Blood Pressure Homeostasis

The circulatory system is a closed-loop system in which cardiac output is dependent upon adequate venous return. Distribution of this output to the various organ systems in amounts appropriate to their needs is accom­ plished by constriction or relaxation of vascular smooth muscle. Within this closed system, a pressure gradient is discerned, with pressure greatest in the aorta and lowest in the vena cavae. The flow of blood through the anatomic complexity of the circulatory system can be described rather simply using hydraulic principles. Several basic interrelationships apply among flow, pressure, and resistance. Flow is a measure of volume per unit of time; blood pressure is a measure of the force exerted by blood per unit area of vessel wall; and resistence to blood flow is the impediment to flow and is due, among other things, to function loss. These three factors are related by the following equation: