URINARY SYSTEM

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URINARY SYSTEM



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16.1 OVERVIEW


The urinary system comprises two kidneys, two ureters, a urinary bladder, and a urethra. Kidneys filter blood and produce urine, by which waste products and foreign substances leave the body. Urine formation involves filtration, secretion, and reabsorption of fluid by renal corpuscles and tubules in kidneys. About 180 L of fluid is filtered daily, but only 1–2 L of urine is produced, with the remaining fluid reabsorbed by renal tubules to reenter the vascular system. Kidneys control acid-base balance, maintain extracellular fluid volume, and regulate total body water. They also produce two hormones: renin aids regulation of systemic arterial blood pressure; erythropoietin stimulates production of erythrocytes in bone marrow. The flattened bean-shaped kidneys have an indented slit, or hilum, on the medial surface through which ureters, blood vessels, lymphatics, and nerves pass. Kidneys are compound tubular glands covered by a thin capsule of dense connective tissue and embedded in a layer of fat. The parenchyma is divided into an outer dark-red cortex, a lighter striated medulla, and a funnel-shaped pelvis that lies in a shallow cavity—the renal sinus. The medulla consists of 12–15 cone-shaped renal pyramids, each with a broad base bordering on the cortex and an apex forming a nipple-like projection, or papilla, which extends into the sinus. Parts of the cortex dip down into spaces between the pyramids to form renal columns. The renal pelvis, a fan-shaped expansion of the ureter, forms two or three cup-like major calyces at its widest border. These divide into minor calyces, each being a drain for the papilla of a pyramid. Parenchyma served by one papilla is a renal lobe; each human kidney has 12–15 lobes. Urine flows from pyramids through calyces into the renal pelvis, then out of the kidneys and into ureters. Ureters deliver it to the bladder, where it is stored before urination via the urethra.



CLINICAL POINT


Based on appearance, hematuria—an abnormal presence of red blood cells in urine—is commonly classified as either gross (visible change in color) or microscopic (usually detected via urine dipstick, light micrograph, or urinalysis). Common benign causes are menstrual bleeding, vigorous exercise, benign prostatic hyperplasia, and urinary tract infection (UTI). More serious causes include renal calculi, kidney or bladder malignancy, prostate or bladder inflammation, glomerulonephritis, polycystic kidney disease, and sickle cell anemia. In certain cases, the differential diagnosis may be made by ultrasonography, intravenous pyelography, or cystoscopy.




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16.2 ORGANIZATION OF THE RENAL VASCULATURE


The highly vascular kidneys receive nearly 25% of cardiac output, and their histologic organization and functions center on blood supply. Blood vessel arrangement provides arterial blood directly to glomeruli (site of ultrafiltration) of renal corpuscles and around all parts of renal tubules (site of reabsorption of substances). Arterial blood from the renal artery—a branch of the aorta—reaches a kidney at the hilum and passes into interlobar arteries, which distribute blood to glomeruli via arcuate arteries at the corticomedullary junction. Blood is then taken to interlobular arteries, which cross the cortical parenchyma radially, and in between medullary rays. Almost all blood goes first to afferent arterioles, which supply renal corpuscles. The capillary network in the corpuscle is unique because it comprises an afferent and an efferent arteriole. The afferent arteriole branches into a tuft of 20–40 loops of fenestrated capillaries—the glomerulus. Filtered blood leaves a glomerulus via an efferent arteriole and travels through the extensive peritubular capillary network around cortical renal tubules to regain some water and solutes. Almost all blood to renal tubules comes from glomeruli. Also, efferent arterioles from juxtamedullary nephrons give off recurrent capillary loops, the vasa recta, which run in parallel into the medulla along medullary rays. Vasa recta drain into arcuate veins at the corticomedullary junction. Venous return from both cortex and medulla drains into interlobular veins, and venous blood follows the course of the arteries to the hilum, where it empties into the renal vein, which takes it to the inferior vena cava and the heart.



CLINICAL POINT


Renovascular hypertension (renal artery stenosis)—high blood pressure caused by narrowing of arteries that deliver blood to the kidneys (chiefly the main renal arteries)—is a major cause of end-stage kidney disease in elderly adults. Left untreated, patients may ultimately require dialysis or kidney transplantation. The most common cause is arteriosclerosis (hardening of the arteries). Risk factors are systemic hypertension, cigarette smoking, diabetes mellitus, high serum cholesterol levels, excessive alcohol use, and increasing age. Diagnosis is via renal arteriography, Doppler ultrasonography, or magnetic resonance angiography (MRA) to assess the site and degree of stenosis. Treatment includes surgical intervention (e.g., balloon angioplasty, vascular stenting) to improve renal blood flow, antihypertensive therapy, and lipid-lowering medications.




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16.3 ANATOMY OF THE URINIFEROUS TUBULE (NEPHRON AND COLLECTING DUCT)


The functional unit of the kidney—the uriniferous tubule—consists of the secretory highly coiled nephron, which is 30–40 mm long and involved in production of urine; and the excretory collecting ducts, which are about 20 mm long and are conduits for urine. They derive embryonically from different sources, which fuse during development: nephrons originate from metanephric diverticulum; collecting ducts, from ureteric buds. Both originate from mesoderm. A nephron is a blind-ended tubule composed of several parts, each with structure reflecting functional differences. The initial blind-ended part of the nephron is the renal corpuscle, which consists of a tuft of glomerular capillaries in the double-walled sac-like Bowman capsule, which is made of epithelium. It receives filtrate of blood from glomerular capillaries. The nephron also consists of a proximal tubule, a segment that makes a hairpin turn and is called the loop of Henle, and a distal tubule. The proximal tubule has convoluted, straight, and thin segments; the distal tubule, straight, macula densa, convoluted, and connecting parts. Henle loop includes descending, thin ascending, medullary thick ascending, and cortical thick ascending limbs. Nephrons empty into collecting tubules, which coalesce to form larger collecting ducts in medullary rays and pyramids that reach the papilla. Collecting ducts take urine to the renal pelvis. Usually, renal corpuscles and convoluted parts of proximal and distal tubules are in the cortex, but their straight parts are in medullary rays. About 80% of nephrons, the cortical nephrons, have short Henle loops and are in the peripheral cortex; the other 20%, the juxtamedullary nephrons, are closer to the corticomedullary junction and have longer Henle loops. Henle loops and most collecting ducts are in the medulla. Each adult kidney has more than 1 million uriniferous tubules.



HISTORICAL POINT


Marcello Malpighi (1628–1694), an Italian histologist and pioneer in use of the microscope, discovered renal corpuscles and tubules, which were named for him (Malpighian). He set the stage for later discoveries about the kidney. German anatomist and pathologist Friedrich Gustav Henle (1809–1885) published the first systematic treatise on histology and contributed to the study of human epithelial tissues. The thin, looped part of the nephron bears his name. Sir William Bowman (1816–1892), an English histologist and ophthalmologist, used the microscope to describe many previously unknown body structures. He identified the capsule of the renal corpuscle—the Bowman capsule.




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16.4 HISTOLOGY AND FUNCTION OF THE RENAL CORTEX


A thin, tough outer capsule of dense fibrous connective tissue invests the kidney. It consists of regularly arranged collagen fibers interspersed with fibroblasts. The capsule is nearly nondistensible, loosely attached, and easily peeled off. The kidney and its capsule lie in a mass of adipose tissue, which cushions and protects the kidney. With routine stains, interstitial connective tissue in the kidney looks quite scanty and inconspicuous. It forms the stroma, which supports many blood vessels of various sizes that are closely associated with the renal parenchyma. The parenchyma consists of long, tortuous, tightly packed renal tubules. The cortex usually appears dark and granular because of its many spherical renal corpuscles and convoluted uriniferous tubules. Its uniform granularity is mostly due to myriad proximal and distal tubules that are sectioned randomly in different planes. Corpuscles in the cortex are scattered between other parts of the uriniferous tubules. Each corpuscle, together with a renal tubule, constitutes a nephron. Corpuscles of cortical nephrons in the outer cortex are uniform in size; the slightly larger juxtamedullary nephrons are especially active in water reabsorption and urine concentration. Parallel groups of loops of Henle and collecting ducts form medullary rays, which extend into the cortex from the deeper medulla. Each medullary ray and its cortical parenchyma make up an ill-defined renal lobule. Blood filters through the glomerular capillary loops of each corpuscle into its renal tubule, and as filtrate passes down the segments of the renal tubule, it is modified by removal or addition of components, the ultimate product being urine.



CLINICAL POINT


Pyelonephritis—infection of the kidney usually caused by ascent of pathogenic organisms (typically Escherichia coli bacteria) from the urinary bladder to the kidney—is characterized by fever, abdominal (flank) pain, frequency of urination, and leukocytosis. Occurring more frequently in women than men because of a shorter female urethra, it may become life threatening when infection enters the bloodstream, leading to sepsis. Acute and chronic forms exist. A major risk factor is long-term indwelling urethral catheterization. Although the usual treatment is aggressive antibiotic therapy, severe cases may require surgical intervention.




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16.5 HISTOLOGY OF RENAL CORPUSCLES


At about 200 μm in diameter, the spherical renal corpuscles are just visible to the naked eye. They are found only in the cortex of the kidney and represent the initial, expanded part of the nephron. They have a vascular pole (where afferent and efferent arterioles enter and leave) and a urinary pole (where the proximal tubule begins). Each corpuscle consists of an epithelial part called Bowman capsule and a vascular part consisting of a tuft of glomerular capillaries formed by a branching afferent arteriole. An efferent arteriole drains this lobulated tuft of 20–40 capillary loops. The double-layered epithelial Bowman capsule forms the corpuscle’s external covering. The outer layer of Bowman capsule, the parietal layer, consists of simple squamous epithelium resting on an indistinct basement membrane. The inner visceral layer of the capsule consists of highly specialized cells called podocytes. Their name derives from the Greek and means foot-like cells. These highly branched podocytes are reflected over the capillary loops in direct contact with the basement membrane of glomerular capillaries. The two layers of Bowman capsule are continuous with each other at the vascular pole. Bowman (urinary) space is between the two layers of the capsule and at the urinary pole becomes continuous with the proximal tubule lumen.



CLINICAL POINT


Immune, circulatory, and metabolic kidney diseases can affect renal corpuscles. These disorders can be evaluated via biopsy and light or electron microscopy. Alport syndrome, or hereditary nephritis

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Jun 18, 2016 | Posted by in HISTOLOGY | Comments Off on URINARY SYSTEM

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