Английский язык для медиков: конспект лекций - Страница 37

Bowman's capsule consists of an inner visceral layer and an outer parietal layer. The space between these layers, the uri nary space, is continuous with the renal tubule.

Visceral layer is apposed to the glomerulus and closely fol lows the branches of the glomerular capillaries. The visceral layer is composed of a single layer of epithelial cells resting on a basal lamina, which is fused with the basal lamina of the capillary endothelium. The cells of the visceral layer, called podocytes, are large and their nuclei bulge into the capsular space.

Cytoplasmic extensions of podocytes, called pedicles, rest on the basal lamina. The pedicles of adjacent podocytes interdigitate along the basal lamina.

Between adjacent pedicles, a thin slit diaphragm assists in preventing large plasma proteins from escaping from the vascular system. Parietal layer is composed of a simple squamous epithelium that is continuous with the proximal convoluted tubule epithelial lining. Proximal convoluted tubule is the longest and most convolut ed segment of the nephron.

It is lined by a single layer of cuboidal to low columnar cells with rounded nuclei and eosinophilic granular cytoplasm. Cell boundaries interdigitate with those of adjacent cells laterally and basally. The proximal convoluted tubule also possesses an apical brush border that provides the cell with a much greater surface area for reabsorption from, and secretion into, the fluid that becomes urine in the kidney tubules. In fact, most of the components of the glomerular filtrate are reabsor-bed in the proximal tubule. Loop of Henle is a hairpin loop of the nephron that extends into the medulla and consists of thick and thin segments. The thick proximal portion of Henle's loop, or the descend ing thick segment, is a direct medullary continuation of the cortical proximal convoluted tubule.

The descending and ascending thin segments of the loop of Henle are lined by a single layer of flat epithelial cells with nuclei that bulge into the lumen.

The thick distal portion of the loop of Henle, the ascending thick segment, ascends to the cortex and is continuous with distal convoluted tubule. It is lined by cuboidal cell; that contain numerous invagina-tions of cytoplasm anc many mitochondria. Distal convoluted tubule is lined by cuboidal cells that contair a granular cytoplasm. Cells of the distal convoluted tubule near the afferent arteriole are taller and more slender than elsewhere in the distal tubule. They constitute the macula densa. Their nuclei are packed closely, so the region appear darker under the light microscope. The macula densa is thought to sense sodium concentration in the tubular fluid. The major function of the distal tubule is to reabsorb soduim and chloride from the tubular filtrate. Collecting tubules consist of arched and straight segments. The arched collecting tubule segments are located in the cortical labyrinths and empty into the straight collecting tubule segments, which pass through the medullary rays. Epithelial cells of the collecting ducts range from cuboidal to columnar. Identification of these tubules is facilitated by their distinct intercellular borders as a result of the lack of complex in-terdigitations seen in the proximal and distal tubules. Vascular supply begins with the renal artery, enters the kidney the hilum, and immediately divides into interlobar arteries. The arteries supply the pelvis and capsule before passing direct between the medullary pyramids to the corticomedullary junction. The interlobar arteries bend almost 90 degrees to form shoarching, arcuate arteries, which run along the cortico-medullary junction. The arcuate arteries subdivide into numerous fine interlobul arteries, which ascend perpendicularly to the arcuate arteries through the cortical labyrinths to the surface of the kidney. Each in-terlobular artery passes midway between two adjacent medullary rays.

The interlobular arteries then give off branches that become the afferent arterioles of the glomeruli.

As the afferent arteriole approaches the glomerulus, some its smooth muscle cells are replaced by myoepithelioid cells, which are part of the juxtaglomerular apparatus. The juxtaglomerular apparatus consists of juxtaglomerular cells, polkissen cells, and the macula densa.

The juxtaglomerular cells secrete an enzyme called renin, which enters the bloodstream and converts the circulating polypeptide angioten-sinogen into angiotensin I. Angiotensin I is converted to angiotensin II, a potent vaso constrictor that stimulates aldosterone secretion from the adrenal cortex. Aldosterone increases sodium and water reabsorption in the distal portion of the nephron.

Polkissen cells are located between the afferent and effer ent arterio-les at the vascular pole of the glomerulus, adja cent to the macula densa.

Their function is unknown. Efferent glomerular arteriole divides into a second system of capillaries, the peritubufar plexus, which forms a dense net work of blood vessels around the tubules of the cortex.

Arterial supply of the medulla is provided by the efferent arte rioles of the glomeruli near the medulla. The arteriolae rectae and the corresponding venae rectae with their respective capillary networks comprise the vasa recta, which supplies the medulla. The endothelium of the venae rectae is fenestrated and plays an important role in maintaining the osmotic gradi ent required for concentrating urine in the kidney tubules.

Ureters

The calyces, renal pelves, and ureters constitute the main excretory ducts of the kidneys. The walls of these structures, in particular the renal pelvis and ureter, consist of three coats: an inner mucosa, middle muscularis, and an outer adventitia.

Mucosa of the calyces and ureter is lined by a transitional epithelium, which varies in thickness with the distention of the ureter. In the collapsed state, the cells are cuboidal with larger с shaped cells in the superficial layer. In the relaxed state, the lumen of the ureter is thrown into folds that generally disappear when the organ dilates during urine transport. Muscularis consists of an inner longitudinal and an outer circular layer of smooth muscle. In the distal ureter, an additional discontinuous outer longitudinal layer is present.

Adventitia consists of loose connective tissue with many large blood vessels. It blends with the connective tissue of the surrounding structures and anchors the ureter to the renal pelvis. The urinary bladder functions as a strong organ for urine. The structure of the wall of the bladder is similar to but thicker than of the ureter. Mucosa of the urinary bladder is usually folded, depending the degree of the bladder distention. The epithelium is transitional and the number of apparent layers depends on the fullness of the bladder. As the organ becomes distended, the superficial epithelial layer and the mucosa become flattened, and the entire epithelium becomes thinner. At its fullest distention, the bladder epithelium maybe only two or three cells thick. Lamina propria consists of connective tissue with abundant elastic fibers. Mus-cularis consists of prominent and thick bundles of smooth muscle that are loosely organized into three layers. Adventitia covers the bladder except on its superior part, where serosa is present. Male urethra serves as an excretory duct for both urine and semen. It is approximately 20 cm in length and has three anatom ic divisions. The prostatic portion is lined by transitional epithelium similar to that of the bladder. The prostatic urethra is surrounded by the fibromuscular tissue of the prostate, which normally keeps the urethral lumen closed. In the membranous and penile portions, the epithelium is pseudostratified up to the glans. At this point, it becomes stratified squamous and is continuous with the epidermis of the external part of the penis. The membranous urethra is encircled by a sphincter of skeletal muscle fibers from the deep transverse perineal muscle of the urogenital diaphragm, which also keeps the urethral lumen closed. The wall of the penile urethra contains little muscle but is surrounded and supported by the cylindrical erectile mass of corpus spongiosum tissue. Female urethra is considerably shorter than that of the male urethra It serves as the terminal urinary passage, conducting urine from the bladder to the vestibule of the vulva. The epithelium begins at the bladder as a transitional variety and becomes stratified squamous with small areas of a pseudostratified columnar epithelium. The muscularis is rather indefinite but does contain both circu lar and longitudinal smooth muscle fibers. A urethral sphincter is formed by skeletal muscle as the female urethra passes through the urogenital diaphragm.