What you need to know
- Compare the structures and functions of arteries, veins and capillaries.
- Link pressure, wall thickness, lumen size and valves to blood flow.
- Explain exchange of substances at capillary beds.
- Describe the formation and return of tissue fluid at an appropriate level.
- Apply vessel adaptations to unfamiliar examples.
Comparing blood vessels
| Feature | Artery | Vein | Capillary |
|---|---|---|---|
| Direction | Away from heart | Towards heart | Between arterioles and venules |
| Pressure | High and pulsatile | Low | Lower than arteries |
| Wall | Thick, muscular and elastic | Thinner, less muscular and elastic | One cell thick |
| Lumen | Relatively narrow | Wide | Very narrow |
| Valves | Not along normal arteries | Present in many veins | Absent |
Misconception AlertArteries always carry oxygenated blood.Select to reveal the correctionSelect to hide the correction
Arteries are defined as vessels that carry blood away from the heart. The pulmonary artery carries deoxygenated blood.
Arteries: high-pressure delivery
Thick walls withstand high pressure generated by ventricular contraction. Elastic fibres stretch as blood is forced in and recoil between beats, helping maintain flow. Smooth muscle changes vessel diameter and distributes blood between organs. A relatively small lumen helps maintain pressure.
Veins: low-pressure return
Blood has lost much of its pressure after passing through capillaries. Veins therefore have thinner walls and wide lumens that reduce resistance. Pocket valves prevent backflow, especially in limbs. Contraction of nearby skeletal muscles squeezes veins and helps move blood towards the heart.
A structure that allows flow in one direction and closes when pressure would otherwise reverse the flow.
Capillaries: exchange with tissues
Capillaries form branching networks close to cells. Their walls consist of a single layer of flattened endothelial cells, giving a short diffusion path. Their narrow lumen slows red blood cells and keeps them close to the wall. Extensive networks create a large total area for exchange.
| Usually moves from blood to cells | Usually moves from cells to blood |
|---|---|
| Oxygen, glucose, amino acids, ions and some hormones | Carbon dioxide, urea and other metabolic products |
Net direction depends on concentration gradients and the needs of the tissue.
Tissue fluid formation and return
At the arterial end of a capillary bed, blood hydrostatic pressure forces some plasma out through small gaps in the capillary wall. Blood cells and most large plasma proteins remain inside. The resulting tissue fluid surrounds cells and provides the medium through which substances diffuse.
As blood moves along the capillary, pressure falls. Much of the fluid returns because plasma proteins help draw water back into capillaries. Excess enters lymph vessels and is eventually returned to the blood.
Tissue fluid is formed from plasma, but it is not identical to plasma because cells and most large proteins remain in the blood.
Quick retrieval check
1. Why are artery walls thick and elastic?
2. How do valves help venous return?
3. Give two adaptations of capillaries for exchange.
4. What happens to excess tissue fluid?
Exam connection
A leg vein valve is damaged. Explain why fluid may accumulate in the lower leg.
Show the mark points
- Damaged valves allow more backflow.
- Venous pressure can rise below the damaged valve.
- Greater pressure promotes movement of fluid from capillaries into tissues.
- Return through capillaries and lymph may not remove it as quickly, causing swelling.