Following filtration, the renal tubules reabsorb most of the filtrate to return valuable substances to the bloodstream. Tubular reabsorption primarily occurs in the proximal convoluted tubule (PCT), which reabsorbs about 65% of water and sodium, along with glucose, amino acids, and vitamins through active transport. Sodium reabsorption drives many of these processes through a sodium-potassium pump, which creates an electrochemical gradient that facilitates the passive movement of other solutes. Water follows sodium through osmosis, ensuring that water reabsorption is closely linked to sodium reabsorption. In the nephron loop (loop of Henle), further reabsorption of water occurs in the descending limb, while sodium, potassium, and chloride are reabsorbed in the ascending limb through a symporter.
The distal convoluted tubule (DCT) and collecting ducts further fine-tune reabsorption, particularly of sodium, chloride, and calcium, under the regulation of hormones like aldosterone and parathyroid hormone (PTH). Aldosterone increases sodium reabsorption by promoting the synthesis of sodium channels, while PTH regulates calcium uptake. In the collecting ducts, antidiuretic hormone (ADH) plays a pivotal role in water reabsorption by increasing the number of aquaporins in the tubular membrane, which allows water to be absorbed from the filtrate back into the bloodstream.
Tubular secretion complements reabsorption by actively removing substances from the bloodstream into the renal tubules. This process is essential for eliminating drugs, metabolic waste products like creatinine, and excess ions such as hydrogen and potassium. Secretion of hydrogen ions helps maintain the body’s acid-base balance, while potassium secretion is regulated by aldosterone in response to the body's electrolyte needs.
