How do nephrons regulate water and electrolyte balance?

Nephrons regulate water and electrolyte balance through the processes of filtration, reabsorption, and secretion.

Nephrons, the functional units of the kidneys, play a crucial role in maintaining the body's water and electrolyte balance. This is achieved through a series of processes that occur within the nephron, namely filtration, reabsorption, and secretion.

Filtration is the first step in the process. Blood enters the nephron via the afferent arteriole and is filtered in the glomerulus, a network of tiny blood vessels. The filtrate, which includes water, glucose, amino acids, and electrolytes, passes into the Bowman's capsule. Larger molecules such as proteins and blood cells are too large to be filtered and remain in the blood.

Reabsorption is the next step. As the filtrate moves through the proximal tubule, loop of Henle, and distal tubule, useful substances such as water, glucose, and electrolytes are reabsorbed back into the bloodstream. This process is highly selective and is regulated by hormones such as antidiuretic hormone (ADH) and aldosterone. ADH increases the permeability of the distal tubules and collecting ducts to water, allowing more water to be reabsorbed. Aldosterone increases the reabsorption of sodium ions, which in turn promotes water reabsorption.

Finally, secretion occurs in the distal tubule and collecting duct. This process involves the active transport of substances from the blood into the tubular fluid. These substances, which include hydrogen ions, potassium ions, and certain drugs, are then excreted in the urine.

In summary, nephrons regulate water and electrolyte balance by filtering the blood, selectively reabsorbing useful substances, and secreting waste products. This ensures that the body's internal environment remains stable, a concept known as homeostasis.