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
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