43 Filtration, Reabsorption, and Secretion
To produce urine, nephrons and collecting ducts carry out three basic functions: glomerular filtration, tubular reabsorption, and tubular secretion.
The first step in urine production is glomerular filtration. In this process, water and most of the solutes in blood plasma pass through the wall of glomerular capillaries, first into the glomerular capsule and then into the renal tubule. The product of glomerular filtration is referred to as glomerular filtrate.
During tubular reabsorption, cells of the tubule reabsorb almost all water and a variety of solutes from the filtrate as it flows through the renal tubule and collecting duct. These reabsorbed substances are then returned to the blood in the peritubular capillaries and vasa recta. In contrast to absorption, in which new substances enter the body (e.g., through the gastrointestinal tract), reabsorption refers to substances that have been removed from the blood being returned to the blood.
In the process of tubular secretion, renal tubule and collecting duct actively transport substances (e.g., wastes, drugs, excess ions) from the blood in the peritubular capillaries. Secretion, in this case, refers to the removal of substances from the blood. In contrast, the secretion of hormones or enzymes, for example, refers to cells releasing substances into blood, ducts, and interstitial fluid.
The normal amount of filtrate produced as a result of glomerular filtration is so high that the amount of fluid entering the proximal convoluted tubules in 30 minutes is greater than the total blood plasma volume. Clearly, some of this fluid must be reclaimed and sent back into the bloodstream to maintain water balance and ion concentrations. This reclamation process occurs through reabsorption in the tubules. Secretion is essentially the opposite of reabsorption. It removes the additional solutes from the plasma and actively transports them to the tubule. Many students get confused initially with this concept that secretion means end up in the urine and reabsorption means ends up back in the blood.