventitia, which is formed from areolar connective tissue. Some extensions of this areolar connective tissue layer also anchor the ureter to the poste- 27.3b Urinary Bladder rior abdominal wall. The urinary bladder is an expandable, muscular container that The ureters project through the posteroinferior bladder wall serves as a reservoir for urine (figure 27.9). The bladder is posi- obliquely, and some smooth muscle fibers of the inner longitudi- tioned immediately posterior to the pubic symphysis. In females, nal layer of the muscularis insert into the lamina propria of the the urinary bladder is anteroinferior to the uterus and directly bladder. Because of the oblique course of the ureters through the anterior to the vagina; in males, the bladder is anterior to the bladder wall, the ureteral walls are compressed as the bladder rectum and superior to the prostate gland. The urinary bladder is distends, decreasing the likelihood of urine refluxing into the a retroperitoneal organ, since only its superior surface is covered ureters from the bladder. with peritoneum.

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Median umbilical ligament

Ureter

Peritoneum

Rugae

Detrusor muscle of muscularis Ureteral openings Transitional Trigone epithelium Mucosa Neck of urinary bladder Lamina propria Submucosa Internal urethral sphincter Detrusor muscle of muscularis Urethra

External urethral sphincter Adventitia (in urogenital diaphragm)

(a) Urinary bladder, anterior view

Transitional epithelium

Lamina propria

LM 78x

Mucosa

Submucosa

Superior wall of distended full bladder

Detrusor muscle of muscularis Superior wall of empty bladder

Urethra LM 18x

(b) Histology of urinary bladder (c)

Figure 27.9 Urinary Bladder. (a) The urinary bladder is an expandable, muscular sac. This view depicts a female bladder. (b) Photomicrographs of the urinary bladder wall show its tunics. (c) Sagittal views show that the urinary bladder expands superiorly and becomes more oval in shape as it fills with urine.

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CLINICAL VIEW Renal Calculi Most stones smaller than about 4 millimeters in diameter eventually pass through the urinary tract on their own once the patient drinks plenty of A renal calculus (kal ́ku¯-lus, pl., calculi; pebbles), or kidney stone, water (2–3 quarts per day) to assist movement. But if the stone is too is formed from crystalline minerals that build up in the kidney. large (more than 8 millimeters in diameter) and doesn’t pass on its own, Normally, urine contains chemicals that prevent these minerals medical intervention is required. The most common treatment is litho- from solidifying, but in some individuals these minerals condense tripsy (lith ó¯-trip-se¯; tresis = boring), whereby ultrasound or shock waves to form hard, rocky structures. Renal calculi can be formed from a are directed toward the stones to pulverize them into smaller particles variety of different minerals. Over 75% of them contain calcium, that can be expelled in the urine. Alternatively, using ureteroscopy, a in combination with either oxalate or phosphate. The second most scope is inserted from the urethra into the urinary bladder and ureter common type are struvite (stroo vı́ ¯t) stones formed from ammonium to break up and remove the stone. and phosphate. If these treatments aren’t viable, traditional surgery may be required. Causes and risk factors for kidney stone formation include inadequate fluid intake and dehydration, reduced urinary flow and volume, and cer- People prone to kidney stones tain abnormal chemical or mineral levels in the urine. Hypercalcuria, are advised to modify their diet Renal calculi a high level of calcium in the urine, leads to calcium stones. Frequent in accordance with the types of urinary tract infections may predispose a person to struvite stones. stones they have. To reduce oxa- Certain medical conditions as well as diet can also be correlated with lates in the blood, people are urged Kidney renal calculi formation. In general, males tend to develop stones more to avoid foods high in oxalates, often than females. such as spinach, Swiss chard, sweet potato, and chocolate. Reduction Ureter If the renal calculi are very small, they are asymptomatic, and the in salt intake and increase in water person excretes them without ever realizing it. However, a larger stone Urinary intake is recommended for all kid- can become obstructed in the kidney, renal pelvis, or ureter. The term bladder ney stone sufferers, as is maintain- urolithiasis (u¯-ro¯-li-thı¯ ́a˘-sis; lithos = stone) refers to the presence ing calcium-rich food intake. Renal of renal calculi anywhere along the urinary tract. Symptoms include calculus severe, cramping pain along the “loin-to-groin” region and possibly nausea and vomiting. The epithelium of the ureter becomes inflamed Renal calculi may become lodged as it tries to push the stone along its path, resulting in blood in the at various sites along the urinary Urethra urine, called hematuria (he¯-ma˘-too ́re¯-a). tract.

When empty, the urinary bladder exhibits an upside-down Additionally, mucosal folds, called rugae, allow for even greater pyramidal shape. Filling with urine distends it superiorly until it distension. Within the trigone region, the mucosa is smooth, thick, assumes an oval shape (figure 27.9c). A fibrous, cordlike median and lacking rugae. The submucosa lies immediately external to umbilical ligament extends toward the umbilicus from its origin the mucosa and is formed by dense irregular connective tissue that on the anterosuperior border of the urinary bladder. It is a rem- supports the urinary bladder wall. nant of the embryologic structure called the urachus. Ureters enter The muscularis consists of three layers of smooth muscle, the posterolateral wall of the urinary bladder through the oblique collectively called the detrusor (dē -troo ́ser, -sō r; detrudo = to drive ureteral openings. The constricted neck of the bladder is located away) muscle. These smooth muscle bundles exhibit such com- inferiorly and connected to the urethra. plex orientations that it is difficult to delineate individual layers in A posteroinferior triangular area of the urinary bladder random histologic sections. At the neck of the urinary bladder, an wall, called the trigone (trı̄ ́gō n; trigonum = triangle), is formed involuntary internal urethral sphincter is formed by the smooth by imaginary lines connecting the two ureteral openings and the muscle that encircles the urethral opening. urethral opening. The trigone does not move and remains in place The adventitia is the outer layer of areolar connective tissue as the urinary bladder fills and evacuates. It functions as a funnel of the urinary bladder. A peritoneal membrane covers only the to direct the stored urine into the urethra as the bladder wall con- superior surface of the urinary bladder, and in this superior region, tracts. The trigone is embryologically different from the rest of the the peritoneum plus the connective tissue forms a serosa. urinary bladder. While the urinary bladder forms from a structure Arterial blood vessels extend to the urinary bladder and called the cloaca, the trigone forms from the distal parts of the penetrate its wall from branches of the internal iliac artery. Venous ureters, which become incorporated into the posterior wall of the blood drains into the internal iliac veins. urinary bladder. The four tunics that form the wall of the bladder are the Micturition mucosa, submucosa, muscularis, and adventitia. The mucosa lines The expulsion of urine from the bladder is called micturition the bladder lumen; it is formed by a transitional epithelium that (mik-choo-rish ́ŭn; micturio = to desire to make water) or urina- accommodates the shape changes occurring with distension, and tion (ū r ́i-nā sh́ ŭn). Micturition is initiated by a complex sequence by a highly vascularized lamina propria that supports the mucosa. of events called the micturition reflex. The bladder is supplied by

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both parasympathetic and sympathetic axons of the autonomic 27.3c Urethra nervous system. The parasympathetic axons (pelvic splanchnic The urethra (ū -rē thŕ ă) is a fibromuscular tube that originates at nerves) come from the micturition reflex center located in spinal the neck of the urinary bladder and conducts urine to the exterior cord segments S2–S4. The pelvic splanchnic nerves relax the inter- of the body (figure 27.10). The luminal lining of the urethra is nal urethral sphincter so that urine can pass through and stimulate a protective mucous membrane that houses clusters of mucin- contraction of the detrusor muscle. Thus, the parasympathetic producing cells called urethral glands. Bundles of smooth muscle axons stimulate micturition. The sympathetic axons are from the fibers surround the mucosa and help propel urine to the outside T11–L2 segments of the spinal cord. These axons cause contrac- of the body. tion of the internal urethral sphincter and inhibit contraction of Two urethral sphincters restrict the release of urine until the the detrusor muscle. Thus, sympathetic axons inhibit micturition. pressure within the urinary bladder is high enough and voluntary The micturition reflex occurs in a series of steps: activities needed to release the urine are activated. The internal 1. When the bladder fills with urine and becomes distended, urethral sphincter is the involuntary, superior sphincter sur- stretch receptors in the bladder wall are activated, and they rounding the neck of the bladder, where the urethra originates. signal the micturition reflex center. This sphincter is a circular thickening of the detrusor muscle and 2. Impulses within the parasympathetic division of the is controlled by the autonomic nervous system. The external ure- autonomic nervous system travel to both the internal urethral thral sphincter is inferior to the internal urethral sphincter and sphincter and the detrusor muscle. is formed by skeletal muscle fibers of the urogenital diaphragm. 3. The smooth muscle in the internal urethral sphincter relaxes, This sphincter is a voluntary sphincter controlled by the somatic and the smooth muscle in the detrusor muscle contracts. nervous system. This is the muscle children learn to control when 4. The person’s conscious decision to urinate causes relaxation they become “toilet-trained.” of the external urethral sphincter. The male and female urethras differ slightly in length and 5. In addition to the squeezing action of the detrusor muscle on morphology. the volume of the urinary bladder, the expulsion of urine is facilitated by contraction of muscles in the abdominal wall Female Urethra and expiratory muscles. The female urethra has a single function: to transport urine to the 6. Upon emptying of the urinary bladder, the detrusor muscle exterior of the body. The lumen of the female urethra is primarily relaxes, and the neurons of the micturition reflex center are lined with a stratified squamous epithelium. The urethra is 3 to inactivated. 5 centimeters long, and opens to the outside of the body at the external urethral orifice located in the female perineum. Male Urethra CLINICAL VIEW The male urethra has two functions—urinary and reproductive— because it serves as a passageway for both urine and semen, but Urinary Tract Infections not at the same time. It is approximately 18 to 20 centimeters long and is partitioned into three segments: the prostatic urethra, the A urinary tract infection (UTI) occurs when bacteria (most membranous urethra, and the spongy urethra. commonly E. coli) or fungi enter and multiply within the urinary The prostatic (pros-tat ́ik) urethra (figure 27.10b) is approxi- tract. Women are more prone to UTIs because they have a short mately 3 to 4 centimeters long and is the most dilatable portion of urethra that is close to the anus, allowing bacteria from the GI the urethra. It extends through the prostate gland, immediately tract to more readily enter the female urethra. Sexual intercourse inferior to the male bladder, where multiple small prostatic ducts also increases the risk of UTIs in both sexes. In addition, UTIs enter it. The urethra in this region is lined by a transitional epi- are associated with the medical use of a urinary catheter, a tube thelium, with many blood vessels in the underlying dense irregu- inserted through the urethra into the urinary bladder to help void lar connective tissue. Two smooth muscle bundles surround the urine. People who have had more than one UTI are at greater risk mucosa: an internal longitudinal bundle and an external circular for developing others. bundle. The external circular muscular bundles are a continuation A UTI often develops first in the urethra, an inflammation called of the thickened circular region of smooth muscle forming the urethritis (u¯-re¯-thrı¯ ́tis). If the infection spreads to the urinary internal urethral sphincter at the bladder outlet. bladder, cystitis (sis-tı¯ ́tis) results. Occasionally, bacteria from The membranous (mem ́bră-nus) urethra is the shortest an untreated UTI can spread up the ureters to the kidneys, a and least dilatable portion of the male urethra. It extends from condition termed pyelonephritis (pı¯ ́e˘ -lo¯-ne-frı¯ ́tis). the inferior surface of the prostate gland through the urogenital diaphragm. As a result, it is surrounded by striated muscle fibers Symptoms of a UTI include difficult and painful urination, called that form the external urethral sphincter of the urinary bladder. dysuria (dis-u¯ ́re¯-a˘ ; dys = bad, difficult); the need to urinate The epithelium in this region is often either stratified columnar or frequently; and a feeling of uncomfortable pressure in the pubic pseudostratified columnar. region. Some people experience little pain or discomfort, but if The spongy (spun ̆ ́jē ) urethra is the longest part (15 cen- the infection spreads to the kidneys, sharp back and flank pain, timeters) of the male urethra. It is encased within a cylinder of fever, and occasionally nausea and vomiting occur. A UTI can be erectile tissue in the penis called the corpus spongiosum, and diagnosed through urinalysis (u¯-ri-nal ́i-sis), a test of the urine extends to the external urethral orifice. The proximal part of that can reveal the presence of inflammatory cells, blood, and the spongy urethra is lined by a pseudostratified columnar epi- bacteria or fungi. Appropriate antibiotic therapy cures most UTIs. thelium, while the distal part has a lining of stratified squamous epithelium.

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Ureteral openings Ureteral openings

Urinary bladder Urinary bladder

Internal urethral sphincter Internal urethral External urethral sphincter sphincter Urethra Prostate gland Prostatic External urethral Urogenital urethra sphincter diaphragm Bulbourethral gland Bulb of the vestibule Membranous Urogenital Bulbospongiosus urethra diaphragm muscle

External urethral orifice Labium Labium minus majus (a) Female urethra

Urethra

Spongy Penis urethra

Figure 27.10 Urethra. The urethra conducts urine from the urinary bladder to the external urethral orifice. These coronal views compare (a) a female urethra and (b) a male urethra.

External urethral orifice (b) Male urethra

27.4 Aging and the Urinary System WHATW DID YOU LEARN? Learning Objective: ●12 Where is the trigone located? How does it assist urinary bladder function? 1. Describe the changes in the urinary system resulting from increasing age. ●13 What is the initiating stimulus that activates the micturition reflex? Changes in the size and functioning of the kidneys normally ●14 Which portion of the male urethra is the shortest and least begin in our third decade of life and continue thereafter. With dilatable? age comes a reduced blood flow to the kidneys and a decrease in

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the number of functional nephrons. The reduction in blood flow drains urine from each mesonephros to the developing urinary contributes to a decreased glomerular filtration rate; consequently, bladder. The mesonephros (or “intermediate” kidney) persists until both reabsorption and secretion are reduced. The loss of nephrons about week 10 of development. results in a diminished ability to filter and cleanse the blood as Early in the fifth week, the final set of excretory organs, well as a decrease in the number of target cells within the kidneys each called a metanephros (or permanent kidney), forms. Each that are capable of responding to stimulation by aldosterone or metanephros (pl., metanephroi) forms a functional adult kidney antidiuretic hormone. Thus, the ability to control blood volume and takes over urine production by week 10 (figure 27.11). The and blood pressure is reduced. components of the metanephros form from two specific structures: Structural changes in the urinary bladder also affect the stor- the ureteric bud and the metanephric mesoderm (table 27.3). age and voiding of urine. The bladder decreases in size, and smooth The ureteric (ū -rē -ter ́ik) bud forms as a separate outgrowth muscle tone in the bladder wall gradually diminishes, which may from the caudal end of the mesonephric duct and gives rise to the lead to more frequent urination. Sometimes, there is a delay in ureter, renal pelvis, calyces, and collecting ducts of the kidney. noticing the urge to urinate. Additionally, control of the urethral The metanephric (met-ă-nef ́rik) mesoderm develops from inter- sphincters—and eventually even control of micturition—may be lost. mediate mesoderm in the sacral region of the embryo, and forms The inability to control the expulsion of urine is called the nephron components of the kidney: the glomerular capsule, incontinence. It most commonly occurs among the elderly, and the proximal convoluted tubule, the nephron loop, and the distal most frequently in women. The extent of the incontinence ranges convoluted tubule. The metanephric mesoderm will not grow and from occasional urine leakage to complete inability to retain urine. develop unless the ureteric bud grows into and merges with it. There are two categories of incontinence: (1) stress incontinence As the ureteric bud grows and merges with the metanephric often occurs during vigorous exercise or strenuous coughing, and mesoderm, the ureteric bud undergoes a branching pattern (fig- (2) urge incontinence results from immediate bladder contraction ure 27.11b). The first set of branches forms the major calyces in after feeling a strong need to urinate. Causes of incontinence week 6, and the second set of branches forms the minor calyces in include severe weight gain, complications following pelvic surgery, week 7. This branching pattern continues until the tiny collecting coincidence with uncontrolled diabetes, severe constipation, pelvic ducts are formed in the kidney, so that by week 32, well over 1 mil- prolapse (in females) as a result of pregnancy, and enlarged pros- lion collecting tubules have formed. As the calyces and collecting tate (in males). ducts form, they signal the metanephric mesoderm to grow and develop into the nephron components. By week 10 of development, WHATW DID YOU LEARN? the metanephros is able to both produce and expel urine. Fetal urine supplements amniotic fluid production. ●15 What are the possible consequences of aging on urine production? During weeks 6–9, the developing kidneys migrate from their position in the pelvic cavity to a more superior position in the lumbar portion of the abdominal cavity (figure 27.11c). The 27.5 Development of the Urinary mechanisms for this ascent are not known, but may be related to differential growth of the embryo. As the kidneys ascend, they System obtain temporary blood vessels from the nearby vasculature. Learning Objective: When the kidneys migrate further, the older temporary blood ves- 1. Explain the embryonic and fetal development of the sels regress and degenerate, and new blood vessels for the kidney urinary system. form. Eventually, by week 9, the kidneys complete migration to the lumbar region and acquire their permanent renal arteries Most of the urinary system and the reproductive system (dis- branching from the descending abdominal aorta. cussed in chapter 28) form from the intermediate mesoderm of the embryo. Intermediate mesoderm is located between the somites (paraxial mesoderm) and the lateral plate mesoderm. Thus, the 27.5b Urinary Bladder and Urethra Development urinary system and the reproductive system are linked in both The urinary bladder and urethra develop from the distal part of the their development and their postnatal functions. hindgut called the cloaca (klō -ā ́kă; sewer). The cloaca forms not only the epithelium of the urinary bladder and urethra, but also the 27.5a Kidney and Ureter Development rectum and anal canal. The cloaca is separated from the outside of Some of the intermediate mesoderm forms bilateral longitudinal the body by a thin cloacal membrane. Extending superiorly from ridges, each called a urogenital ridge. The urogenital ridges are located on either side of the vertebral column and give rise to three sets of excretory organs, which become increasingly more advanced. The first excretory organs, each called a pronephros Table 27.3 Embryonic Derivations of Ureter and Kidney Structures (prō -nef ́ros; before kidney; pl., pronephroi), appear in the cervical region of the embryo and seem to be vestigial—that is, they have Ureteric Bud Origin Metanephric Mesoderm Origin no known function in humans. They appear in the early fourth Ureter Renal corpuscles week and quickly degenerate by the end of that same week. Renal pelvis Proximal convoluted tubules The second set of organs, each called a mesonephros (pl., mesonephroi), appears just before the pronephroi degenerate. These Major calyces Nephron loops structures are formed from tissue comprising the urogenital ridge Minor calyces Distal convoluted tubules in the thoracic and lumbar regions. Each mesonephros is composed Collecting tubules and ducts of multiple saclike segments. A mesonephric (mez-ō -nef rik)́ duct

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Degenerating pronephros Mesonephric Ureteric bud duct

Metanephric mesoderm Week 5

Renal pelvis Major calyx Mesonephros

Week 6 Ureter Renal pelvis

Allantois Major calyces Hindgut Mesonephric Minor calyces duct Cloaca MMetanephricetanephric mesoderm Metanephros Ureteric bud Week 7

(a) Week 5 (b) Metanephric kidney formation

Figure 27.11 Kidney Kidney Development. The kidneys form from the urogenital ridges of the embryo. (a) By week 5, the Urinary bladder Ureter pronephroi have regressed, and the mesonephroi begin producing urine. The metanephric kidneys, which eventually Kidney migration form the permanent kidneys, also begin to form. (b) The path Rectum development of the metanephric kidney is shown between weeks 5 and 7. (c) Between weeks 6 and 9, the kidney migrates superiorly from its initial location in the pelvic cavity to the lumbar region on the posterior abdominal wall.

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CLINICAL VIEW Kidney Variations the abdominal cavity. The single, large kidney looks like a horse- and Anomalies shoe. The kidney’s superior migration is usually halted as it gets stuck around the origin of the inferior mesenteric artery. Horseshoe Because the kidneys develop from multiple embryologic structures kidneys are fairly common, occurring in about 1 per 600 births. Like and must migrate from the pelvic cavity to the lumbar region of the a pelvic kidney, a horseshoe kidney typically is asymptomatic and abdominal cavity, variations in both kidney development and place- functions normally. ment are common. Supernumerary (soo-per-noo mer-á ¯r-e¯) kidneys are extra kidneys Renal agenesis is the failure of a kidney to develop. Kidney development that develop. They appear to be caused by ureteric bud duplication. ceases if the ureteric bud and the metanephric mesoderm do not grow However, supernumerary kidneys are very rare. It is more common toward each other and meet. Failure of one kidney to develop, called to see a duplicated or bifid ureter, from a duplicated ureteric unilateral renal agenesis, occurs in about 1 per 1000 births, while bilateral bud, traveling to a single kidney. Typically, these have no clinical renal agenesis occurs in about 1 per 3000 births. Unilateral renal agenesis significance. is often asymptomatic, whereas bilateral renal agenesis is invariably fatal. A pelvic kidney can be present if the developing kidney fails to migrate from the pelvic cavity to the abdominal cavity. The pelvic kidney receives its blood supply from branches of the common iliac artery, as opposed to a renal artery that branches from the aorta. A pelvic kidney usually has normal function and causes no problems for the individual. A horseshoe kidney develops when the inferior parts of the left and right kidneys fuse as they try to ascend from the pelvic cavity into

Bifid ureter (arrows).

Finally, because the kidneys must migrate from the pelvic cavity into the abdominal cavity, it is fairly common for multiple renal vessels to supply the kidney. Normally, as the kidney migrates, it acquires temporary vessels from the nearby blood supply, and its older, inferior vessels regress and degenerate. If these temporary vessels fail to Horseshoe kidney is a fairly common variation. degenerate, the kidney is left with multiple vessels.

the cloaca to the umbilicus is a thin, tubular hindgut extension develop into the urinary bladder and urethra. The urorectal septum called the allantois (ă-lan ́tō -is). (By week 6, the allantois becomes grows toward and attaches to the cloacal membrane, subdividing it a fibrous cord called the urachus, and in adults the remnant of this into an anterior urogenital membrane and a posterior anal mem- cord is called the median umbilical ligament.) brane. Both of these membranes rupture by week 8, allowing the Between weeks 4 and 7 of development, a mass of mesodermal urethra and the anal canal to each communicate with the outside tissue called the urorectal (u ŕ ō -rek t́ ăl) septum grows through the of the body. cloaca and toward the cloacal membrane (figure 27.12). This septum subdivides the cloaca into an anterior urogenital (u¯ ŕ ō -jen ́i-tăl) WHATW DID YOU LEARN? sinus and a posterior anorectal (ā ń ō -rek t́ ăl) canal. The urogenital sinus (formed from endoderm) and some surrounding mesoderm ●16 What structures are formed by the cloaca?

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Urachus Mesonephros Mesonephros

Gut tube Mesonephric Mesonephric duct Allantois duct Urinary bladder Developing Urogenital sinus metanephros Cloacal Urogenital membrane membrane Cloaca Urorectal septum Ureteric bud Anorectal canal Developing Anal membrane metanephros

(a) Week 4 (b) Week 7: Urorectal septum divides cloaca into urogenital sinus and anorectal canal Figure 27.12 Bladder Development. (a) A common cloaca unites the distal part of the gut tube with the future urinary bladder. The cloaca is separated from the outside of the body by a thin cloacal membrane. (b) A tissue mass called the urorectal septum grows inferiorly and partitions the cloaca into an anteriorly located urogenital sinus (which forms the urinary bladder and urethra) and an anorectal canal. The cloacal membrane is also subdivided into an anteriorly located urogenital membrane and a posteriorly located anal membrane.

Clinical Terms

diuretic (dı̄-ū -ret ́ik; dia = throughout) Any chemical or substance nocturia (nok-too ́rē- ă; nox = night, ouron = urine) Excessive that increases urine output. Examples are alcohol, caffeinated urinating at night. drinks, and diuretic drugs prescribed for high blood pressure. polyuria (pol-ē- ū ŕ ē- ă; polys = much) Excessive secretion of urine resulting in frequent, copious urination.

Chapter Summary

27.1 General ■ The urinary system is composed of the kidneys and the urinary tract (the ureters, the urinary bladder, and the urethra). Structure and ■ The urinary system filters the blood, transports and stores urine, excretes urine, helps maintain blood volume, and Functions of the regulates erythrocyte production. Urinary System 818 27.2 Kidneys 820 27.2a Gross and Sectional Anatomy of the Kidney 820 ■ Usually the right kidney is slightly inferior to the left kidney because of the space occupied by the liver. ■ The kidneys are surrounded by a strong fibrous capsule, perinephric fat, renal fascia, and paranephric fat. ■ The kidney contains the renal cortex (peripheral) and the renal medulla (central). The renal medulla is subdivided into renal pyramids, each with an apex called a renal papilla that projects into the funnel-shaped minor calyx. ■ Urine deposited within the minor calyx flows into the major calyx and then into the renal pelvis.

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Kidneys 27.2b Blood Supply to the Kidney 821 (continued) 820 ■ Blood is transported to the kidneys in renal arteries, which divide into segmental arteries that form interlobar arteries, which give rise to arcuate arteries. Arcuate arteries branch into interlobular arteries, which supply the glomerulus via the afferent arteriole. ■ Blood leaving the glomerulus follows this path: efferent arteriole, peritubular capillaries or vasa recta, interlobular veins, arcuate veins, interlobar veins, and renal vein. 27.2c Nephrons 824 ■ The nephron is the structural and functional unit of the kidney. Nephrons are composed of a renal corpuscle and a renal tubule. ■ The renal corpuscle is the site of blood filtration. ■ The proximal convoluted tubule and nephron loop reabsorb almost all plasma proteins, nutrients, and ions from the tubular fluid. ■ The distal convoluted tubules originate from the nephron loop. 27.2d How Tubular Fluid Becomes Urine 828 ■ Distal convoluted tubules empty into collecting tubules that empty into collecting ducts, which merge to empty into the papillary duct that opens at the edge of the renal papilla and releases urine into the minor calyx. 27.2e Juxtaglomerular Apparatus 828 ■ The juxtaglomerular apparatus has juxtaglomerular cells (smooth muscle cells of the afferent arteriole) and a macula densa (distal convoluted tubule epithelial cells adjacent to the afferent arteriole). It releases renin to regulate blood pressure. 27.2f Innervation of the Kidney 828 ■ The renal plexus innervates each kidney with sympathetic and parasympathetic axons.

27.3 Urinary ■ The urinary tract consists of paired ureters, a urinary bladder, and a single urethra. Tract 829 27.3a Ureters 829 ■ The ureters are retroperitoneal, fibromuscular tubes that conduct urine from the kidneys to the urinary bladder. ■ The ureteral mucosa has a transitional epithelium; the muscularis consists of two layers of smooth muscle that contract to squeeze urine to the urinary bladder. 27.3b Urinary Bladder 830 ■ The urinary bladder stores urine. ■ The mucosa is formed by a transitional epithelium; the submucosa is formed by dense irregular connective tissue, and the muscularis is composed of three layers of smooth muscle bundles (the detrusor muscle). ■ The adventitia is the external layer of the urinary bladder. ■ Parasympathetic stimulation promotes micturition, and sympathetic stimulation inhibits it. 27.3c Urethra 833 ■ The urethra conducts urine from the bladder to the exterior of the body. ■ An internal urethral sphincter (involuntary) and an external urethral sphincter (voluntary) control the voiding of urine. ■ The female urethra is relatively short, and has a single function: to transport urine. ■ The male urethra serves as a passageway for both urine and semen. It is about 18 to 20 centimeters long and is partitioned into the prostatic urethra, the membranous urethra, and the spongy urethra.

27.4 Aging and ■ Structural changes in the kidneys and urinary bladder affect the production, storage, and voiding of urine. the Urinary ■ Incontinence is the inability to control urine expulsion. System 834

27.5 Development ■ Most of the early urinary system and the reproductive system form from the intermediate mesoderm. of the Urinary System 835 27.5a Kidney and Ureter Development 835 ■ The intermediate mesoderm condenses along the posterior body wall and forms a urogenital ridge on each side of the vertebral column. ■ Three successive sets of excretory organs form: the pronephros, the mesonephros, and the metanephros (or permanent kidney). 27.5b Urinary Bladder and Urethra Development 835 ■ The urinary bladder and urethra develop from the distal part of the hindgut called the cloaca.

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Challenge Yourself

Matching ______5. The arteries located at the corticomedullary junction Match each numbered item with the most closely related lettered of the kidney are the item. a. arcuate arteries. b. segmental arteries. ______1. distal convoluted tubule a. location of renal c. interlobar arteries. corpuscle ______2. urethra d. renal arteries. b. expels urine outside the ______6. Which statement is false about the kidneys? ______3. ureter body a. The right kidney is positioned more inferiorly than ______4. peritubular capillaries the left kidney. c. major calyces empty into b. The cortex is subdivided into renal pyramids. ______5. glomerulus this funnel-shaped region c. The renal artery, renal vein, and ureter connect to ______6. efferent arteriole d. most secretion occurs in the kidney at its hilum. this nephron segment d. The kidney is covered by a fibrous capsule. ______7. urinary bladder e. stores urine until it is ______7. Urine in a major calyx of the kidney next travels to the ______8. renal pyramid voided a. ureter. ______9. cortex b. minor calyx. f. structural units that c. urinary bladder. ______10. renal pelvis constitute the medulla d. renal pelvis. g. conducts blood out of ______8. Which structure is not controlled by the autonomic the glomerulus nervous system? a. muscularis of the ureter h. vessels involved in b. external urethral sphincter reabsorption c. internal urethral sphincter i. site of plasma filtration d. detrusor muscle of the urinary bladder j. conducts urine from ______9. Reabsorption is the movement of fluid and solutes kidney to bladder from the a. filtrate into the glomerular capillaries. b. tubular fluid into the capsular space. Multiple Choice c. tubular fluid into the peritubular capillaries. d. blood vessels into the collecting ducts. Select the best answer from the four choices provided. ______10. The micturition reflex controls ______1. Which organ is responsible for filtering the blood? a. urine formation. a. ureter b. voiding of the filled bladder. b. urinary bladder c. reabsorption of glucose from filtrate. c. kidney d. filling of the urinary bladder. d. urethra ______2. Which statement is true about the urinary bladder? Content Review a. The bladder neck is surrounded by the external 1. What are the basic functions of the urinary system? urethral sphincter. 2. Describe the connective tissue coverings that surround the b. The detrusor muscle contains only two layers of kidney, from internal to external. Why are these coverings smooth muscle. especially important to kidney structure and function? c. The bladder is lined with transitional 3. Map the flow of blood into and out of the kidney. List epithelium. which structures carry oxygenated blood and which d. The bladder receives urine from the kidneys via carry deoxygenated blood. In addition, list the structures the two urethras. responsible for gas exchange and reabsorption of materials ______3. Tubular fluid from the proximal convoluted tubule from the filtrate. next travels to the 4. Describe the anatomic structure of the glomerulus and the a. capsular space. visceral layer of the glomerular capsule. b. collecting duct. c. distal convoluted tubule. 5. Why are microvilli prominent on the apical surface of the d. nephron loop. proximal convoluted tubule epithelium but not in the distal convoluted tubule? ______4. The apex of a renal pyramid is called the renal a. calyx. 6. What do the cells of the juxtaglomerular apparatus secrete? b. papilla. What function does this product perform? c. column. 7. What prevents urine stored in the urinary bladder from d. capsule. being forced back through the ureters to the kidney?

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8. Describe the innervation of the ureters and urinary bladder. Jason that his symptoms were the result of dehydration. 9. Trace the course of fluid movement, beginning with the Based on your knowledge of the urinary system, how and production of filtrate in the renal corpuscle and ending with why did Jason become dehydrated? What hormone normally the expulsion of urine from the urethra. regulates the amount of water in the urine, and how did the alcohol interfere with this hormone’s function? 10. What is the cause of a urinary tract infection? Why are these infections more common in women? 2. Males who suffer from either benign prostatic hypertrophy (noncancerous prostate gland enlargement) or prostate Developing Critical Reasoning cancer often have problems with urination. Based on your knowledge of the male urethra, hypothesize why these 1. While drinking many beers one night, Jason noticed that urination problems occur. he had to urinate more frequently. The following morning, Jason’s mouth felt dry, and he had a headache. A friend told

Answers to “What Do You Think?”

1. Without functioning kidneys, the blood would not be able 3. When we are lying down, gravity is unable to passively to be filtered, so waste products would accumulate. This transport urine to the urinary bladder. Thus, peristalsis is accumulation of toxic material in the blood leads to death also needed so that urine can be actively pumped from the unless the materials are filtered out. ureters to the urinary bladder no matter what position the 2. ADH is secreted when the body is dehydrated, so the body body is in. can conserve what remaining water it has.

www.mhhe.com/mckinley3 Enhance your study with practice tests and activities to assess your understanding. Your instructor may also recommend the interactive eBook, individualized learning tools, and more.

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