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In this topic of IBDP Biology, we will learn about circulatory system.
Blood vessels
- Three layers surrounding a central blood carrying canal (the lumen).
- The outer layer.
- Composed mainly of collagen – tough.
- Smooth muscle cells may be present, particularly in veins.
- Often the most prominent layer in the walls of veins.
- The middle layer.
- Composed mainly of smooth muscle.
- Reinforced by layers of elastic tissue.
- Inner lining:
- A lining layer of flattened epithelial cells termed endothelium.
- Sits on a basement membrane.
- The only layer that is present in all blood vessels

- Arteries:
- When it comes to IBDP Biology, arteries carry blood away from the heart
- Relatively high pressure.
- Vary in diameter.
- Thick muscle to maintain high blood pressure.
- Elastic to even out the blood pressure.
- Endothelium is very smooth to recue resistance.
- Arterioles:
- Less than 0.3mm thick
- Small arteries.
- Less elastic tissue.
- Middle layer consists almost entirely of smooth muscle:
- Constricts to reduce blood flow by narrowing lumen.
- Dilates to increase blood flow by enlarging lumen.
- Veins:
- Small veins = venules.
- Carry blood back to the heart.
- Very little elastic and muscle tissue.
- Wider lumen than arteries.
- Relatively low pressure.
- Contraction of skeletal muscle squeeszes veins, enabling movement of blood
- Contain valves:
- Delicate projections of the endothelium.
- Mainly in limbs.
- Ensure blood flows towards the heart.
Tissue fluid & Capillaries
- When it comes to IBDP Biology, capillaries provide the link between the arterial and venous blood vessels.
- Diameter approx 7micrometers.
- This enables them to fit between cells in tissues.
- It also ensures that red blood cells need to squeeze through:
- This increases their surface areas in contact with the endothelium.
- They are highly branched.
- Wall of single layer of flattened endothelial cells.
- Quite loose fitting with fenestrations (pores).
- Held together by basement membrane.
- Water and small solutes can pass through creating tissue fluid.
- Capillaries allow efficient exchange between blood and tissue fluid.
- Its composition remains fairly constant because it is constantly formed and removed.
- All cells must be bathed in fluid to survive
- Proteins remain in blood
- Tissue fluid provides respiring cells with:
- Water
- Oxygen
- Glucose
- Amino acids
- Fatty acids
- Hormones
- Salts
- Vitamins
- Tissue fluid enables various substances to move back into the blood:
- Water
- Carbon dioxide
- Urea
- Lactic acid
Exchange of materials at the capillary bed
Arteriole end
- As the blood enters the capillary, it is under high hydrostatic pressure
- There is opposed by the osmotic effect of the plasma proteins and the hydrostatic pressure of the tissue fluid
- Capillary hydrostatic pressure is much greater.
- There is a net outwards force
- This forces water and solutes out of the capillaries
Venule end
- When it comes to IBDP Biology, as blood passes through the capillary the hydrostatic pressure drops.
- Hydrostatic pressure of tissue fluid increases.
- Fluid returns into the capillaries due to this pressure gradient.
- Concentration of plasma proteins increases due to reduction in water content.
- This creates a water potential gradient from tissue fluid to blood
- Water is also drawn in by osmosis

This is the end of this topic.
