Some substances, including by-products of metabolism that become toxic in high concentrations and drugs or other molecules not nor-mally produced by the body, are secreted into the nephron from the peritubular capillaries. As with tubular reabsorption, tubular secretion can be either active or passive. For example, ammonia diffuses into the lumen of the nephron, whereas H+, K+, creatinine, histamine, and penicillin are actively transported into the nephron.
Hydrogen ions are actively transported into the proximal con-voluted tubule. The epithelial cells actively transport large quanti-ties of H+ across the nephron wall into the filtrate. The secretion of H+ plays an important role in regulating the body fluid pH.
In the proximal convoluted tubule, K+ is reabsorbed. However, in the distal convoluted tubule and collecting duct, K+ is secreted, resulting in a net loss of K+ in the urine.