Excretion and kidney function

Excretion removes metabolic wastes, such as urea and carbon dioxide. Egestion removes undigested material from the gut. The kidneys contain nephrons. Pressure filters water and small solutes from blood at the glomerulus into the nephron; most blood cells and large proteins remain in the circulation. Tubules selectively reabsorb useful substances and much of the filtered water.

Urine contains materials not returned to blood, including urea and variable water and salts. Antidiuretic hormone increases water reabsorption in suitable nephron regions, producing less, more concentrated urine. Dialysis can remove selected wastes and balance some solutes by exchange across a membrane, but it does not reproduce every kidney function. Plants can store wastes or remove them through processes such as leaf shedding or salt secretion.

Watch out: Much filtered water and many useful solutes are selectively reabsorbed into blood.