^ 1.5-2 million per kidney ^ Renal corpuscle
• Located in the cortical labyrinth
♦ Glomerulus. A tuft of fenestrated capillaries, whose pores lack diaphragms; filter blood. Formed by an afferent arteriole, the glomerulus indents into Bowman's capsule like a baseball fits into a baseball glove. Blood leaves the glomerulus via the efferent arteriole.
♦ Bowman's capsule. Double-walled, epithelial capsule with central space called Bowman's space; surrounds the glomerulus and receives the fluid filtered from the blood
■ Parietal layer. Outer layer, simple squamous epithelium which is reflected at the vascular pole of the renal corpuscle to become the visceral layer; continuous with the proximal tubule at the urinary pole
■ Visceral layer. Inner layer surrounding the glomerulus. Consists of a single layer of modified epithelial cells called podocytes. The radiating foot processes of these cells give rise to many secondary processes called pedicels. Pedicels of adjacent podocytes interdigitate and surround the glomerular capillaries. The slits (filtration slits) between the pedicels are bridged by slit diaphragms.
♦ Filtration barrier. Barrier between blood in glomerular capillary and space of Bowman's capsule
■ Fenestrated endothelium of glomerular capillary
■ Thick, fused basal laminae of the podocytes and the glomeru-lar endothelial cells
■ Slit diaphragms between pedicels of visceral layer of epithelium
♦ Poles of the glomerulus
■ Vascular pole. Where afferent and efferent arterioles enter and leave the renal corpuscle, respectively
■ Urinary pole. Where the parietal layer of Bowman's capsule is continuous with the proximal convoluted tubule
^ Renal tubule
• The glomerular filtrate of the blood continues from Bowman's space into the renal tubule, which meanders first through the cortex, then the medulla, then back to the cortex, and finally enters the collecting duct.
• Regions of the renal tubule
♦ Listed in order are regions of the renal tubule through which urine passes
■ Proximal convoluted tubule
■ Proximal straight tubule
■ Distal straight tubule
■ Distal convoluted tubule
♦ Proximal tubule, convoluted portion
■ Located in labyrinth of cortex; highly convoluted
■ Interconnects parietal epithelium of Bowman's capsule with straight portion of proximal tubule
■ Composed of a simple cuboidal epithelium with microvilli; cells possess numerous infoldings of the basal plasma membrane and many mitochondria
■ Absorption of glucose, amino acids, and the majority of salt and water occur here.
♦ Loop of Henle. Located in medullary tissue (i.e., medullary ray and medulla)
■ Proximal tubule, straight portion (thick descending limb of loop of Henle)
- Located either in medullary ray (in cortex) or in medulla
- Interconnects proximal convoluted tubule with thin limb of Henle's loop
- Histology is identical to that of the proximal convoluted tubule
- Absorption of same substances as in proximal convoluted tubule
- Found in medulla
- Interconnects proximal straight tubule with distal straight tubule
- Frequently makes the "loop" in the loop of Henle
- Composed of a simple squamous epithelium
- Actively pumps out chloride, with sodium following passively, to produce a hypertonic urine
■ Distal tubule, straight portion (thick ascending limb of Henle's loop).
- Located either in medulla or in medullary ray (in cortex)
- Interconnects thin segment with distal convoluted tubule
- Composed of a simple cuboidal epithelium with inconsistent microvilli. The cytoplasm is less acidophilic and the lumen is wider than in the proximal tubule. The basal plasma membrane is extensively infolded with numerous mitochondria between the folds.
♦ Distal tubule, convoluted portion
■ Located in the labyrinth portion of cortex; highly convoluted
■ Interconnects the distal straight tubule with collecting tubule
■ Histology is identical with the distal straight tubule
■ Returns to a glomerulus to form part of the juxtaglomerular apparatus
■ Major site of salt and water control in the body
• Juxtaglomerular (JG) apparatus
♦ Located at the vascular pole of a nephron; helps regulate blood pressure
♦ JG cells. Modified smooth muscle cells in wall of an afferent arteriole
♦ Macula densa. Cluster of modified cells in the wall of a distal convoluted tubule adjacent to the JG cells. The clustering of cells, and therefore of their nuclei, gives the appearance of a "dense spot" in the wall of the distal convoluted tubule.
♦ Monitors the tonicity of the urine in the distal tubule. The macula densa affects the adjacent JG cells to adjust their production of renin, a hormone that aids in regulating blood pressure.
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