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 urine, the vasa recta are long, straight capillaries that reach deep into the medulla alongside the longest nephron loops where they can collect reabsorbed substances from the loop segments.

Because blood in the renal circulation flows through two arterioles (where a majority of the manipulation of vascular resistance is found), renal blood pressure drops from about 95 mm Hg in the renal arteries to less than 10 mm Hg in the renal veins. Resistance in the afferent arterioles fluctuates in order to help maintain a relatively constant glomerular capillary hydrostatic pressure even if there are substantial changes in systemic blood pressure. The resistance of the efferent arterioles also contributes to maintenance of glomerular capillary hydrostatic pressure and also contributes to a low hydrostatic pressure in the peritubular capillaries.

Specialized Cells Associated with the Nephron

In all nephrons, the last part of the ascending limb of the nephron loop transitions into the distal convoluted tubule and comes in contact with the afferent arteriole supplying the renal corpuscle. In this region the columnar epithelial cells at the beginning of the distal convoluted tubule are very crowded, leading to the name macula densa (“dense spot”). The macula densa is believed to monitor sodium chloride concentration of the filtrate entering the distal convoluted tubule. The wall of the afferent arteriole that is adjacent to the macula densa contains granular cells (also known as juxtaglomerular cells ). The granular cells produce and secrete the enzyme renin and are also capable of contracting when stimulated. These cells and the macular densa make up the juxtaglomerular apparatus. The action of the juxtaglomerular apparatus helps control glomerular hydrostatic pressure by sending signals to the afferent arteriole. There are also special smooth muscle cells called interglomerular mesangial cells in in the spaces between the loops of the glomerulus. These cells help regulate blood flow through the glomerulus.

Collecting Ducts

As the functional units of the kidneys, the primary role of the nephrons is to filter plasma, reabsorb what the body would like to keep, and excrete the rest. Any substances not reabsorbed in the tubules of the nephrons flows into one of thousands of collecting ducts in the kidney. A short collecting tubule forms the juncture between a distal convoluted tubule and a collecting duct. Each collecting duct receives fluid from several nephrons and then transports it to the renal pelvis. The collecting tubules and ducts have two types of cells: intercalated cells, cuboidal cells with plentiful microvilli, and the more populous principal cells, cuboidal cells with limited, short microvilli. Principal cells help maintain water and sodium and potassium ion balance in the body. Intercalated cells help regulate the acid-base balance of the blood.

Urinary Structures and Functions