Characteristics of glomerulotubular balance.
Glomerulotubular balance (GTB) is defined as the ability of each successive segment of the proximal tubule to reabsorb a constant fraction of glomerular filtrate and solutes delivered to it. For maintenance of GTB the coupling of peritubular blood flow and intratubular fluid flow to the process of glomerular filtration seems to be of functional importance since tubular fluid reabsorption is significantly altered when either or both parameters are experimentally changed. In the case of peritubular blood flow, variations of tubular fluid reabsorption have been ascribed to variations of the mean net colloid osmotic pressure in the peritubular blood and its effects on the paracellular backleak of tubular resorbate. This relationship has, however, been clearly demonstrated only in volume expansion, when GTB is impaired. Under nondiuretic conditions, in which GTB typically occurs, the importance of the peritubular colloid osmotic pressure in control of tubular fluid reabsorption is less clear since variations of peritubular colloid osmotic pressure within a physiologic range exert only a negligible influence on tubular fluid transport. Pharmacologically induced alterations of peritubular hemodynamics or mean net colloid osmotic pressure can affect tubular fluid reabsorption without consistently altering the net interstitial pressure. In the case of intratubular flow rate, variations of tubular fluid reabsorption are comparable to changes seen with GTB. Such an interrelation is found only in tubules perfused by natural tubular fluid. In individual nephrons flow-dependent reabsorption cannot always be observed even when it appears that GTB is preserved in that kidney. Flow dependency of tubular fluid reabsorption might be attributed to some properties or constituents in tubular fluid rather than to some intrinsic characteristics of the tubular epithelium. Because flow dependence and tubular fluid transport are homogeneous along the tubule, fluid reabsorption might be controlled by a mechanism akin to a flow reactor. As yet it is not possible to explain GTB exclusively by peritubular or luminal control alone.
D000818:Animals; D006801:Humans; D007668:Kidney / Q000098:blood supply; D007678:Kidney Glomerulus / Q000502:physiology*; D007684:Kidney Tubules / Q000502:physiology*; D007687:Kidney Tubules, Proximal / Q000502:physiology; D008833:Microcirculation; D008954:Models, Biological