Excretion is the act of passing waste matter — faeces, urine, sweat — out of the body: the noun form of the body’s waste-disposal function. The excretory systems each handle their share: the large intestine compacts and expels the indigestible residue of food; the kidneys filter blood continuously, excreting urea, creatinine, excess electrolytes, and water; the lungs breathe out carbon dioxide, the gaseous waste of metabolism; the skin releases sweat carrying salts and water. Together they maintain the internal constancy — fluid balance, pH, waste clearance — on which every cell depends. Diet shapes excretion directly: protein intake sets urea load, fibre sets stool bulk, salt and water set urine volume. It is the unglamorous half of metabolism, and its smooth running is the definition of a body in balance.