39 The Nephron Loop
The renal corpuscle, the proximal convoluted tubule, and the juncture between the proximal convoluted tubule and the nephron loop are located in the renal cortex. The first part of the nephron loop, the descending limb of the nephron loop, drops into the renal medulla. In the renal medulla, the loop makes a sharp, almost 180-degree turn back toward the renal cortex as the ascending limb of the nephron loop. The ascending limb is continuous with the distal convoluted tubule. The ascending and descending limbs of the nephron loop have two distinct parts: a thin section of the limb and a thick section of the limb. In the thin section of the limb, the diameter of the tubule is distinctly smaller than the diameter of the rest of the nephron tubules. In the thin sections of the limbs, the epithelium is thinner simple squamous epithelium that is permeable to water. In the thick sections of the limbs, the epithelium is simple cuboidal epithelium that is highly impermeable to water. Regardless of being in the thin or the thick segments, the lumen is the same size as the lumen in the rest of the renal tubule.
The Distal Convoluted Tubule
The final segment of the nephron is the distal convoluted tubule. As the ascending limb of the nephron loop reaches the renal cortex, it becomes the distal convoluted tubule. The distal convoluted tubule extends to the collecting tubule, the short connection with a collecting duct. The distal convoluted tubule is composed of simple cuboidal epithelium with very few microvilli and no brush border.
Capillaries of the Nephron
The glomerulus is not the only capillary bed associated with nephrons. Peritubular capillaries are branches of the efferent arterioles that drain the glomeruli and recover most of the filtrate produced in the renal corpuscle. Glomerular capillaries differ from other capillary beds in the body, because they are both supplied by and drained by arterioles. The feeder afferent arterioles are branches of the cortical radiate arteries. The draining efferent arterioles branch into the peritubular capillary network around the proximal and distal convoluted tubules or the vasa recta around the nephron loop. The diameter of the draining efferent arterioles is smaller than that of the afferent arterioles, giving the efferent arterioles higher resistance. Because of this, the glomerulus has a high blood pressure that allows it to filter high volumes of fluid and solutes out of the blood and into the glomerular capsule. The nephrons segments reabsorb approximately 99 percent of this filtrate. The peritubular capillaries adhere to neighboring convoluted tubules and drain into neighboring venules. These low-pressure and porous capillaries easily reabsorb the water and solutes that the tubule recovers from the filtrate. In some nephrons, rather than breaking up into peritubular capillaries, the efferent arterioles form clusters of thin-walled vasa recta. Important for the formation of concentrated ur