Arteries, Veins & Capillaries: Structures & Functions
Blood vessels structure & function table
| Structure | Explanation | |
| Muscular artery | Thicker tunica media mainly composed of smooth muscle | Able to perform vasoconstriction and vasodilation |
| Much less elastin than elastic arteries | Cannot stretch and recoil | |
| Narrow lumen | Blood flows under high pressure | |
| Elastic artery | Thinner tunica media (compared to muscular artery) mainly composed of elastin and collagen | Gives artery the ability to stretch in response to each pulse |
| Relatively few smooth muscle fibres | Cannot perform vasoconstriction or vasodilation | |
| Narrow lumen | Blood flows under high pressure | |
| Vein | One-way valves | Close to prevent the backflow of blood |
| Wide lumen | Blood pressure is reduced in veins with no surges | |
| Less smooth muscle and elastin | No need for veins to stretch and recoil | |
| Lots of collagen | Increased strength and structure | |
| Capillary | Very small diameter | Blood travels relatively slowly, giving more opportunity for diffusion to occur |
| Capillaries branch between cells | Substances can diffuse quickly between cells and the blood quickly | |
| Thin walls, no elastic, smooth muscle or collagen | Capillaries can fit between individual cells and diffusion is rapid |
Worked example
The table below shows features of blood vessels.
Which row of the table is correct?
| Feature of elastic artery | Feature of muscular artery | Feature of vein | Feature of capillary | |
| A | One-way valves | Wide lumen | Narrow lumen | Very small diameter |
| B | Relatively few smooth muscle fibres | Tunica media composed of smooth muscle | Lots of collagen | No elastic, smooth muscle or collagen |
| C | Wide lumen | Lots of collagen | Little elastic | Tunica media composed of collagen |
| D | One-cell thick | Narrow lumen | Narrow lumen | Lots of elastic |
The correct answer is B.
This is because elastic arteries do not contain many smooth muscle fibres and are not able to perform vasoconstriction or vasodilation. In contrast, muscular arteries contain much more smooth muscle. Veins have large amounts of collagen for strength and keeping their shape, while capillaries are only one-cell thick and so do not contain any elastic, smooth muscle or collagen.
Examiner Tip
For “Explain” questions, remember to pair a description of a structural feature to an explanation of how it helps the blood vessel to function. For example, “Capillaries are one-cell thick, which enables quick and efficient diffusion of substances.”
