Transport Properties of the Renal Tubule
The filtrate formed by glomerular filtration is essentially identical to blood plasma in overall osmolarity and in the concentrations of small solutes. Filtrate becomes urine as a result of the serial processing of the fluid as it flows along the tubule and collecting duct.
1. Proximal convoluted tubule. The transport epithelium making up the wall of the proximal convoluted tubule alters the volume and composition of filtrate substantially, both by reabsorption and secretion. For example, ammonia is secreted, passing from the peritubular capillaries into the interstitial fluid and then across the epithelium of the tubule to join the filtrate.
The proximal tubule also helps to maintain a constant pH in body fluids by the controlled secretion of hydrogen ions. And drugs and other poisons that have been processed in the liver are secreted into the filtrate by the epithelium of the proximal convoluted tubule. On the other hand, nutrients, including glucose and amino acids, are actively transported from the filtrate to the interstitial fluid, and then into the blood within the peritubular capillaries. Without this reabsorption, these nutrients would be lost with the urine. Potassium is also reabsorbed.
One of the most important functions of the proximal convoluted tubule is the reabsorption of NaCl and water. In fact, about 75% of the NaCl and about 70% of the water that is pushed by filtration from the blood into the renal tubule is reabsorbed across the epithelium of the proximal convoluted tubule. The epithelial cells in this region of the tubule have a structure well suited to this voluminous reabsorption. Facing the lumen of the tubule is a brush-border, so named for the numerous microvilli projecting from the epithelial cells.
This adaptation provides an extensive surface area for reabsorption. Salt and water present in the filtrate diffuse across the brush-border and enter the epithelial cells. On the opposite side of the epithelium-the side facing the interstitial fluid outside the tubule-active transport occurs. The membranes pump Na + out of the cells and into the interstitial fluid. This transfer of positive charge is balanced by the passive transport of Cl- out of the tubule. As salt moves from the filtrate to the interstitial fluid, water follows passively by osmosis. The surface of the epithelium facing the exterior of the tubule has a much smaller area than does the brush-border, which minimizes the leakage of salt and water back into the tubule. Instead, the salt and water now diffuse from the interstitial fluid into the peritubular capillaries.
2. Descending limb of the loop of Henle. Reabsorption of water continues as the filtrate moves along the tubule to the descending limb of the loop of Henle. Here, the transport epithelium is freely permeable to water, but not very permeable to salt and other small solutes. For water to move out of the tubule by osmosis, the interstitial fluid bathing the tubule must be hyperosmotic to the filtrate. The osmolarity of the interstitial fluid does in fact increase gradually, becoming progressively greater along a radial axis from the outer cortex to the inner medulla of the kidney (the mechanism that maintains this gradient will be discussed shortly).
Thus, filtrate moving downward from the cortex to the medulla within the descending limb of the loop of Henle continues to lose water to interstitial fluid of greater and greater osmolarity. At the same time, the NaCl concentration of the filtrate increases as water departs by osmosis.
3 Ascending limb of the loop of Henle. The filtrate reaches the tip of the loop, located deep in the kidney medulla in the case of juxtamedullary nephrons, and then heads up to the cortex again within the ascend-ing limb of the loop. In contrast to the descending limb, the transport epithelium of the ascending limb is permeable to salt, but not very permeable to water The ascending limb actually has two specialized regions, a thin segment near the loop tip and a thick segment leading to the distal convoluted tubule. As filtrate ascends in the thin segment, NaCl, which became concentrated in the descending limb, diffuses out of the tubule into the interstitial fluid. This loss of salt contributes to the high osmolarity of the interstitial fluid in the medulla. The exodus of salt from the filtrate continues in the thick segment of the ascending limb, but here the transport is active. The epithelium of the thick segment pumps Cl- out of the tubule, and Na + responds to this efflux of negative charge by following the Cl- passively. By losing salt without giving up water, the filtrate becomes progressively more dilute as it moves up to the medulla again in the ascending limb of the loop of Henle.
4. Distal convoluted tubule. The distal tubule is another important site of selective secretion and absorption. For example, the distal tubule plays a key role in regulating the K+ concentration of body fluids, especially by varying the amount of the K+ that is secreted into the filtrate. The distal tubule also contributes to pH regulation, by the quantitative secretion of H + and by the reabsorption of bicarbonate (НС03-), an important buffer in the blood and interstitial fluid.
5. Collecting duct. The collecting duct now carries the filtrate back in the direction of the medulla and renal pelvis. The epithelium of the collecting duct is permeable to water but not to salt. Thus, as the collecting duct traverses the gradient of osmolarity that exists in the interstitial fluid, the filtrate loses more and more water by osmosis to the hyperosmotic fluid outside the duct. Loss of water concentrates the urea in the filtrate, but not all of this urea is immediately passed along to the renal pelvis in the urine. At the bottom of the collecting duct, in the inner medulla, the epithelium of the duct is permeable to urea.
Because of the high concentration of urea in the filtrate at this point, some of the urea diffuses out of the duct and into the interstitial fluid bathing the portions of nephrons in the medulla. This interstitial urea is a major solute contributing, along with the salt, to the high osmolarity of the interstitial fluid in the kidney medulla. And, it is this high osmolarity of the interstitial fluid that enables the kidney to conserve water by excreting a urine that is hyperosmotic to the general body fluids.