ABSTRACT
Most of the energy in the circulation ultimately derives from the hydrolysis of ATP in heart muscle and conversion of high energy phosphate bonds by sarcomeres into heart muscle work. Energy in the circulation will be defined first and then flow will be discussed, including a discussion of gravity's effects on the circulation. In a person at rest, kinetic energy in the circulation relative to the other terms is small, approximately 3% to 5% of total energy, but it increases with, exercise. The siphon model is pertinent for the circulation because in the circulation the key energy term is pressure potential energy. But the physiological effects of gravity acting on the blood are most evident on the venous side of the circulation due to veins being much more compliant than arteries. Turbulent flow is chaotic and disorganized rather than laminar and the black dye spreads throughout the cross-section of the tube due to the whorls and eddies of turbulence.
