In this topic of IBDP Biology, we will learn about circulatory system.

•     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.

• 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

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.

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