The same basic mechanism applies to solutes, such as potassium, calcium, and other salts, the concentrations of which are maintained in very narrow ranges in the extracellular and intracellular spaces. The kidneys filter these solutes at a largely constant rate, but they alter their rate of excretion based on input from homeostatic sensor mechanisms. Solute excretion can be adjusted by altering the rate of either reabsorption or secretion. Many substances are reabsorbed or secreted using active, transcellular mechanisms that can be very finely tuned.

The signals that modulate these processes differ depending on the substance in question. For example, aldosterone is released in response to elevated extracellular potassium levels and promotes increased potassium secretion (see Plate 3-10). In contrast, parathyroid hormone (PTH) is released in response to decreased calcium levels and promotes a net increase in calcium reabsorption (see Plate 3-11). The details of these homeostatic mechanisms, as well as some of the complications that occur when they are disturbed, are discussed in detail later in this section.