Formation Excretion of Urine 尿液的形成和排泄.pptVIP

* The macula densa senses changes in the composition of tubular fluid that occur when the tubular fluid flow rate changes and signals the afferent arteriole to alter constriction. Changes in the tubular fluid flow rate are brought about by changes in GFR and/or the rate of reabsorption in the proximal tubule. For example a rise in GFR increases the tubular fluid flow rate at the macula densa. That triggers an increase in afferent arteriolar resistance that reduces pressure in the glomerular capillaries and brings GFR back down. The rise in resistance also reduces RBF. * In the absence of other changes, the tubule tends to reabsorb a constant fraction of the filtered salt and water (glomerulotubular balance). The filtration fraction increases and this results in a rise in the protein concentration in the plasma flowing into the peritubular capillary bed, that is, b increases. At the same time the arteriolar constriction reduces Pc downstream in the peritubular capillaries. The change in both pressures increases the net driving force for fluid uptake from the paracellular spaces, thereby increasing the rate of reabsorption by the proximal tubule. * * * * * There are two pathways through which, tubular substances are reabsorbed into the interstitial fluid * Transport through the tubular wall is same as the transport across cell membrane as discussed in the second chapter. through the cellular pathway usually involves specific carriers and channels. Transport through the paracellular pathway occurs by diffusion driven by electrical and chemical gradients between the tubular fluid and the interstitial fluid. A transport mechanism which directly utilizes metabolic energy is called a primary active mechanism. The Na-K pump for instance utilizes the metabolic energy stored in ATP. These systems may operate either as capacity-limited systems or as gradient-limited systems. A secondary active system utilizes the chemical or electrical energy resulting from the work of a p