Blood vessels include arteries, capillaries, and veins which are responsible for transporting blood throughout the body.
The blood from the heart is carried through the body by a complex network of blood vessels. Arteries take blood away from the heart whilst veins return deoxygenated blood to the heart. Arteries are connected to veins through capillary beds where an exchange of metabolites occurs.
The structure of the different types of blood vessels reflects their function or layers. There are three distinct layers, or tunics, that form the walls of blood vessels. The inner, tunica intima is a smooth, inner lining of endothelial cells that are in contact with the red blood cells. Veins and arteries both have two tunics that surround the endothelium: the middle, tunica media is composed of smooth muscle, while the outer tunica externa is connective tissue (collagen and elastic fibers). The elastic, connective tissue stretches and supports the blood vessels, while the smooth muscle layer helps regulate blood flow by altering vascular resistance through vasoconstriction and vasodilation.
The arteries have thicker smooth muscle and connective tissue than the veins to accommodate the higher pressure and speed of freshly-pumped blood. The veins are thinner walled as the pressure and rate of flow are much lower. In addition, veins are structurally different from arteries in that veins have valves to prevent the backflow of blood. Because veins have to work against gravity to get blood back to the heart, contraction of skeletal muscle assists with the flow of blood back to the heart.
Practice Questions
Khan Academy
MCAT Official Prep (AAMC)
Biology Question Pack, Vol 2. Question 117
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Key Points
• Arteries carry blood away from the heart; the main artery is the aorta.
• Smaller arteries called arterioles diverge into capillary beds, which contain 10-100 capillaries that branch among the cells and tissues of the body.
• Veins are blood vessels that bring blood back to the heart and drain blood from organs and limbs.
• Veins and arteries have two more tunics that surround the endothelium: the middle tunica media is composed of smooth muscle that regulates blood flow, while the outer tunica externa is the connective tissue that supports blood vessels.
Key Terms
Vasodilation: dilation of the blood vessels
Vasoconstriction: constriction of a blood vessel
Venule: small vein, especially one that connects capillaries to a larger vein
Tunic: layers in the lining of blood vessels
Veins: blood vessels carrying deoxygenated blood back to the heart
Arteries: muscular blood vessels that carry oxygenated blood from the heart to the rest of the body
Aorta: the main artery in the body
Capillary beds: collections of narrow blood vessels where gaseous exchange is possible
Tunica intima: a smooth, inner lining of endothelial cells that are in contact with the red blood cells
Tunica media: composed of smooth muscle
Tunica external: consisting of connective tissue (collagen and elastic fibers)
