Biology 13.6 Blood Vessels and their Functions

Job of blood vessels = allow rapid transport of blood to control flow so blood is confined to vessels.

Structure of blood vessels

• Arteries = carry blood away from heart into arterioles.
• Arterioles = smaller arteries, control blood flow from arteries to capillaries.
• Capillaries = tiny vessels, link arterioles to veins.
• Veins = carry blood from capillaries back to heart.

All these have the same basic structure:

• Tough outer layer = resists pressure changes from within and outside.
• Muscle layer = can contract so control the flow of blood.
• Elastic layer = stretches and springs back to help maintain blood pressure.
• Thin inner lining (endothelium) = smooth to prevent friction, thin diffusion pathway.
• Lumen = central cavity to allow blood through.

Relative proportions of each layer differ in each type of blood vessel.

Arterioles differ from arteries by being smaller and having a relatively larger muscle layer.

NB: Arteries, arterioles and veins = transport not exchange (only capillaries carry out exchange).

NB: Elastic tissue stretches and recoils, not contracts and relaxes.

Artery structure related to function

Function of arteries = transport blood rapidly under high pressure from heart to tissues.

• Muscle layer is thick compared to veins = smaller arteries constrict and dilate to control volume of blood through them.
• Elastic layer is relatively thick compared to veins = important that blood pressure in arteries is high so blood can reach extremities. Elastic wall stretches at systole (contraction of heart) and springs back at diastole (relaxation of heart) - helps maintain high and smooth pressure surges created by heartbeat.
• Overall thickness of wall is large = resists bursting under pressure.
• No valves (except in arteries leaving heart) = high pressure prevents backflow.

Arteriole structure related to function

Function of arterioles = carry blood (under lower pressure) from arteries to capillaries and control flow of blood between the two.

• Muscle layer is thicker than in arteries = contraction allows constriction of lumen - restricts blood flow, controls movement into capillaries that supply tissue with blood.
• Elastic layer thinner than in arteries = blood pressure is lower.

Vein structure related to function

Function of veins = to transport blood slowly (under low pressure) from tissues to heart.

• Muscle layer is relatively thin = carry blood away from tissues so constriction and…