Amino acids are reabsorbed by sodium dependent transporters in the proximal tubule. Substances are reabsorbed from the tubule into the peritubular capillaries. Glucose at normal plasma levels is completely reabsorbed in the proximal tubule. Therefore, sodium and water are reabsorbed into the plasma by renal tubules. Some is reabsorbed into the bloodstream and then delivered to kidney. Potassium is secreted into the tubule in exchange for the sodium, which is reabsorbed. Water is 55% reabsorbed in the proximal tubule. Volume depletion occurs when the fluid ordinarily secreted by the GI tract cannot be reabsorbed. Normally Na and water are reabsorbed from the tubules and dumped into the interstitium. When aldosterone is present, more sodium is reabsorbed and more potassium secreted. This is because BUN is readily reabsorbed by the kidneys while creatinine is not. The stercobilinogen can not be reabsorbed and remains in the intestine. However, 92-97% is reabsorbed in the intestines and recycled via enterohepatic circulation. Potassium is reabsorbed to maintain serum potassium concentration within narrow limits. In most other mammals, the endometrium is reabsorbed in the estrous cycle.