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OUTLINE 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 mck78097_ch27_817-841.indd 817 2/25/11 2:24 PM 818 Chapter Twenty-Seven Urinary System 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. mck78097_ch27_817-841.indd 818 2/25/11 2:24 PM Chapter Twenty-Seven Urinary System 819 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 mck78097_ch27_817-841.indd 819 2/25/11 2:24 PM 820 Chapter Twenty-Seven Urinary System 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 Anterior 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. mck78097_ch27_817-841.indd 820 2/25/11 2:24 PM Chapter Twenty-Seven Urinary System 821 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.
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