Convoluted tubules

The epithelia of the proximal tubules (P), both convoluted and straight, have long microvilli. We identified this epithelia as simple cuboidal microvillus epithelia. The microvilli in the distal tubules (D) is not as prominent, giving the epithelium a shorter appearance. The striped appearance and acidophilia of these epithelial cells is largely due to infoldings of the basal plasmalemma and mitochondria. In the proximal tubules, many of the solutes present must be reabsorbed and returned to the circulation (can you list some of these substances?). In the distal tubules, primarily sodium is reabsorbed. In either case, the epithelium must transport substances across a concentration gradient. The energy produced by the mitochondria is essential for this to take place. Sodium passes through ion channels in the apical plasmalemma to enter the cells. ATP-dependant Na⁺/K⁺ pumps on the basal plasmalemma pump sodium into the interstitia to be taken up by the peritubular capillaries. Much of the reabsorption of the other solutes is dependent on the movement of sodium. The microvilli and basal plasmalemma infoldings increases the surface area on both sides of the cell for this work.