Reabsorption and secretion

About 180 litres of filtrate are formed a day but only about 1.5 litres of urine are passed, so nearly 99 per cent of the filtrate must be taken back by the renal tubules. This is reabsorption.

The epithelial cells of the different tubule segments reabsorb by active or passive mechanisms.

Glucose, amino acids, Na⁺ and similar substances are reabsorbed actively; nitrogenous wastes are absorbed passively.

In the initial segments of the nephron, water is also reabsorbed passively.

In secretion, tubular cells add substances such as H⁺, K⁺ and ammonia to the filtrate.

Secretion is an important step too, because it keeps the ionic and acid–base balance of the body fluids.