Reabsorption of nutrients, water and ions
The reabsorption of Na+ by primary active transport provides the energy and the means for reabsorbing almost every other substance, including water. Substances reabsorbed by secondary active transport (the “push” comes from the gradient created by Na+K+ pumping at the basolateral membrane) include glucose, amino acids, lactate and vitamins. In nearly all these cases, a luminal carrier moves Na+ down its concentration gradient as it cotransports (symports) another solute. Cotransported solutes diffuse (via different transport proteins) across the basolateral membrane before moving into the peritubular capillaries. Although there is some overlap of carriers, the transport systems for the various solutes are quite specific and limited.
There is a transport maximum (Tm) for nearly every substance that is reabsorbed using a transport protein in the membrane. The Tm (reported in mg/min) reflects the number of transport protein in the renal tubules available to ferry each particular substance. In general, there are plenty of transporters and therefore high Tm values for substances such as glucose that need to be retained, and few or no transporters for substances of no use to the body.
When the transportes are saturated,-that is, all bound to the substance they transport-the excess is excreted in urine. This is what happens in individuals who become hyperglycemic because of uncontrolled diabetes mellitus. As plasma levels of glucose approach and exceed 180 mg/dl, the glucose Tm is exceeded and large amounts of glucose may be lost in the urine even though the renal tubules are still functioning normally.
In passive tubular reabsorption, which encompasses osmosis, diffussion, and facilitated diffusion, substances move down their electrochemical gradients without the use of ATP. The movement of Na+ and other solutes establishes a strong osmotic gradient, and water moves by osmosis into the peritubular capillaries, a process aided by transmembrane proteins called aquaporins that form water channels across cell membrane. In continuosly water-permeable regions, aquaporins are constant components of the tubule cell membranes. Because these channels are always present, the body is “obliged” to absorb water in the proximal nepron regardless of its state of over or underhydration. This water flow is referred to as obligatory water reabsorption. Aquaporins are virtually absent in the luminal membranes of the collecting duct unless antidiuretic hormone (ADH) is present.
As water leaves the tubules, the concentration of solutes in the filtrate increases and, if able, they to begin to follow their concentration gradients into the peritubular capillaries. This phenomenon of solutes following solvents explains the passive reabsorption of a number of solutes present in the filtrate, such as lipid-soluble substances, certain ions and some urea. It also explains in part why lipid-soluble drugs and enviromental toxins are difficult to excrete, since lipid-soluble compounds can generally pass through membranes, they will follow their concentration gradients and be reabsorbed, even if this is “not desirable”.
As they move through the tubule cells into the peritubular capillary blood, Na+ ions also establish an electrical gradient that favors passive reabsorption of anions (primarily Cl-) to restore electrical neutrality in the filtrate and plasma.
Any plasma proteins that squeeze through the filtration membrane are removed from the filtrate in the proximal tubule by endocytosis and digested to their amino acids, which are moved into the peritubular blood.