7.6 Blood Vessels and their Function

• Arteries carry blood away from the heart and into the arterioles.
• Arterioles are smaller arteries that control blood flow from arteries to capillaries.
• Capillaries are tiny vessels that link arterioles back to veins.
• Veins carry blood from capillaries back to the heart.
• Arteries, arterioles and veins all have the same basic layered structure. This consists of a tough fibrous outer layer (to resist pressure changes), a muscle layer (to contract and control blood flow), an elastic layer (can recoil to help maintain blood pressure), endothelium (thin, smooth inner lining to reduce friction and allow diffusion) and lumen (central cavity for blood to flow through).
• The proportions of each layer in each blood vessel varies depending on their role in the circulatory system.
• The function of arteries is to transport blood rapidly under very high pressure from the heart to the tissues.
• Arteries have a thick muscular layer compared to veins. This is that smaller arteries can be constricted and dilated in order to control the volume of blood passing through them.
• Arteries have a thick elastic layer compared to veins. This is because blood pressure in the arteries is kept high to reach extremities of the body. The elastic wall is stretched at the end of each beat of the heart (systole). It then springs back when the heart relaxes (diastole), and this recoil action helps maintain a high pressure and smooth pressure surges.
• Arteries have a thick wall overall, which also helps resist the vessel bursting under pressure.
• Arteries have no valves (besides arteries leaving the heart) because blood is under constant high pressure, so blood can’t flow backwards.
• The function of arterioles is to carry blood under lower pressure than the arteries, from the arteries to the capillaries.
• Arterioles have a muscle layer that is relatively thicker than in the arteries. The contraction of this layer allows constriction of the lumen of the arteriole. This restricts the flow of blood and so controls its movement into the capillaries that supply the tissues with blood.
• Arterioles have a relatively thinner elastic layer than in the arteries. This is because the blood pressure is lower.
• The function of veins is to transport blood under low pressure from the capillaries in tissues to the heart.
• Veins have a thin muscle layer compared to arteries, since veins carry blood away from tissues and therefore their constriction and dilation cannot control the flow of blood to the tissues.
• Veins have a thin elastic layer compared to arteries because the low pressure of blood within the veins will not cause them to burst and the pressure is too low to produce a recoil action.
• Overall, veins have walls with a low thickness. This is because a thick wall isn’t needed, since low blood pressure doesn’t pose the risk of bursting the vessel. Furthermore, thinner walls allow veins to be flattened easily to aid blood flow.
• Veins contain valves throughout to ensure that blood doesn’t flow backwards due to low pressure. When body muscles contract, veins are compressed, pressurising the blood within them. The valves ensure that this pressure directs the blood in one direction only - towards the heart.
• The function of capillaries is to exchange metabolic materials such as oxygen, carbon dioxide and glucose between the blood and cells of the body. Blood flow in the capillaries is slow.
• Capillary walls consist mostly of the lining layer, making them extremely thin. This provides a short diffusion pathway and allows for rapid diffusion of metabolic materials between the blood and cells.
• Capillaries are numerous and highly branched, providing a large surface area for exchange.
• Capillaries have a narrow diameter to permeate tissues, meaning that all cells are close enough to a capillary and there is a short diffusion pathway.
• Capillaries have a narrow lumen so that red blood cells are squeezed flat against the walls. This brings the red blood cells closer to the cells for supplying oxygen, which further reduces diffusion distance.
• Capillaries contain spaces between the lining (endothelial) cells that allow white blood cells to escape in order to deal with infections within tissues.
• Capillaries are fairly small, however they cannot serve every single cell directly. Therefore, tissue fluid is formed from metabolic materials to bathe the cells.