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When it comes to AS/A-level Biology, can you distinguish arteries from veins?
Arteries
- Carry blood away from heart.
- They have thick and multi-layered walls.
- Walls are capable of stretching to accommodate the ‘pulse’ of blood.
- They are capable of expanding and contracting to change and maintain blood pressure.
- They do not contain valves.
- Blood inside is high in oxygen and low in carbon dioxide except in pulmonary artery.
- Blood inside is bright red and is under high pressure.
- E.g. coronary (heart), brachial (arms), carotid (head), femoral (legs), renal (kidney)

Arterioles
- Tiny branches off the arteries.
- They have thinner and less muscular walls.
- They cannot be seen with unaided eye.
- They feed blood into the capillaries.
- They are narrower than arteries but wider than capillaries.

Capillaries
- The flow and pressure of blood is controlled by sphincters (outside of capillaries).
- They are microscopic vessels (8 um).
- Erythrocytes pass through in single file.
- Walls only one cell thick so as to facilitate diffusion.
- Thinness allows for easy diffusion outward or inwards through single cell layer.
- They form capillary beds, i.e., network of vessels linking arterial and venous blood.
- Oxygen, nutrients and other materials move out of the capillaries and into the extra-cellular fluid and then into cells.
- Carbon dioxide, wastes and other materials are picked up and move into the capillaries (both involve diffusion).

Venules
- Vessels larger than capillaries but smaller (narrower and thinner walls) than veins.

Veins
- They are larger inside diameter compared to arteries.
- They take blood towards heart.
- They have thinner, less muscular walls than arteries, but still 3 layers.
- No stretching or contracting of walls except by external valves.
- They contain valves to help return the blood to the heart.
- Blood moves smoothly.
- They are low in oxygen except pulmonary circulation.
- Blood inside is dull red and is under low pressure.
- E.g. cardiac (heart), renal (kidney)

Varicose Veins
- When valves do not function properly, blood leaks backwards and pools in veins.
- Veins sag, stretch and swell, creating bulging knarled knees.
Differences between Arteries and Veins

Do you know the difference between an open system and a closed system in AS/A-level Biology?
Open System
- Blood not always inside vessels.
- Tissues directly in contact with blood.
- Heart is just under dorsal surface.
- E.g. insects
- Blood enters via ostia.
- Heart pumps towards head via peristalsis.
Closed System
- Blood stays entirely in vessels.
- Tissue fluid leaves vessels to bathe tissues and cells.
- Blood can be pumped at higher pressure so higher flow = waste can be removed quicker and the supply is quicker.
- E.g. fish
- Heart 👉🏻 arteries 👉🏻 gills 👉🏻 veins 👉🏻 body tissues 👉🏻 veins 👉🏻 heart

References:
1. https://teachmeanatomy.info/the-basics/ultrastructure/blood-vessels/
That's the end of the topic!

Drafted by Bonnie (Biology)