General Physiology of the Nephron
Nephrons regulate the composition of blood by a combination of three processes that transfer material between the renal tubules and the capillaries that serve them:
1. Filtration. Blood pressure forces fluid from the capillaries of the glomerulus across the epithelium of Bowman's capsule into the lumen of the renal tubule. The porous capillaries, along with specialized cells of the capsule called podocytes, function as a filter, being permeable to water and small solutes but not to blood cells or large molecules such as plasma proteins (Figure 40.11). Filtration is nonselective with regard to small molecules; any substance small enough to be forced through the capillary wall by blood pressure enters the renal tubule. Thus, at this point, the filtrate contains a mixture of solutes such as salts, glucose, vitamins, NITROGENOUS WASTES . such as urea, and other small molecules that mirrors the concentrations of these substances in blood plasma.
2. Secretion. As filtrate travels through the renal tubule, it is joined by substances that are transported across the tubule epithelium from the surrounding interstitial fluid. Since small molecules pass freely from the plasma within capillaries into the interstitial fluid. the net effect of renal secretion is the addition of plasma solutes to the filtrate within the tubule. The proximal and distal convoluted tubules are the most common sites of secretion. Unlike the nonselective filtration that occurs at the glomerulus-capsule interface, secretion is a very selective process involving both passive and active transport. For example, the controlled secretion of hydrogen ions from the interstitial fluid into the nephron tubule is important in maintaining a constant pH for the body fluids.
3. Reabsorption. Since filtration is nonselective, it is important that small molecules essential to the body be returned to the interstitial fluid and blood plasma. This selective transport of substances across the epithelium of the renal tubule from the filtrate to the interstitial fluid is called reabsorption. The convoluted tubules and the loop of Henle all contribute to reabsorption, as does the collecting duct that receives filtrate from the tubule. Nearly all the sugar, vitamins, and other organic nutrients present in the initial filtrate are reabsorbed. Most of the water of the filtrate is also reabsorbed in the kidneys of mammals and birds, thus enabling these terrestrial animals to save water by decreasing urine volume. Together, reabsorption and secretion control the concentrations of various salts in the body fluids, responding to imbalances by causing the kidneys to excrete more or less of a particular ion.
Thus, selective secretion and reabsorption modify the composition of the filtrate, increasing the concentrations of some substances and decreasing the concentrations of others in the urine that is finally excreted, The overall effect of filtration, secretion, and reabsorption is analogous to cleaning out a drawer (blood) by first removing all the small articles (filtration), returning useful items to the drawer (reabsorption), adding additional useless household items to the refuse pile (secretion), and then discarding all the unwanted objects (excretion).
To better understand the versatility of the nephron as an apparatus for controlling the composition of body fluids, it is necessary to examine the specialized functions of the transport epithelium that forms the wall of the various regions of the renal tubule and collecting duct.