Venomotor tone and central venous pressure.
A discussion concerned primarily with responses of capacitance vessels to physiologic stimuli and with the role of these vessels in the pathophysiology of some clinical conditions is presented. The venous system functions as a dynamic reservoir from which blood is pumped by the heart. The systemic veins contain approximately 75% of the circulating blood volume. Pressure at the central end of the venous reservoir (central venous pressure) is an important factor determining cardiac filling and cardiac output. Central venous pressure, in turn, is determined by the balance between the amount of blood in the veins and the capacity of the venous system. Active changes of venous smooth muscle tone can alter both venous capacity and blood volume. Increases in tone of large veins or capacitance vessels tend to decrease venous capacity and increases venous pressure. Decreases in tone tend to increase venous capacity and lower pressure. The large veins provide little resistance to blood flow, but the smaller veins or postcapillary resistence vessels influence capillary hydrostatic pressure, capillary filtration, and blood volume. Increase in tone of postcapillary resistance vessels tend to increase capillary pressure and filtration and decrease blood volume. Decrease in tone in these vessels tend to produce opposite effects. There appears to be a generalized increase in venous distensibility associated with pregnancy. Several studies have demonstrated decreased venous tone and decreased velocity of blood flow through veins during pregnancy. It is proposed that this might be caused be a relaxing effect of progesterone on venous smooth muscle. Oral contraceptives also cause increased venous distensibility and decreased velocity of venous blood flow. These changes appear to be related more to progesterone and progestins than to estrogens. It is emphasized that the effects of drugs, reflexes, and pathologic states on venomotor tone must be considered in the management of patients with cardiovascular diseases.