Blood vessels
| Capillaries |
|---|
| Very thin wall (one endothelial cell thick) |
| Very narrow lumen (allows only one red blood cell to pass at a time) |
| No valves |
Key Structural Differences:
- Arteries have thick walls to withstand high pressure and a narrow lumen to maintain that pressure.
- Veins have thin walls because blood pressure is low, but they possess valves to ensure one-way flow against gravity or low pressure gradients.
- Capillaries are microscopic vessels connecting arteries and veins. Their structure is optimized for exchange rather than transport.
State the function: The primary function of capillaries is to facilitate the exchange of materials between the blood and tissue cells.
Specifically, capillaries:
- Deliver oxygen and nutrients (e.g., glucose) from the blood to the tissues.
- Remove carbon dioxide and other waste products from the tissues into the blood.
| Blood Type |
|---|
| Deoxygenated |
| Oxygenated |
| Deoxygenated |
| Oxygenated |
| Oxygenated (and contains urea) |
| Deoxygenated (and contains less urea) |
| Oxygenated |
| Deoxygenated |
| Contains high concentration of glucose/amino acids |
Note on Naming:
- Arteries generally carry oxygenated blood, except the pulmonary artery.
- Veins generally carry deoxygenated blood, except the pulmonary vein and hepatic portal vein (which carries nutrient-rich blood from the gut).
Why are arteries structured this way?
Arteries transport blood away from the heart under high pressure generated by ventricular contraction.
- Thick, muscular walls: Withstand the high pressure without bursting.
- Elastic fibres: Stretch during systole (heart contraction) and recoil during diastole (heart relaxation), helping to maintain blood pressure and smooth out flow.
- Narrow lumen: Helps maintain high pressure within the vessel.
Why are veins structured this way?
Veins transport blood back to the heart under low pressure.
- Thin walls: Do not need to withstand high pressure.
- Wide lumen: Offers low resistance to flow, allowing blood to return easily despite low pressure.
- Valves: Prevent backflow of blood. This is crucial because veins often carry blood against gravity (e.g., from legs to heart) and rely on muscle contractions to push blood forward; valves ensure it does not flow backward.
| Functional Advantage |
|---|
| Creates a short diffusion distance for substances to pass between blood and tissues. |
| Allows plasma and small solutes to leak out and form tissue fluid, facilitating exchange. |
| Forces red blood cells close to the vessel wall, reducing diffusion distance for oxygen. |
| Provides a large surface area for efficient exchange of materials. |
Correction: Arteries have a narrow lumen relative to their wall thickness. This narrowness helps maintain the high pressure required to pump blood to the body. Veins have a wide lumen because pressure is low and they need minimal resistance.
Correction: Remember the exceptions: The pulmonary artery carries deoxygenated blood to the lungs, and the pulmonary vein carries oxygenated blood to the heart.
Why examiners accept this: Examiners look for a direct link between a specific structural feature and its physiological advantage. Simply listing features is insufficient; you must explain the 'why'.
Correct Usage Example:
Incorrect: "Arteries have thick walls."
Correct: "Arteries have thick, muscular walls to withstand the high pressure of blood pumped from the heart without bursting."
Correct: "Veins have valves to prevent backflow of blood as it moves under low pressure towards the heart."
- Thicker wall (muscular/elastic layer). 2. Narrower lumen. (Or: Absence of valves).