Urinary System Objectives * Describe the histologic features of the kidneys, ureters and bladder. * Describe the structures that comprise the renal filtration barrier and their role in formation of glomerular filtrate (provisional urine). * Describe the role of the loop of Henle in concentrating urine. * Describe how aldosterone and antidiuretic hormone (ADH) affect the renal tubules. * Trace the pathway of urine flow along the nephron and urinary tract. OVERVIEW OF THE URINARY SYSTEM * The urinary system consists of - The paired kidneys; - Paired ureters, which lead from the kidneys to - The urinary bladder; and - The urethra, which leads from the bladder to the exterior of the body. Functions of the Urinary System * Filtration & excretion of cellular wastes from blood * Regulation of fluid and electrolyte balance by selective reabsorption and excretion of water and solutes * Production of the hormones renin and erythropoietin Extend from the 12th thoracic to the 3rd lumbar vertebrae, Reddish, bean-shaped organs Renal hilum The Urinary System Kidney Organization Kidney Organization Parenchyma Renal sinus * Cortex - Renal pelvis - Renal corpuscles - Major and minor calyces - Medullary rays - Nerves and vessels * Medulla - Connective tissues - Renal pyramids * Renal columns Kidney Organization Kidney Organization Kidney Cortex Medullary Rays Kidney Cortex A labyrinth of tubules Kidney Medulla and Renal Papillae Photomicrograph of human kidney capsule. This photomicrograph of a Mallory- Azan–stained section shows the capsule (cap) and part of the underlying cortex. The outer layer of the capsule (OLC ) is composed of dense connective tissue. The fibroblasts in this part of the capsule are relatively few in number; their nuclei appear as narrow, elongate, red-staining profiles against a blue background representing the stained collagen fibers. The inner layer of the capsule (ILC ) consists of large numbers of myofibroblasts whose nuclei appear as round or elongate, red-staining profiles, depending on their orientation within the section. Note that the collagen fibers in this layer are relatively sparse and that the myofibroblast nuclei are more abundant than those of the fibroblasts in the outer layer of the capsule. X180. Cortex Medulla Kidney Cortex Medullary Rays Kidney Cortex Medullary Rays (Collect tubules) Diagram showing standard nomenclature for structures in the kidney. The two types of nephrons in the kidney are shown with their associated duct systems. A long-looped nephron is shown on the left, and a short-looped nephron is shown on the right. The relative position of the cortex, medulla, papilla, and capsule are indicated (not drawn to scale). The inverted cone-shaped area in the cortex represents a medullary ray. The parts of the nephron are indicated by number: 1, renal corpuscle including the glomerulus and Bowman’s capsule; 2, proximal convoluted tubule; 3, proximal straight tubule; 4, descending thin limb; 5, ascending thin limb; 6, thick ascending limb (distal straight tubule); 7, macula densa located in the final portion of the thick ascending limb; 8, distal convoluted tubule; 9, connecting tubule; 9*, connecting tubule of the juxtamedullary nephron that forms an arch (arched connecting tubule); 10, cortical collecting duct; 11, outer medullary collecting duct; and 12, inner medullary collecting duct. (Modified from Kriz W, Bankir L. A standard nomenclature for structures of the kidney. The Renal Commission of the International Union of Physiological Sciences (IUPS). Kidney Int 1988;33:1–7.) * The renal columns represent cortical tissue contained within the medulla. - The caps of cortical tissue that lie over the pyramids are sufficiently extensive that they extend peripherally around the lateral portion of the pyramid, forming the renal columns (of Bertin). Renal columns contain the same components as the rest of the cortical tissue, they are regarded as part of the medulla. Kidney Lobes and Lobules * The number of lobes in a kidney equals the number of medullary pyramids. - Each medullary pyramid and associated cortical tissue at its base an sides (one half of each adjacent renal column) constitute a lobe of the kidney. - The lobar organization of the kidney is conspicuous in the developing fetal kidney. Each lobe is reflected as a convexity on the outer surface of the organ, but they usually disappear after birth. The surface convexities typical of the fetal kidney may persist, however, until the teenage years and, in some cases, into adulthood. - Each human kidney contains 8 to 18 lobes. - Kidneys of some animals possess only one pyramid; these kidneys are classified as unilobar, in contrast to the multilobar kidney of the human. Photomicrograph of fetal kidney. This photomicrograph of an H&E–stained human fetal kidney shows the cortex, the medulla, and two associated pyramids. Note each surface convexity corresponds to a kidney lobe. During postnatal life, the lobar convexities disappear and the kidney then exhibits a smooth surface. X30. * A lobule consists of a collecting duct and all the nephrons that it drains. - The lobes of the kidney are further subdivided into lobules consisting of a central medullary ray and surrounding cortical material. - the center or axis of a lobule is readily identifiable, the boundaries between adjacent lobules are not obviously . - The concept of the lobule has an important physiologic basis; the medullary ray containing the collecting duct for a group of nephrons. The Urinary System Kidney Circulation Blood Supply to the Kidney Most important for you to know . Approximately 90% to 95% of the blood passing through the kidney is in the cortex; 5% to 10% is in the medulla. Efferent arterioles Afferent arterioles LM – injected kidney vascular system SEM - Ilu, intralobular artery Af, afferent arteriole; Ef, efferent arteriole; Gl, glomerulus Blood Flow in the Kidney Afferent arteriole Glomerular capillaries Efferent arteriole or Peritubular capillaries Vasa recta Supply cortical nephrons and Supply juxtamedullary nephrons, proximal and distal structures in the medulla, and then convoluted tubules loop back to cortex-medullary boundary Blood Supply to a Renal Corpuscle The Nephron * The nephron is the structural and functional unit of the kidney. - The nephron is the fundamental structural and functional unit of the kidney. - Both human kidneys contain approximately 2 million nephrons. - Nephrons are responsible for the production of urine. - A nephron includes: . The renal corpuscle . Tubules - proximal convoluted tubule - loop of Henle - distal convoluted tubule General Organization of the Nephron * The nephron consists of the renal corpuscle and a tubule system. - The renal corpuscle represents the beginning of the nephron. - It consists of the glomerulus, . a tuft of capillaries composed of 10 to 20 capillary loops, surrounded by a double-layered epithelial cup, the renal or Bowman’s capsule. Bowman’s capsule is the initial portion of the nephron, where blood flowing through the glomerular capillaries undergoes filtration to produce the glomerular ultrafiltrate. - The glomerular capillaries are supplied by an afferent arteriole and are drained by an efferent arteriole that then branches, forming a new capillary network to supply the kidney tubules. - The site where the afferent and efferent arterioles penetrate and exit from the parietal layer of Bowman’s capsule is called the vascular pole. - Opposite this site is the urinary pole of the renal corpuscle, where the proximal convoluted tubule begins. Efferent arteriole Bowman’s capsule Afferent arteriole Glomerulus Interlobular artery Arcuate v & a Ascending & descending loops (Henle) Collecting tubule Renal Corpuscle A renal corpuscle consists of three things: Glomerulus - A tuft of capillaries Bowman’s capsule - The container that surrounds the glomerulus - Two layers: visceral (inner) and parietal (outer) Urinary space - The space between the two layers of Bowman’s capsule (where urine collects). Distal tubule Macula densa of distal tubule Afferent arteriole Efferent arteriole Juxtaglomerular cells (modified smooth muscle) Vascular pole Bowman’s capsule (parietal layer) Bowman’s capsule (visceral layer of podocytes) Parietal layer Urinary space Urinary pole Brush border (microvilli) Proximal convoluted tubule Glomerulus Bowman’s capsule (parietal layer) Urinary space Renal corpuscle Vascular and urinary poles Capillaries (blue) and podocytes (green) Podocytes * Podocytes have a central cell body with primary processes. * Each primary process gives rise to secondary processes called pedicels (foot processes) that embrace the glomerular capillaries. * Foot processes from different podocytes interdigitate. * Spaces between pedicels are called filtration slits - A little diaphragm covers each filtration slit. Primary processes Interdigitating secondary foot processes Foot processes Podocyte Glomerular capillaries & podocyte Mesangial Cells * In addition to podocytes and glomerular capillary endothelial cells, mesangial cells are a third cell type present in the glomerulus. * Functions of mesangial cells: - Physical support for glomerulus. - Phagocytosis of proteins and other debris from the glomerular basal lamina. - Secretion of cytokines and other substances for immune defense and repair. Diagram and photomicrograph showing the relationship between the intraglomerular mesangial cells and the glomerular capillaries. a. The mesangial cell and its surrounding matrix are enclosed by the glomerular basement membrane