a colorless, slightly opalescent fluid secreted by the sweat glands.
Human sweat contains 98 to 99 percent water, about 0.1 percent urea, uric acid, creatinine, serine, fats, volatile fatty acids, cholesterol, and alkaline metal salts, including chlorides (NaCl predominates— about 0.3 percent), phosphates, and sulfates, as well as sulfuric acid esters and aromatic oxygen acids. The secretion of the sebaceous glands is always mixed with the sweat that gathers on the skin surface. The composition of sweat depends on the condition of the body, the intensity of sweat excretion, and the presence of various substances in the blood. Sweat may be acid, with a pH of 3.8–6.2, or alkaline, when there is decomposition of urea and production of ammonia. In man from 0.5 to 10 liters of sweat or more per day are excreted, depending on intensity of muscle work, temperature of the external environment, and the quantity of water imbibed. Thus, with heavy muscle work, sweat contains a significant quantity of lactic acid and nitrogenous substances. In pathological states, sweat may contain glucose (sugar diabetes), bile pigments, cystine (cystinuria), and sometimes erythrocytes (bloody sweat).