A2 is the initial and major component of S2. This system consists of arteries, arterioles, the capillary system, and the venous system. The arteries are strong, compliant, elastic walled vessels that carry blood away from the heart and distribute it to capillary beds throughout the body. This is a high-pressure circuit. The elastic fibers located within arterial wall stretch during systole and recoil during diastole.
Arterioles maintain blood pressure.
They contain smooth muscle and are innervated by the autonomic nervous system. This stimulation causes constriction of the vessels. Decreased adrenergic discharge dilates vessels, best controlling the lead distribution to various capillary beds. They comprise the major vessels controlling vascular resistance. The capillary system allows for exchange of oxygen, carbon dioxide and solutes between the blood and tissues as well as permitting fluid volume transfer between plasma and interstitial cells.