The proximal tubule reabsorbs between 40 and 60% of the glomerular ultrafiltrate. Here, glucose and amino acids are reabsorbed in its totality along. The proximal convoluted tubule also reabsorbs 70% of the filtered potassium (K) and 75% of the bicarbonate (HCO3).
The cytoplasm of the cells that make up the proximal tubule is densely packed with mitochondria, which are largely found in the basal region within the infoldings of the basal plasma membrane. The high quantity of mitochondria gives the cells an acidophilic appearance. The high number of mitochondria is necessary to supply the energy for the active transport of sodium ions out of the proximal tubule. Water passively follows the sodium out of the cell along its concentration gradient.
The filtrate which accumulates in Bowman's space drains into the proximal tubule, and hence to the loop of Henle, the distal tubule, and the collecting duct. In these various segments of the renal tubule, the filtrate is modified into urine, chiefly by reabsorption of non-waste components.