48 Tubular Secretion
There are two methods of removing unwanted substances from filtrate. First, tubular cells simply do not reabsorb some solutes. The second method is tubular secretion. Secretion transfers unwanted substances from the blood and tubule cells into the tubular fluid. With the exception of potassium ions, most secretion occurs in the proximal convoluted tubule. However, some secretion also occurs in the cortical regions of the collecting ducts and in the late distal convoluted tubule.
Tubular secretion is important for a number of reasons. It removes substances that are not easily filtered, such as some drugs and metabolites that are securely attached to plasma proteins. Second, it disposes of unwanted substances or end products that have been reabsorbed passively, such as urea and uric acid. Third, it eliminates excess potassium ions. Finally, it regulates blood pH. As an example, when blood pH becomes too acidic, secretion adds more hydrogen ions (acid) into the filtrate and retains more bicarbonate ions (a base).
Secretion in the Proximal Convoluted Tubule
With the exception of potassium ions, the proximal convoluted tubule is the primary site of secretion. This tubule secretes a range of organic ions, including hydrogen ions and ammonium ions. Ammonium ions are a poisonous waste product of deamination (the process that removes an amino group from an amino acid). Urea is also secreted in this tubule.
Secretion in the Distal Convoluted Tubule and Collecting Duct
Secretion and reabsorption of sodium, potassium, hydrogen, and bicarbonate ions are regulated and adjusted depending on various circumstances. For instance, the rate of potassium ion secretion is adjusted for the dietary intake of potassium, with the goal of maintaining a stable potassium ion level in body fluids. Potassium ions are regularly being brought into principal cells by basolateral sodium-potassium pumps. This process results in a high intracellular concentration of potassium ions. There are leakage channels in the apical (luminal) membrane of principal cells through which sodium ions enter and potassium ions escape. This mechanism of secretion is the primary source of the potassium ions that are excreted in the urine.
Summary of Tubular Reabsorption and Secretion
Reabsorption removes useful substances from the glomerular filtrate and returns them to the blood. The proximal convoluted tubule is the primary site of absorption; it is where all glucose, most amino acids, and the majority of salts are reabsorbed. Water reabsorption is coupled with the reabsorption of solutes in the proximal convoluted tubule in a process called obligatory water reabsorption. Only about 10 percent of water is reabsorbed in the collecting ducts, with the help of antidiuretic hormone, in a process called facultative water reabsorption. Secretion removes unwanted substances from the filtrate, such as urea and uric acid, so they can be excreted in urine. Secretion also helps regulate blood pH by adjusting the hydrogen ion and bicarbonate ion content of the filtrate. Several hormones affect reabsorption and secretion.