Introduction

The urinary system is composed of kidneys, ureters, urinary bladder and urethra and functions by removing metabolic wastes from the blood and by maintaining the blood's ionic balance. The two main units of the system are 1) the nephron which produces the urine and 2) a duct system which conveys the urine to the exterior.

A. KIDNEYS

The nephrons and the proximal portions of the duct system lie within the kidneys. These organs are covered by a thin outer capsule of fibrous connective tissue, and when cut, they display two distinct regions, an outer cortex and an inner medulla. The cortex contains the structures which filter the blood and the convoluted portions of the duct system. The medullary region has a striated appearance resulting from its content of the straight portions of the duct system. Since the medulla has the shape of an inverted pyramid with a convex base, the striations appear fan-shaped. The convex base of the medullary pyramid is adjacent to the cortex; while the apex of the pyramid can unite with the apical regions of two or three other pyramids to form a renal papilla. The surface of the papilla is perforated with 10 to 25 pores which are the openings of the duct system. Urine drains from the papillae to a minor calyx which in turn is drained by a major calyx. From the major calyx the urine moves into the upper, expanded portion of the ureter known as the renal pelvis.

The functional unit of the kidney is the uriniferous tubule which is composed of a nephron and a series of collecting ducts. The nephron consists of a glomerulus with its blood vessels, a capsule part of which is reflected over the glomerular vessels, and a series of tubules which have been divided into proximal and distal portions. The glomerulus and its capsule (Bowman's capsule) are known as the renal corpuscle (Malpighian corpuscle).

1. Renal Corpuscle

The glomerular tuft of capillaries is formed from the afferent arteriole which enters the Bowman's capsule. At the entrance the elastic lamina of the arteriole ends and the muscle cells become the juxtaglomerular cells which secrete the hormone renin. Within the glomerulus the capillary divisions unite and form the efferent arteriole which carries blood to the cortical and medullary regions of the kidney. The glomerulus is covered by a capsule which is a double-walled cup of squamous epithelium. The squamous epithelium of the outer parietal layer of the capsule is continuous with the cuboidal epithelium of the proximal convoluted tubule. The epithelium of the inner layer, that layer closest to the glomerulus and its capillaries (visceral layer), consists of specialized cells called podocytes. These cells have a number of primary and secondary processes that are closely applied to the basement membrane (basal lamina) that separates them from the fenestrated endothelium of the glomerular capillaries. Between the outer squamous epithelium of the parietal layer of Bowman's capsule and the inner podocytes is the subcapsular space of Bowman which is continuous with the lumen of the proximal convoluted tubule where squamous epithelium changes to the cuboidal type

2. Proximal Convoluted Tubules

The proximal convoluted tubules are lined with cuboidal epithelium which is made up of eosinophilic cells that have numerous microvilli. These microvilli can be distinguished in histological sections as a brush border on the lumen surface of the cells. The nuclei are round or oval, and there are no distinct boundaries between the cells. The proximal convoluted tubule becomes straight towards the renal medulla and is known as the descending limb of the loop of Henle. The cuboidal epithelium changes abruptly to squamous epithelium as the tube narrows to form the thin segment of Henle's loop.

The brush border is no longer visible. From the loop of Henle the tubule ascends toward the cortical region. The ascending limb has been divided into thin and thick regions. The thin region is closest to the loop of Henle and is composed of squamous epithelium. The thick region contains a taller, cuboidal epithelium with an absence of brush borders, and the ascending limb of the loop of Henle continues into the distal convoluted tubule

3. Distal Convoluted Tubules

The straightened portion of the ascending limb of the loop of Henle becomes twisted, and the epithelium is the cuboidal type, but lacks brush borders. The absence of a brush border on the luminal surface of the cuboidal epithelial cells is used to distinguish them histologically from the cells of the proximal convoluted tubules. Near the juxtaglomerular cells of the afferent arteriole the distal convoluted tubule comes in contact with the renal corpuscle. This region is the macula densa. In a given section of the renal cortex there will be fewer distal convoluted tubules than proximal convoluted tubules.

4. Collecting Tubules

These structures are not considered to be part of the nephron, but are part of the uriniferous tubule. The distal convoluted tubules join into the collecting tubules which run through both the collecting and medullary regions of the kidney. The structures in the cortex through which the collecting tubules course are known as the medullary rays. In addition to collecting tubules, a medullary ray willl also contain the descending and ascending portions of Henle's loop of the cortical nephrons which empty into the collecting tubules. Thus, the medullary ray and surrounding glomeruli and associated convoluted tubules constitute a renal tubule. The collecting tubules pass through the outer to the inner zone of the medulla and open onto the surface of the renal papillae. The collecting tubules are composed of cuboidal epithelium consisting of cells with small, rounded nuclei. The presence of sharply defined boundaries between the cells serves to distinguish this tubule from those discussed above. The cuboidal epithelial cells of the collecting tubules also lack a brush border.

5. Renal Blood Supply

The renal artery at its junction with the aorta is of the elastic type and then becomes a muscular artery. This artery enters the hilus and divides into the interlobar, arcuate and interlobular arteries, all of which are muscular arteries and decrease in size. The interlobular arteries give rise to the afferent arterioles of the glomerulus , and the blood leaves the glomerulus via the efferent arterioles. In the outer part of the cortex these efferent arterioles lead to capillaries surrounding the convoluted tubules. In the cortical region close to the medulla, the efferent arterioles pass into the medulla as the straight arterioles. Renal veins arise from cortical and medullary plexuses. The renal capsule has a rich network of capillaries which anastomose with the cortical vessels.

B. URETER

The calyces, renal pelvis, ureter and bladder possess a similar histological structure. The mucosal epithelium is lined with a basement membrane and the lamina propria is attached to layers of smooth muscle, all of which are covered by the connective tissue of the adventitia. The number of cell layers in the transitional epithelium varies from 2 - 3 in the calyx to 4 -5 in the ureter to 6 - 8 in the bladder. In the ureter the transitional epithelium lies on the connective tissue of the lamina propria. Muscle contraction during the process of fixation imparts a stellate shape to the lumen. A muscularis mucosae is absent. The muscular layers of the ureter form an inner longitudinal and outer circular layer, but at this duct nears the bladder, an additional outer longitudinal layer of smooth muscle is added. The adventitia is connective tissue that contains elastic fibers. The calyces, renal pelvis, ureter and bladder possess a similar histological structure. The mucosal epithelium is lined with a basement membrane and the lamina propria is attached to layers of smooth muscle, all of which are covered by the connective tissue of the adventitia. The number of cell layers in the transitional epithelium varies from 2 - 3 in the calyx to 4 -5 in the ureter to 6 - 8 in the bladder. In the ureter the transitional epithelium lies on the connective tissue of the lamina propria. Muscle contraction during the process of fixation imparts a stellate shape to the lumen. A muscularis mucosae is absent. The muscular layers of the ureter form an inner longitudinal and outer circular layer, but at this duct nears the bladder, an additional outer longitudinal layer of smooth muscle is added. The adventitia is connective tissue that contains elastic fibers.

C. URINARY BLADDER

The urinary bladder is a muscular sac with a mucosa of transitional epithelium. The mucosa has a folded surface, and the epithelium is 6 -8 cells thick in the empty bladder and 2 cells thick when the bladder becomes distended with urine. A muscularis mucosae is again absent. The three muscle layers are arranged as those of the portion of the ureter closest to the bladder. The loose fibrous adventitia contains blood vessels, lymphatics, nerves and elastic fibers.

D. URETHRA

The lumen of the female urethra is crescent or U-shaped and usually appears collapsed. The female urethra is lined with transitional epithelium in that portion near the bladder and this changes to stratified squamous epithelium in the distal portions. The lamina propria contains small mucous glands and numerous blood vessels. There are two coats of muscle, an inner longitudinal and an outer circular coat. The external sphincter is composed of striated muscle.

The male urethra has been divided into prostatic, membranous, cavernous, and terminal portions. In the prostatic portion the mucosa is composed of transitional epithelium, but some portions possess pseudostratified or stratified columnar epithelium. The lamina propria contains vascular and fibroelastic tissue, and the muscularis has an inner longitudal and outer circular layer. The mucosa of the membranous part consists of stratified columnar epithelium and basement membrane. The muscularis is similar to that of the prostatic portion. Cowper's glands (bulbourethral glands) are present near the cavernous part, and their ducts are lined with squamous epithelium surrounded by a thin layer of smooth muscle. In the cavernous portion of the male urethra the epithelium is of two types, stratified columnar and stratified squamous. The epithelial surface shows many glandular recesses called lacunae of Morgagni which continue into branching tubules known as the gland of Littre. The terminal part of the urethra is lined by a mucosa of nonkeratinized stratified squamous epithelium.

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