The major by-products of cellular metabolism are heat, carbon dioxide, water, inorganic salts, and nitrogen compounds. In excess quantities, any of these byproducts are toxic, and unless they are excreted from the body, the cells would eventually be destroyed by their own metabolic processes. The skin (Chapter 5) plays a major role in excreting excess heat, and the respiratory system (Chapter 6) plays a major role in excreting carbon dioxide. Except in extremely hot environments, the skin and lungs combined do not excrete sufficient water or salts to balance the quantities produced by cellular metabolism. Moreover, neither system excretes nitrogen compounds such as urea in any quantity. The urinary system performs the complex task of maintaining the chemical balance in the body. Without a functioning urinary system, excess nitrogen compounds accumulate in the blood (uremia), the blood becomes acidic (acidosis), and excess water accumulates in the tissues (edema). Within a few days, the individual lapses into a coma, and death occurs within 2 weeks.