ANATOMY

Detailed explanation-1: -Reflex venoconstriction is defined as a neurogenically induced contraction of the smooth muscle in the walls of veins; it causes a reduction in vascular capacity which is seen as a decrease in the volume of blood contained at the same pressure, an increase in the pressure at the same volume, or a combination of both.

Detailed explanation-2: -Norepinephrine generally causes vasoconstriction and increases blood pressure, with variable effects on heart rate; cardiac output may increase, 16, 19-24 decrease, 17, 24 or remain unchanged.

Detailed explanation-3: -Constriction of the veins decreases their capacity and expels blood from them into the systemic circulation. However, venoconstriction may increase venous resistance and subsequently decrease VR and CO.

Detailed explanation-4: -Thus, venoconstriction increases the return of blood to the heart. Another way of stating this is that venoconstriction increases the preload or stretch of the cardiac muscle and increases contraction.