Section 12: Urinary System

Ronald A. Bergman, Ph.D., Adel K. Afifi, M.D., Paul M. Heidger, Jr., Ph.D.
Peer Review Status: Externally Peer Reviewed

Plates

The urinary system consists of the kidneys, ureters, urinary bladder, and urethra.

The volume and composition of body fluids are maintained remarkably constant regardless of fluid and solute intake. The regulation of body fluid and the constancy of the internal environment is the primary role of the kidneys.

Each kidney resembles the bean of the same name. At the center of the concave side, where the ureters and veins issue and arterial vessels and nerves usually enter, is the hilus. The kidneys are covered by a dense collagenous capsule called the renal capsule. The organ has a cortex and an inner medulla. The cortical area is clearly defined from the inverted conical or pyramid-shaped medulla. The base and sides are surrounded by cortical tissue, and the apex protrudes into the renal calyces. These protrusions are called renal papillae, whose surface is perforated by openings of numerous papillary ducts (of Bellini) and is thus called the area cribrosa.

From the base of the medullary pyramid, several hundred elongated parallel arrays of tubules enter the cortex. These so-called medullary rays consist of individual units composed of a straight collecting duct surrounded by many parallel tubular parts of the nephron. A renal lobule is defined as a single medullary ray and the cortical tissue that surrounds it. The cortical tissue in the area between the medullary pyramids constitutes the renal columns of Bertin. The cortex is punctuated by many renal or Malpighian corpuscles. The renal cortex consists of nephrons and blood vessels.

The human kidney has about four million filtering units called nephrons. The nephron consists of four parts: (1) renal corpuscle (capillary loops surrounded by an epithelial cover), (2) proximal convoluted tubule, (3) thin and thick limbs of Henle, and (4) distal convoluted tubule. The components of the nephron are completely covered by a basal lamina. The renal corpuscle is composed of two parts: a "tuft" of capillaries and a double-walled epithelial capsule called Bowman's capsule. The interior of Bowman's capsule (Bowman's space) is continuous with the proximal convoluted tubule.

Every minute, approximately one fourth of the cardiac output of blood enters the kidneys, and all the circulating blood passes through the kidneys every 5 minutes. The blood flows through the glomerular capillaries under 45 mm Hg pressure, and about one fifth of the plasma water leaves the capillaries and enters Bowman's space and the proximal tubules. The blood remaining in the capillaries flows into the efferent arterioles, which break up into capillary peritubular networks closely applied to renal tubules.

Plasma water in Bowman's space is termed the glomerular filtrate and normally contains all the major ions, amino acids, glucose, urea, and other substances in approximately the same concentration that exists in the blood plasma. However, this ultrafiltrate does not normally contain red blood corpuscles or significant amounts of protein. Both water and solutes are reabsorbed across the renal tubule cells to the fluid-filled interstitial space and into the peritubular capillaries to reenter the blood stream. This is called tubular reabsorption. Other substances leave the peritubular capillaries, enter the interstitial fluid, and are transported across the tubule cells, where they enter the tubule to be added to the glomerular filtrate. This is called tubular secretion.

The proximal convoluted tubule reabsorbs the filtered glucose and amino acids and some of the sodium, chloride, and bicarbonate ions. This active process, enhanced by the numerous microvilli that increase the surface area of proximal tubule cells, results in an osmotic gradient, which causes the passive reabsorption of water. The content of the proximal tubule is isotonic with respect to the blood plasma. The distal straight portion of the proximal tubule is in continuity with the thin limb of the loop of Henle, which extends into the kidney medulla and returns as the thick limb to the cortex. The content of the descending thin limb becomes increasingly hypertonic as salt (active chloride pump) is actively transported from the ascending limb into the interstitial space surrounding the descending thin limb. Chloride, together with sodium ions, passively enters the descending limbs, as water is drawn to the interstitium. Unlike the descending limb, the ascending limb of the loop of Henle is impermeable to water, and the content of the ascending limb of the tubule becomes increasingly hypotonic as salt is actively transported from it. Sodium ions are actively reabsorbed, and water is passively reabsorbed in the distal tubule.

The ascending thin limb of the loop is in continuity with the distal tubule. The distal tubule and the collecting duct are engaged primarily in the regulation of acid-base and potassium ion balance in the blood. Sodium ions are conserved (under the influence of the hormone, aldosterone) and exchanged for hydrogen or potassium ion, or both. The distal convoluted tubule cells do not have numerous microvilli but rather are characterized by infoldings of their basal cell membrane forming compartments containing mitochondria. These result in the appearance in light microscopy of so-called basal striations. These cellular architectural modifications facilitate active transport between the interstitial fluid and the lumen of the distal tubule.

The final concentration of urine, by passive reabsorption of water to form a hypertonic urine, occurs in the collecting ducts as they pass through the region of increasing osmolality in the medulla produced by the thin loop of Henle. This is facilitated by the action of antidiuretic hormone on the membrane permeability of the cells lining the collecting ducts. The urine leaves the kidney via papillary ducts, enters the ureter, and is stored'in the urinary bladder. Contraction of the urinary bladder forces the urine into the urethra to be eliminated from the body. A single kidney produces roughly 63 ml of glomerular filtrate per minute and reabsorbs 62 ml. Hence, 1 ml is drained into the bladder as urine, resulting in an output of 1440 ml per 24 hours.

The kidneys are also the site of synthesis of the hormone renin. Adjacent to the renal corpuscle and distal convoluted tubule, the internal elastic lamina of the afferent arteriole disappears and the smooth muscle cells become structurally modified (epithelioid in appearance) and synthesize and store the hormone renin in the form of granules. These granules stain intensely with periodic acid-Schiff method and also following Zenker's fixation with several common histologic dyes. These cells are called juxtaglomerular (JG) cells. The distal tubule cells immediately adjacent to JG cells are also modified, appearing with tightly packed nuclei, and the area is called the macula densa. The macula densa and JG cells are jointly called the juxtaglomerular apparatus. In addition to the juxtaglomerular apparatus, there are some pale staining cells of unknown function associated with afferent arterioles that are variously given several names, including extraglomerular mesangial, lacis, or polkissen cells. Anything that results in a reduction of blood flow to the kidney, such as an aneurysm or significant blood loss, will activate the secretion of stored renin. Renin acts on a plasma protein termed angiotensinogen, resulting in the formation of an inactive decapeptide, angiotensin I. In the presence of a converting enzyme (found in high concentration in lung endothelium), angiotensin I becomes an octapeptide termed angiotensin II. Angiotensin II is a powerful vasoconstrictor. It also causes enhanced secretion of the adrenal cortical hormone, aldosterone, which acts on the cells of distal tubules to increase the reabsorption of sodium and chloride ions. This, in effect, increases blood pressure by increasing the volume of extravascular fluid. Chronic deficiency in blood flow to the kidneys can result in a serious, life-threatening hypertension.

Some names associated with the urinary system follow: Bellini was a seventeenth -century Italian anatomist; Bertin, an eighteen th-century French anatomist, Malpighi, a seventeen th-century Italian anatomist, Henle, a nineteenth-century German anatomist; Bowman, a nineteenth-century English anatomist; Schiff, a twentieth-century German chemist in Florence, and Zenker, a nineteenth-century German pathologist.

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