To excrete is to pass waste matter out of the body — especially to discharge faeces, the solid waste of digestion. The body excretes through several routes: the bowel expels undigested food residue, gut bacteria, and shed cells; the kidneys excrete urea, excess salts, and water as urine; the lungs excrete carbon dioxide and water vapour; the skin excretes sweat. Each route serves the same purpose — removing what the body cannot use or must not keep — and nutrition supplies the material for all of them: fibre bulks the faeces, protein metabolism generates the urea, water carries the load. Disordered excretion signals disease — constipation, kidney failure, retention. Excretion is the necessary counterpart of nutrition: what goes in must, in some form, come out.