a method used to study the filling of a part of the body with blood by graphically recording the fluctuations in the resistance of that part of the body. It is used in physiology and medicine.
Rheography is based on the fact that when an alternating current of sonic or ultrasonic frequency (16–300 kilohertz) passes through a part of the body, the organic fluids (chiefly blood in the large blood vessels) act as conductors. This makes it possible to determine the condition of the blood circulation in a particular region of the body or in an organ, for example, in an extremity or in the brain, heart, liver, or lungs. Since the filling of a part of the body with blood is influenced by vascular tone and the total quantity of blood, rheography provides an indirect idea of both the peripheral resistance to the blood flow in the blood vessels and the volume of circulating blood.
A rheogram is recorded by a rheograph, which consists of a power unit, a high-frequency current oscillator, an amplifier, a recording device, and electrodes. Rheography is used in medicine in the diagnosis of diseases of the heart and blood vessels and of other internal organs; it is also used in cases of blood loss and shock.