AS/A-level Biology - Circulatory System

When it comes to AS/A-level Biology, can you distinguish arteries from veins?

• 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)

• 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.

• 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).

• Vessels larger than capillaries but smaller (narrower and thinner walls) than 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)

• When valves do not function properly, blood leaks backwards and pools in veins.
• Veins sag, stretch and swell, creating bulging knarled knees.

Do you know the difference between an open system and a closed system in AS/A-level Biology?

• 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.

• 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!

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