Structure-Function Relationships in the Cardiovascular System

Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. The venous circulation is a low-pressure system with much lower amounts of smooth muscle and elastic tissue, thinner walls, and larger lumens than arteries. Veins are capacitance vessels with significant compliance and the ability to distend and hold up to 70%–80% of blood volume at rest.

Capillaries are the primary structures in the circulatory system that allow the exchange of gas, nutrients, and other materials between the blood and the extracellular fluid (ECF). Capillaries are the smallest of the blood vessels. Because a capillary diameter is so small, only 1 RBC may pass through at a time. Capillaries are organized into capillary beds, which are extensive networks of branches and anastomoses. Blood flows from the metarterioles, into the capillaries, out the thoroughfare channel, and into venules. Continuous, fenestrated, and sinusoid (discontinuous) capillaries are the 3 primary types, and each has a slightly different structure. Capillary dysfunction can occur either as a result of or a contribution to the clinical manifestation of many clinical conditions.

Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Each of these types of arteries contain 3 primary layers: the tunica intima, tunica media, and tunica adventitia. In particular, the tunica media contains smooth muscle cells (allowing for vasoconstriction), and in the larger vessels, a significant amount of elastin (allowing these vessels to stretch and recoil in response to changes in pressure during systole and diastole).