Juxtaglomerular Apparatus

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 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 influence 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. Urinary system

Collecting Tubules and 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 block like 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.

Distal convoluted tubule

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 epithelium 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

5. Cortical and juxtamedullary nephrons, renal 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.

Loop of Henle

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.

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.

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 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 proximal tubule measure about 14 mm, making this the longest portion of the nephron.

Kidney Parts

B. Nephrons: Nephrons are the functional subunits of the kidney.
Each includes a renal corpuscle, 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. Modified smooth muscle cells, mesangial cells, lie between the capillary 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 between 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 includes
(l)the diaphragm-covered capillary fenestrations,
(2) the fused basal laminae of the capillary endothelial cells and podocytes, and
(3) the diaphragm-covered filtration slits that lie between the interdigitating 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.

KIDNEYS

A. General Organization: The kidneys 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 comprise 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