nephron and concentrations of interstital fluid image

In the second half of the proximal tubule, the higher chloride concentration favors the diffusion of this ion from the tubule lumen through the intercellular junctions into the renal inter-stitial fluid.Ĭoncentrations of Solutes Along the Proximal Tubule. The second half of the prox-imal tubule has a relatively high concentration of chloride (around 140 mEq/L) compared with the early proximal tubule (about 105 mEq/L) because when sodium is reabsorbed, it preferentially carries with it glucose, bicarbonate, and organic ions in the early proximal tubule, leaving behind a solution that has a higher concentration of chloride. Instead, sodium is now reabsorbed mainly with chloride ions. But in the second half of the proximal tubule, little glucose and amino acids remain to be reabsorbed.

In the first half of the proximal tubule, sodium is reabsorbed by co-transport along with glucose, amino acids, and other solutes. As discussed, the secretion of hydrogen ions into the tubular lumen is an important step in the removal of bicarbonate ions from the tubule (by combining H + with the HCO 3 _ to form H 2 CO 3, which then dissociates into H 2 O and CO 2 ).Īlthough the sodium-potassium ATPase pump pro-vides the major force for reabsorption of sodium, chloride, and water throughout the proximal tubule, there are some differences in the mechanisms by which sodium and chloride are transported through the luminal side of the early and late portions of the proximal tubular membrane. The remainder of the sodium is transported from the tubular lumen into the cell by counter-transport mechanisms, which reabsorb sodium while secreting other substances into the tubular lumen, especially hydrogen ions. The extensive membrane surface of the epithelial brush border is also loaded with protein carrier mole-cules that transport a large fraction of the sodium ions across the luminal membrane linked by way of the co-transport mechanism with multiple organic nutrientssuch as amino acids and glucose. Tubular cells have an extensive brush border on the luminal (apical) side of the membrane as well as an extensive labyrinth of intercellular and basal channels, all of which together provide an extensive membrane surface area on the luminal and basolateral sides of the epithelium for rapid transport of sodium ions and other substances.

The proximal tubule epithelial cells are highly metabolic and have large numbers of mitochondria to support potent active transport processes. The high capacity of the proximal tubulefor reabsorption results from its special cellular char-acteristics, as shown in Figure 27–6. Proximal Tubules Have a High Capacity for Active and Passive Reabsorption. These percentages can be increased or decreased in different physiologic conditions, as discussed later. Normally, about 65 per cent of the filtered load of sodium and water and a slightly lower percentage of filtered chloride are reabsorbed by the proximal tubule before the filtrate reaches the loops of Henle. In subsequent, we discuss the reabsorption and secretion of other specific substances in different parts of the tubular system. Only the tubular transport functions that are quantitatively most important are discussed, especially as they relate to the reabsorption of sodium, chloride, and water. With these generalizations in mind, we can now discuss the different characteristics of the individual tubular segments that enable them to perform their specific excretory functions.

In the previous sections, we discussed the basic principles by which water and solutes are transported across the tubular membrane. Reabsorption and Secretion Along Different Parts of the Nephron

Nephron and concentrations of interstital fluid image