Factors Regulating Arterial Blood Pressure
Mean arterial pressure is regulated by changes in cardiac output and systemic vascular resistance. The following scheme summarizes the factors that regulate cardiac output and systemic vascular resistance.
Cardiac output is determined by the product of stroke volume and heart rate. Stroke volume is determined by inotropy and ventricular preload. (The effects of afterload on stroke volume are not shown in this figure.) Ventricular preload is altered by changes in venous compliance and blood volume. A decrease in venous compliance, as occurs when the veins constrict, increases ventricular preload by increasing central venous pressure. Total blood volume is regulated by renal function, particularly renal handling of sodium and water. Blood volume shifts within the body (not shown in figure) as occurs when changing body posture, also change central venous pressure and preload. Heart rate, inotropy, venous compliance, and renal function are all strongly influenced by neurohumoral mechanisms.
Systemic vascular resistance is determined by the anatomy of the vascular network (series versus parallel resistance elements). Generally, vascular structure remains relatively unchanged; however, pathological conditions (e.g., vascular thrombosis) can affect the number of perfused blood vessels. Furthermore, changes can occur in the relative number of parallel and series resistance elements. In hypertension, there is evidence that rarefaction occurs - that is, a decrease in the anatomical number arterioles and capillaries.
The most important mechanism for changing systemic vascular resistance involves changes in vessel lumen diameter. The Poiseuille relationship shows that resistance is inversely related to the fourth power of the vessel radius. In chronic hypertension, vessel radius is often reduced due to a thickening of the vessel wall - this leads to a reduction in lumen size. Vascular factors such as nitric oxide, endothelin, and prostacyclin have important influences on vessel diameter. Furthermore, myogenic mechanisms intrinsic to the vascular smooth muscle also can alter vessel diameter. Tissue factors (e.g., adenosine, potassium ion, hydrogen ion, histamine) are chemicals released by parenchymal cells surrounding blood vessels and can significantly alter vessel diameter. In general, tissue factors are more concerned with regulating organ blood flow than systemic arterial pressure; however, any change in vessel tone will affect both organ blood flow and systemic arterial pressure. Finally, neurohumoral mechanisms play a very important role in regulating systemic vascular resistance and arterial pressure, particularly in certain forms of secondary hypertension. Neurohumoral mechanisms are regulated principally by arterial baroreceptors and to a lesser extent by chemoreceptors. Many of the therapies used for reducing arterial pressure involve inhibiting the action of neurohumoral mechanisms.