OBJECTIVES

This chapter should help you to:

· List the organs and functions of the urinary system and describe the role of each organ in the system's main functions.
· Identify the structures and regions seen grossly in a frontal section of a kidney and describe their organization and general function.
· Describe the structure, function, and location of each component of a nephron and the urinary tract into which the nephrons empty and be able to identify these structures in histologic sections.
· Describe the function of the juxtaglomerular apparatus and identify its components.
· Trace the flow of blood through the kidney and identify the various renal vascular elements in histologic sections.
· Trace the flow of urinary filtrate from Bowman's space to the exterior, naming in order the tubules and urinary tract components through which it flows and describing any changes in filtrate composition that occur in each component of the system.
· Compare the countercurrent exchange system with the countercurrent multiplier system of the kidney.
· Compare the roles of ADH and aldosterone in renal function.
· Compare the urethras of males and females and describe the structure, function, and location of the sphincters that surround each.
· Identify the components of the glomerular Liltration barrier in an electron micrograph or diagram of a portion of a renal corpuscle.

SYNOPSIS

I. GENERAL FEATURES OF THE URINARY SYSTEM

A. Components of the System: The urinary system includes the kidneys and the urinary tract.

1. Kidneys. These paired, bean-shaped, retroperitoneal organs are located in the posterior wall of the abdominal cavity. a. Structural and functional subdivisions, When sliced in the frontal plane (Fig 19-1), each kidney shows a dark-staining outer cortex and a light-staining inner medulla that partly surrounds the renal hilum, The hilum consists of a space, the renal sinus, that contains the larger renal blood vessels, the renal pelvis, and adipose tissue. Each human kidney consists of several pyramid-shaped subunits--renal lobes--whose bases lie in the cortex and whose apices lie in the medulla. The apices are cupped by minor calyces that collect and empty the urine from each lobe into the larger major calyces, These in turn empty into the single, funnel-shaped renal pelvis, which is continuous with the ureter, Each lobe consists of numerous renal lobules, each containing hundreds of nephrons, These are largely tubular structures that filter the blood, modify the filtrate to form urine, and empty into a series of collecting tubules and ducts which converge on the medulla and empty into the minor calyces. b. Blood supply. Because the kidneys are blood-filtering organs, their blood supply is crucial to their function. A pair of renal arteries--one to each kidney--branches from the aorta in the upper abdomen. Each artery undergoes successive branching to feed specialized capillary beds in both the cortex (glomeruli and peritubular capillaries) and medulla (vasa rectal. Knowledge of the renal artery's branching pattern within the kidney is important. It aids in understanding how the blood reaches the capillaries that play specific and crucial roles in renal function. In addition, the structure, route, and location of the branches provide clues to the way in which the structural and functional subdivisions are arranged. 2. Urinary tract. The structure of the ureters, urinary bladder, and urethra, which constitute the urinary tract, is described mainly in terms of their wall structure. Except for certain portions of the urethra, the lumen of the tract is characteristically lined by transitional epithelium.

B. General Functions of the System: The kidneys filter metabolic wastes and foreign sub stances from the blood; regulate the ion, salt, and water concentrations of the fluids that bathe the body's tissues; and produce renin and erythropoietin. The collection of raw filtrate from the blood in the glomerular capillaries is only the first step in urine production. It is followed by the reabsorption of important ions, small proteins, nutrients, and much of the water. These are returned to the blood in the peritubular capillaries and vasa recta in precise proportions. The portion of the raw filtrate that is not reabsorbed constitutes the urine; it is carried by the ureters from the kidneys to the urinary bladder, where it is temporarily stored and later released through the urethra.

II. KIDNEYS

A. General Organization: The kidneys, which measure about Il x 6 cm, are bean-shaped retroperitoneal organs encapsulated by dense connective tissue and surrounded by adipose tissue. Several components can be distinguished without the aid of a microscope.

1. Renal sinus. This medial concavity of each kidney contains the renal pelvis, the entering and exiting blood vessels and nerves, and adipose tissue.
2. Hilum, This consists of the renal sinus and its contents.
3. Cortex, This is the kidney's dark-staining outer region; it underlies the capsule. It contains the renal corpuscles, proximal and distal convoluted tubules, peritubular capillaries, and medullary rays.
4. Medulla, This is the kidney's light-staining inner region, which partly surrounds the renal sinus. It consists of 8-18 conical medullary pyramids whose bases abut the cortex and whose apices (renal papillae) point inward, toward the renal sinus. It also contains the collecting ducts, loops of Henle, and vasa recta. Each renal papilla, perforated by openings of the collecting ducts, is cradled by a minor calyx into which the ducts empty. Several minor calyces empty into a major calyx. The major calyces empty into the renal pelvis, which in turn drains into the ureter.
5. Medullary rays. These fingerlike extensions of medullary tissue that enter the cortex com prise clusters of collecting tubules and ducts. One medullary ray occupies the center of each renal lobule.
6. Renal lobes, Each human kidney has 8-18 lobes, the kidney's largest subdivisions. Each lobe, which consists of a medullary pyramid and its associated cortex, contains numerous renal lobules.
7. Reual lobules, Each of these subdivisions of the lobes consists of a central medullary ray and all the nephrons that empty into its collecting tubules. The borders between adjacent renal lobules are marked by interlobular arteries and veins.

B. Nephrons: Nephrons are the functional subunits of the kidney. Each includes a renal corpus cle, a proximal convoluted tubule, a loop of Henle, and a distal convoluted tubule.


1. Renal corpuscle. As the blood-filtering unit of the nephron, each renal corpuscle consists of a glomerulus covered by Bowman's capsule. Together these structures form the filtration barrier. Each corpuscle has both a urinary and a vascular pole. a. Glomerulus, This is a small tuft of fenestrated capillaries whose fenestrae are covered by thin diaphragms. Modified smooth muscle cells, mesangial cells, lie between the capill- ary loops. b. Bowman's capsule is a double-walled epithelial chamber. Its inner wall, or visceral layer, consists of podocytes, These cells have long primary processes, from which arise interdigitating foot processes (pedicels) that grasp the glomerular capillaries like fingers around a broom handle and adhere tightly to the fused capillary-podocyte basal lamina. The outer wall--the parietal layer--is simple squamous epithelium. The chamber be tween the visceral and parietal layers is known as the urinary or Bowman's space, c. Filtration barrier. Consisting of the structures that separate the capillary lumen from the urinary space, the filtration barrier(Fig 19-2) includes (l)the diaphragm-covered capil lary fenestrations, (2) the fused basal laminae of the capillary endothelial cells and podocytes, and (3) the diaphragm-covered filtration slits that lie between the interdigitat ing pedicels. d. Vascular pole. This side of the corpuscle is where the afferent arterioles that feed the glomerular capillaries enter and the efferent arterioles that drain them leave. It lies opposite the urinary pole. e. Urinary pole. This side of the corpuscle is where the proximal convoluted tubule exits. f, Filtration mechanism. Blood is delivered to the glomerulus by the afferent arteriole, Arterial pressure forces fluid from the blood through the filtration barrier and into the urinary space. Each component of the barrier (fenestrae, diaphragms, basal lamina, filtration slits) aids in limiting the passage of blood components by size, thus preventing blood cells and large proteins from entering the urinary space. Molecules trapped in the basal lamina are periodically removed by the mesangial cells. A reduced volume of blood leaves the glornerulus via the efferent arteriole, and the raw filtrate in the urinary space enters the proximal convoluted tubule for further processing.

2. Proximal convoluted tubule. This epithelial tube begins at the urinary pole of the rcnal corpuscle. Its lining is simple low columnar-to-cuboidal epithelium. The lining cells have abundant long microvilli. Together they form a brush border that partly obscures the lumen and increases the surface area available for absorption. The lining cells absorb about 85% of the sodium from the filtrate. Water follows passively, reducing the amount of filtrate by about 25%. All the glucose (unless present in great excess), amino acids, acetoacetate, and vitamins are reabsorbed by facilitated transport (see Chapter 3) and small proteins are reab sorbed by pinocytosis. The many mitochondria required for the energy-intensive absorptive function interdigitate with numerous basal membrane infoldings and make the lining cells acidophilic. The convoluted part of the proximal tubule lies in the cortex and empties into its straight portion (also called the thick descending limb of the loop of Henle), which has a similar epithelium and function. Together, the convoluted and straight portions of the proxi mal tubule measure about 14 mm, making this the longest portion of the nephron and the most frequently encountered tubule type in cortical sections.

3. Loop of Henle a. Structure. Henle's loop, a U-shaped epithelial tube, includes thick and thin descending limbs and thin and thick ascending limbs. It extends from the proximal convoluted tubule in the cortex, dips into the medulla, and returns to the cortex, where it empties into the distal convoluted tubule. The abrupt transition from thick to thin in both arms of the U is the result of changes from low columnar or cuboidal to squamous and back to cuboidal epithelium. The change in the luminal diameter is less dramatic than that in the external diameter. b. Function. A prerequisite for the production of hypertonic urine, the loop acts as a countercurrent multiplier to establish an osmotic gradient in the interstitial fluid of the medulla. Hypertonic (concentrated) and hypotonic (dilute) are relative terms. The point of reference assumed here is the tonicity of normal tissue fluid or blood (isotonic). The medullary interstitium, for example, is approximately isotonic near the corticomedullary junction and gradually becomes most hypertonic near the tips of the medullary papillae. The descending and ascending portions of the Loop of Henle play an important role in establishing and maintaining this osmotic gradient. (1) Descending portion. The first part of Henle's loop, it plays a passive role in making the medullary interstitium hypertonic and helps maintain the gradient. The filtrate entering the descending part of the thin loop is isotonic, but the removal of salt, nutrients, and water in the proximal tubule causes a reduction in volume from its raw state in the urinary space. Although the descending portion of the loop is permeable to water, it is impermeable to salt. As the fluid in its lumen passes deeper into the hypertonic medulla. it loses water to the interstitium and gradually becomes more hypertonic. As water is lost, the filtrate decreases in volume. Its tonicity equilibrates with the hypertonic interstitium, peaking at the bottom of the U as it enters the ascending portion. (2) Ascending portion. Two functional properties of the lining epithelium give this part of Henle's loop a more active role. (a) Cells in the thick ascending portion are structurally similar to those in the distal convoluted tubule. They constantly pump chloride ions from the filtrate into the interstitial fluid around the tubules. Sodium passively follows the chloride, increasing the salt concentration (and tonicity [osmolarityl) in the interstitium. (b) Because this part of the loop is impermeable to water, the water in the filtrate cannot follow the salt into the interstitium and dilute it. As the reduced volume of filtrate ascends toward the distal convoluted tubule in the cortex, the removal of chloride and sodium (but not water) by the cells lining this portion of the loop causes the fluid in its lumen to gradually become isotonic or hypotonic. (The importance of the osmotic gradient to the production of hypertonic urine will become clearer after considering the events occurring in the collecting ducts that pass through the medulla en route to the calyces.)

4. Distal convoluted tubule. This final segment of the nephron lies in the cortex. Its epithelial lining is low cuboidal, with no brush border, making its lumen appear wider. Its lining cells are more basophilic than those lining the proximal convoluted tubules. The lateral cell boundaries are indistinct as a result of extensive lateral membrane interdigitations with their neighbors. The distal tubule cpithelium forms a disk of tightly packed columnar cells called a macula densa at the point near the vascular pole of a renal corpuscle where it contacts an afferent arteriole. This disk may monitor the osmolarity of the fluid in the tubule lumen; the osmolarity of the fluid delivered by the loop of Henle changes little here. The distal con voluted tubule makes final adjustments of the salt, water, and acid balance. The presence of aldosterone (a product of the adrenal cortex) causes the lining cells to remove more sodium and add phosphate to the fluid. The cells may further adjust the pH by secreting hydrogen and ammonium ions into the lumen. 5. Cortical and juxtamedullary nephrons, penal corpuscles are found throughout the cortex. While most belong to the cortical nephrons, the 15% closest to the medulla belong to the juxtamedullary nephrons. The latter group has short thick descending limbs and longer thin limbs that extend deeper into the medulla. The juxtamedullary nephrons bear the primary responsibility for setting up the osmotic gradient in the medulla.

C. CollectingTubulesand Ducts:


1. Structure. These differ from the nephrons in their embryonic origin and can be easily distinguished from the proximal and distal tubules in sections. Their blocklike lining cells have distinct intercellular borders; they are cuboidal in the smaller tubules and columnar in the larger leg, papillary) ducts of the medulla. Since their cytoplasm stains poorly, the cells appear clear or white.
2. Function. The cortical collecting tubules receive a reduced volume of hypotonic or isotonic urine from the nephrons and empty it into larger collecting ducts. These leave the cortex in medullary rays and enter the medulla, increasing in size until they open into a minor calyx through the tips of papillae. The medullary collecting ducts play the final role in forming hypertonic urine. Under the influence of the posterior pituitary hormone, antidiuretic hor mone (ADH or vasopressin), they become permeable to water. As they pass through the osmotic gradient of the medulla, water diffuses passively from their lumens into the hyper tonic medullary interstitium, causing the osmolarity of the fluid in their lumens to equilibrate with that of the interstitium; only a small volume of hypertonic urine is released. Without ADH, however, the collecting ducts remain impermeable to water and a larger amount of hypotonic or isotonic urine is produced.

D. Juxtaglomerular Apparatus: Located near the vascular pole of a renal corpuscle at the point of contact between a distal convoluted tubule and an afferent arteriole, this includes juxtaglomerular (JG) cells, a macula densa, and polkissen (extraglomerular mesangial cells). The JG cells, modified smooth muscle cells in the wall of the afferent arteriole, exhibit typical secretory ultrastructure and numerous PAS-positive cytoplasmic granules. Although the influ ence of the macula densa on the JG cells is poorly understood, below-normal blood volume, blood pressure, or levels of blood sodium causes the JG cells to secrete renin, This enzyme cleaves plasma angiotensinogen to produce angiotensin I, which is converted to its active form, angiotensin II, by enzymes in the lungs. Angiotensin II, a vasoconstrictor, increases blood pressure and stimulates aldosterone production by the adrenal cortex, thereby increasing chloride and sodium reabsorption by the distal tubule. The sodium and chloride enter the blood in the peritubular capillaries. While the distal tubules are impermeable to water, the increased tonicity of blood leaving the kidneys draws water into the blood as it passes through other tissues, increasing blood volume and pressure. Increased blood pressure distends the afferent arterioles, stretching the JCj cells and halting renin secretion. Polkissen function is unknown.

E. Blood Supply and Circulation: The arteries below are accompanied by similarly named veins.


1. Each kidney receives a renal artery--a branch from the abdominal aorta.
2. Anterior and posterior branches arise from the renal artery before it reaches the renal hilum .
3. Interlobar arteries arise from the anterior and posterior branches in the renal hilum and penetrate the medulla between the medullary pyramids.
4. Arcuate arteries arise from the interlobular arteries and course along the arched border between the cortex and medulla.
5. Interlobular arteries arise at right angles from the arcuates; they penetrate the cortex between the medullary rays and lie at the borders between neighboring renal lobules.
6. Many afferent arterioles arise from each interlobular artery. Each afferent arteriole supplies a glomerulus (II.B. l.a).
7. An efferent arteriole carries blood away from the glomerulus. a. Efferent arterioles of cortical nephrons branch to form a profusion of peritubular capillaries that carry absorbed products away from the proximal and distal tubules and converge to form the stellate veins of the peripheral cortex. These drain into the inter lobular veins. b, Efferent arterioles ofjuxtamedullary nephrons give rise to numerous straight capillary loops--vasa recta-that descend into the medulla.
8. Vasa recta arise mainly from the efferent arterioles of the juxtamedullary nephrons; some may arise from the arcuate artery. The descending parts of the vasa recta carry isotonic blood into the medulla. The blood loses water and picks up sodium as it passes deeper into the medulla. Unlike the loop of Henle, the ascending parts of the vasa recta are as permeable to salt and water as are its descending parts. As the blood ascends through the gradient, its tonicity equilibrates with that of its surroundings. The blood carried away from the medulla is thus once again isotonic. The passive exchange of salt and water between the vasa recta and the interstitium is known as the countercurrent exchange mechanism. It is important in carrying away water lost to the filtrate during its descent into the medulla and thus in maintaining the osmotic gradient set up by the countercurrent multiplier system of Henle's loop. Blood in the ascending portions of these vessels drains into interlobular veins and exits through the veins that accompany the larger arteries.

F. Summary of Renal Function: An organized system of arteries carries blood to the glomeruli of the renal corpuscles. Each corpuscle acts as both filter and funnel, collecting raw filtrate and directing it to the proximal convoluted tubule where glucose, amino acids, acetoacetate, small proteins, vitamins, sodium, and water are rcabsorbed. The remaining fluid enters the loop of Henle, which sets up a hypertonic osmotic gradient in the medulla. The fluid leaves the loop and enters the distal convoluted tubule. Here, aided by the juxtaglomerular apparatus and al dosterone, the salt, ion, and water balance between the blood and urine is adjusted. The urine then exits the nephron through the collecting ducts, which pass back through the medulla. ADH renders the medullary collecting ducts permeable to water, allowing the water to flow out of the lumens of the collecting ducts and into the medullary interstitium. This results in the release of a reduced volume of hypertonic urine into the minor calyx.

III. RENAL CALYCES & RENAL PELVIS

The walls of each renal calyx and pelvis consist of mucosa, muscularis, and adventitia; no submucosa is present. The mucosa, consisting of typical urinary (transitional) epithclium (see Chapter 4), attaches to an underlying helical meshwork of smooth muscle (muscularis) by a connective tissue lamina propria of variable density. The cpithelium forms an osmotic barrier that protects the sur rounding tissues from the hypertonic urine and the urine from dilution. The adventitia blends into the adipose tissue contained in the renal sinus.

IV. URETERS

These carry urine from the renal pelvis to the urinary bladder. While the lumen is narrower than that of the renal pelvis, the wall structure is similar, including the lining of transitional epithelium. The ureter wall thickens and the muscle cells change from a helical to a longitudinal array near the bladder before fanning out in the bladder wall to form the superficial and deep trigones of the bladder.

V. URINARY BLADDER

This distensible muscular sac, lined by transitional epithelium underlain by a dense lamina propria, has walls similar to those of the ureter, pelvis, and calyces but with a thicker muscularis. The smooth muscle fibers run in many directions and are not organized in layers except near the urethral orifice, where they form an involuntary internal sphincter.

VI. URETHRA

This differs in length, epithelium, and function in males and females.

A. Male Urethra: Longer than the female urethra, this conducts both urine and seminal fluid; it has 3 main parts.

1. Prostatic portion. The most proximal part of the male urethra, it exits directly from the neck of the urinary bladder. It is surrounded by the prostate gland (see Chapter 22) and is lined by transitional epithelium. This portion receives the prostatic and ejaculatory ducts and empties into the membranous portion
2. Membranous portion. This is the shortest segment. It is encircled by the skeletal muscle of the membranelike urogenital diaphragm, whose fibers form a voluntary external sphincter. This portion of the urethra is lined by pseudostratified columnar epithelium and empties into the cavernous portion.
3. Cavernous portion. Passing through the corpus cavernosum urethrae (corpus spongiosum) of the penis (see Chapter 22), this part of the male urethra is divided into bulbous and pendulous parts. Within the glans, near the tip of the penis, the urethral lumen widens to form the fossa navicularis. The epithelium changes here from pseudostratified columnar to stratified squamous. The urethra opens at the end of the penis through the urethral meatus. Numerous glands of Littre empty mucous secretions into the lumen all along the urethra; they are more numerous in the pendulous part.

B. Female Urethra: Shorter than the male urethra, this carries only urine. It is lined by stratified squamous epithelium with patches of pseudostratified columnar. Midway along its path from the bladder to the exterior, it is surrounded by a voluntary external sphincter formed by the urogenital diaphragm.

Back to Histology Notes Page

Back to Histology Home Page