18.2: Structures of Blood Vessels

Structures of Blood Vessels

Blood vessels leaving the heart are large in size and gradually decrease in size until they become capillaries in the tissues where exchanges occur. Capillaries drain into larger blood vessels and these vessels gradually increase until they are large in size and connect with the heart. From the heart and then back again, blood vessels occur in the following order: 1. arteries, 2. arterioles, 3. capillaries, 4. venules, 5. veins. The word lumen is used to describe an internal space within an organ. As it applies to blood vessels, the lumen is the space inside a blood vessel where blood flows through the vessel.

Blood vessels are composed of three layers, although in veins and smaller blood vessels these may be so narrow to be indistinguishable. The exact structure of the blood vessel depends on its location and function. The three layers of blood vessels are (from inside to outside):

1. tunica intima - This is the innermost layer which is made of a single layer of endothelial cells (the endothelium) lined up in the direction of blood flow. These cells provide a smooth internal surface which is important in allowing the free flow of blood without clotting. The endothelial cells lie on a basement membrane which contains collagen and occasionally elastic fibers. In large blood vessels, these elastic fibers may be aggregated at the lower end of the intima to form the internal elastic lamina.
2. tunica media - This is the middle layer of the blood vessel consisting of smooth muscle collagen, and elastic fibers. It is particular prominent in arteries although less obvious in veins and non-existent in capillaries.
3. tunica adventitia - The outermost layer is composed largely of collagen. In arteries, the elastic fibers form a distinct layer between the tunica adventitia and tunica media known as the external elastic lamina. Within the adventitia of vessels with thick walls lie the vasa vasorum, small blood vessels which supply the walls of the vessel with the nutrients and waste removal the tissues of the vessel requires.

Above: The structures of blood vessels are modified depending on their type and location. These structural modifications reflect different functional properties of the vessels. Capillaries are shown as purple since this is where exchange of materials occurs in tissues and therefore they are between an oxygenated and deoxygenated state.

Arteries and Arterioles

The aorta is the largest artery in the body. In the tunica media of the aorta, there is a large amount of elastic tissue. This is essential for the aorta to stretch to accommodate the output of blood from the left ventricle and then to recoil to maintain an adequate diastolic pressure and generate a more continuous flow of blood. The aorta is therefore an example of an elastic artery.

The transition from an elastic artery to a muscular one is gradual, so transitional arteries may show features of both arterial types. The tunica media of these arteries have characteristics of a medium, muscular artery (inner portion of smooth muscle) and large, elastic artery (outer portion with elastic lamellae).

Muscular arteries, such as the femoral artery in the thigh, occur as the elastic arteries gradually merge into muscular arteries. The tunica media of the muscular arteries contains few elastic fibers and large amounts of smooth muscle. The tunica media has a layer of elastic tissue on its internal and external aspects; they are known as the internal elastic lamina and external elastic lamina, respectively.

Above: Cross sections of (left) a medium artery and (right) a large elastic artery, both at the same magnification of 40x.

Arterioles vary in diameter from 30 nm to 400 nm. The tunica media is composed of one or two layers of smooth muscle cells. These layers become fragmented as arterioles decrease in size. Arterioles make a big contribution to resistance to blood flow in tissues. This is important for maintaining blood pressure.