When oxygen level decreases, dilatation of the blood vessels occurs. This increases blood flow. With autonomic regulation of the vessels, adrenergic sympathetic nervous system regulation secretes norepinephrine at the nerve endings, which produces vasoconstriction. Arterioles help to regulate blood flow and arterial pressure. Veins help to regulate the amount of blood flow.
Vasoconstriction causes the increase in venous return to the heart.
The parasympathetic nervous system secretes acetycholine at the nerve ending producing vasodilation. Stretch receptors or baroreceptors are located at the aortic arch, carotid sinus, vena cava, pulmonary arteries, and atria. They are activated by elevated blood pressure usually greater than 60 millimeters hg. or blood volume. They respond to the stretch of arterial walls. Activation results in inhibitions of sympathetic action.