Mechanical and metabolic interrelations among the three major adult organ systems and adipose tissue. Liver is shown representative of the four metabolic system components. Metabolic loading conditions include, for example, tissue oxygen consumption, intravascular fluid volume, and arterial blood pressure effects. Although small in magnitude, brain contributes to the mechanical load on the musculoskeletal system. Abbreviations: EI, energy intake; AT, adipose tissue; EO, energy output or expenditure; MET, metabolic system; MS, musculoskeletal systems; and NS, nervous system.