How do kidneys filter and reclaim useful substances?

If everything passing through a kidney's first filter left the body as urine, the body would lose useful glucose, salts and far too much water. Its remarkable solution is to filter broadly and then recover selectively. What becomes urine is the small, changing remainder after a much larger exchange between blood and tiny tubes.

Each kidney contains about a million working units called nephrons. A nephron begins with the glomerulus, a tuft of capillaries surrounded by a collecting capsule. Pressure drives water and small dissolved substances across a filtration barrier into the capsule. Blood cells and most large proteins normally stay in the vessels. The fluid that enters the tubule is filtrate, not finished urine, and it includes substances the body will reclaim.

Blood leaving the glomerulus continues into vessels around the tubule. This creates two nearby routes: tubular fluid moving through a narrow passage and blood moving through capillaries. Reabsorption transfers material out of the tubular fluid, through or between its lining cells, and back toward blood. Secretion transfers selected substances the other way, from blood toward the tubular fluid. These opposite directions let a nephron alter the filtrate as it travels.

The lining does the sorting. In the proximal tubule, transport proteins recover glucose and many other solutes. Sodium gradients help power some of this movement: pumps that use ATP keep intracellular sodium relatively low, allowing sodium entering from the tubule to carry certain other substances with it. Water follows osmotic gradients where the tissue allows it. Recovery therefore depends on living cells, selective transport proteins and energy; a passive sieve alone could not do this job.

Farther along, different properties of the nephron's loop help establish a concentrated environment deeper in the kidney. The descending limb readily loses water, whereas the ascending limb recovers salts while being largely impermeable to water. Opposite fluid directions and different wall properties help sustain a concentration gradient. Nearby blood vessels carry away recovered water and solutes while helping preserve that gradient.

The collecting duct then passes through this concentrated region. Antidiuretic hormone, or ADH, can increase the number of water channels in its lining. More water can then leave by osmosis and return toward blood, leaving less, more concentrated fluid to become urine. With less ADH, the duct is less permeable to water, so more can remain in the fluid. This adjustment links a microscopic membrane change to the body's overall water balance.

For any substance, the final accounting is what was filtered, minus what was reabsorbed, plus what was secreted. These terms describe amounts over the same time interval; they are not three separate batches of urine. The kidney's achievement is continuous selective exchange. The same basic network can conserve water and useful nutrients while allowing wastes and excess substances to leave, helping keep the fluid around the body's cells within workable limits.