Blood Vessels

Blood vessels form a tubular network that allows blood to travels from the heart to the tissues and back to the heart again. Blood that leaves the heart passes into arteries. Large arteries branch into progressively smaller arteries that function to deliver blood to various regions of the body. Small arteries branch into even smaller vessels called arterioles, which function to regulate the flow of blood into different tissues. Arterioles branch into capillaries, the smallest of all blood vessels. Capillaries are the sites of nutrient and waste exchange between the blood and body cells. Capillaries are microscopic vessels that join the arterial system with the venous system. Blood coming out of the capillaries passes into vessels of increasing diameter as it flows back toward the heart. Capillaries join to form venules, which then merge to form small veins. Small veins unite to form large veins that eventually deliver blood back to the heart.

Arteries

Arteries serve as (1) efficient conduits for the movement of blood and (2) pressure reservoirs that keep blood moving during diastole. Arteries have a large internal diameter and thus offer little resistance to the flow of blood. Arteries also contain an elastic layer in their walls. Elastin is a protein fiber that has elastic qualities. During systole, large arteries distend with blood as their elastic walls stretch. During diastole, the walls rebound, thus pushing blood along. In this way the arteries act as a pressure reservoir that maintains a constant flow of blood through the capillaries despite pressure fluctuation during the cardiac cycle. Arteries also have a smooth muscular layer that functions to regulate the flow of blood through the artery. Contraction of the smooth muscle decreases the internal diameter of the vessel in a process called vasoconstriction. Relaxation of the smooth muscle increases the intermnal diameter in a process called vasodilation.

Arterioles
Arterioles serve as (1) the major determinant of blood pressure and (2) as the major determinant of blood flow to the individual organs Arterioles have a much smaller diameter than arteries and thus provide significant resistance to the flow of blood. This resistance creates pressure in the circulatory system. Pressure is required to provide adequate flow of blood to all parts of the body. Blood flow to individual organs can be regulated by controlling the diameter of the arterioles. Vasodilation of an arteriole lowers resistance and results in an increase in flow through that particular arteriole. Vasoconstriction of an arteriole increases resistance and results in decreased flow through that particular arteriole.

Capillaries

Capillaries are the smallest and most numerous of blood vessels. Capillaries function as the site of exchange of nutrients and wastes between blood and tissues. The anatomy of capillaries is well suited to the task of efficient exchange. Capillary walls are composed of a single layer of epithelial cells surrounded by a basement layer of connective tissue. The thin nature of the walls facilitates efficient diffusion of oxygen and carbon dioxide. Most capillaries also have pores between cells that allow for bulk transport of fluid and dissolved substances from the blood into the tissues and visa versa.
Although capillaries are extremely numerous (40 billion in the body), collectively they hold only about 5% of the total blood volume at any one time. This is because most capillaries are closed most of the time. Precapillary sphincters, which are bands of smooth muscle that wrap around arterioles, control the amount of blood flowing in a particular capillary bed. Contraction of the sphincter shuts off blood flow to a capillary bed, while relaxation of the sphincter allows blood to flow.

Veins

Veins are larger and more compliant (stretchable) than arteries, thus they can hold more blood. In fact, the veins act somewhat like a blood reservoir, containing 60% of the total blood volume at rest. As physical activity increases, the veins undergo vasoconstriction, driving more blood back to the heart and increasing circulation. Also, the return of venous blood to the heart is aided by one-way valves that insure unidirectional flow of blood