Morphology & Characteristics of Blood Vessels

Blood vessels are an intricate part of the circulatory system, and are the "tubes" through which blood flows throughout the body. While blood vessels are do not actively push blood through the system, they are involved in every medical condition, including cancer, inflammation and cardiovascular disease. There are several types of vessels, each designed for a special purpose in various parts of the body. All blood vessels are not the same in the thickness of their walls, the diameters of their center openings or their elasticity.

Types

Arteries are the largest types of vessels. As they branch away from the heart, they become smaller arterioles, and finally the tiniest of all vessels, capillaries. The capillaries then connect to venules, then to veins that return blood to the heart and lungs, and then back to the arteries.

Structure

Both arteries and veins have a similar structure. The center opening through which blood flows in veins and arteries is called the "lumen." Surrounding the lumen is the innermost layer called the "tunica intima," which is the thinnest layer and is made up of simple squamous endothelial cells. Also included in this thin layer are tiny, circular, elastic bands called "internal elastic lamina."
The second layer, called the "tunica intermedia," is the thickest layer, and is made up of more circular elastic bands, the "external elastic lamina." In some arteries, the tunica intermedia contains vascular smooth muscle which helps regulate the size of the lumen.
The third and outermost layer is the "tunica adventitia," made up of connective tissue.

Capillaries

Capillaries, the smallest of all the blood vessels, have walls that are only one cell thick. While these vessels are tiny, they do more than simply transport blood. The thin walls of capillaries allow oxygen and nutrients that come through the arterioles to cross through capillary walls to the cells of the body. They also allow waste and carbon dioxide to move from the body's cells through the capillary walls and into the blood that is returning to the heart via the venules and veins. In addition, the capillaries allow salts, minerals, water and other substances to cross in and out of the capillaries to maintain a proper balance in the body.

Function

Arteries transport blood away from the heart to the body, and supply the body with oxygen-rich blood. The exception to this rule is the pulmonary artery which takes blood from the heart to the lungs. Veins take oxygen-depleted blood from the body back to the heart. The exception to this rule is the pulmonary vein, which carries blood from the lungs to the heart and from there to the arteries.

Differences Between Veins and Arteries

While veins and arteries both have a similar three-layer structure, arteries contain smooth muscle that is regulated by the sympathetic nervous system. When necessary, the sympathetic nervous system signals arteries to relax or contract, which changes the size of the lumen and thereby regulates blood flow and also affects blood pressure. Although veins have larger lumens, lack muscle tissue and are less elastic than arteries, veins have structures that are not found in arteries: valves. After the heart sends a full "pump" of blood through the arteries, it floods down into the arterioles and washes through the capillaries. Since the capillaries are so small, blood must pass through them once cell at a time, and this slows the flow. As the blood flows out of the tiny capillaries and into the larger venules and finally into the big veins, the pressure is less than what it was in the arteries. Since the blood flow is more sluggish, it tends to flow backward in the veins between heart beats. The valves in the veins allow the blood to flow into the veins, but then quickly close until the next heart beat forces another wave of blood into the veins, opening the valves again.

Resources

• Blood Vessels--The Human Heart
• Blood Vessel Color Images
• Structure and Function of Blood Vessels