When considering venous return, the pressure gradient is mean systemic pressure minus the right atrial pressure, and resistance is the total peripheral vascular resistance. These extensive series of experiments laid the groundwork for an in-depth understanding of the basic factors controlling venous return.Įverywhere in the body, pressure gradients and resistances determine blood flow rate. Some of the experiments were conducted with closed chest animals that were breathing again negative pressure as low as –10 mm Hg, lowering the intrathoracic and right atrial pressures to the lowest physiological levels. They could also augment flow into the atrium from a reservoir, thereby increasing right atrial pressure.
By varying the rate of flow around the atrium, they were able to reduce right atrial pressure in a controlled manner while measuring cardiac output in the systemic circulation, from which they knew the value of venous return. Because right atrial pressure is a function of output from the right atrium into the right ventricle and flow of blood into the atrium from the vena cava, they had to develop a preparation in which outflow could be augmented by mechanically pumping from the atrium through an extracorporal shunt into the pulmonary artery or through a heart lung bypass machine to the aorta.
Analysis of the effects of changes in the right atrial pressure on venous return required a means to vary the right atrial pressure in a controlled manner. They found that the technique could be repeated many times without affecting the value of mean systemic pressure. He and his coworkers developed techniques to temporarily stop the heart by electrical fibrillation or other means while blood was mechanically pumped rapidly through an arterial to venous shunt in order to bring the arterial and venous pressures to equilibrium in less than 7 s. Mean systemic pressure can only be measured when pressures throughout the systemic circulation have come to equilibrium after the heart has stopped beating. Guyton recognized the importance of determining the role of both mean systemic pressure and right atrial pressure in controlling venous return, and measuring both accurately proved to be very difficult. However, full understanding of the venous side has been challenging because of the complex nature of some of its characteristics. Because clinicians and investigators have long observed that factors affecting primarily the venous side of the circulation can have profound influence on cardiac output, mechanisms governing the flow of blood to the heart have been studied in some depth. Venous return refers to the flow of blood from the periphery back to the right atrium, and except for periods of a few seconds, it is equal to cardiac output.
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