here are basically three categories of chemicals normally functioning in the body—elements, inorganic compounds, and organic compounds: {draw:frame} Elements are components of all chemical compounds. Of the 92 naturally occurring elements, only 20 are normally found in the body. Seven of these, carbon, oxygen, hydrogen, calcium, nitrogen, phosphorous, and sulfur make up approximately 99% of the human body weight. In most cases, the elements are components of inorganic or organic compounds. In a few cases, however, elements themselves may enter into chemical reactions in the body, e.g., oxygen during cell respiration, sodium in neurotransmission, and arsenic and lead in impaired mitochondrial metabolism. Inorganic compounds are important in the body and responsible for many simple functions. The major inorganic compounds are water (H2O), bimolecular oxygen (O2), carbon dioxide (CO2), and some acids, bases, and salts. The body is composed of 60-75% water. Oxygen is required by all cells for cellular metabolism and circulating blood must be well oxygenated for maintenance of life. Carbon dioxide is a waste product of cells and must be eliminated or a serious change in pH can occur, known as acidosis. A balance in acids, bases, and salts must be maintained to assure homeostasis of blood pH and electrolyte balance. Organic compounds are involved in nearly all biochemical activities involved in normal cellular metabolism and function. The mechanisms by which xenobiotics cause cellular and biochemical toxicity are predominantly related to changes to organic compounds. The main feature that differentiates organic compounds from inorganic compounds is that organic compounds always contain carbon. Most organic compounds are also relatively large molecules. There are five major categories of organic compounds involved in normal physiology of the body, namely