Exchange Surfaces (SQA National 5 Biology): Revision Note

The need for exchange

• For the body to function, cells need a constant supply of oxygen and nutrients for respiration, which releases energy in the form of ATP

  • Oxygen is absorbed into the bloodstream in the lungs, where it diffuses from the alveoli into the blood

  • Nutrients, such as glucose and amino acids, are absorbed from digested food in the small intestine and enter the bloodstream through the villi

Delivery to the cells

• Once key nutrients and oxygen have been absorbed into the bloodstream, they must then be delivered to the respiring cells

• The heart pumps oxygenated, nutrient-rich blood via arteries, arterioles and capillaries

  • Oxygen is carried by red blood cells

  • Nutrients are carried in the blood plasma

• At the capillaries in tissues, substances move between blood and cells, mostly by diffusion

Removing waste

• Waste materials, such as carbon dioxide from respiration, diffuse from cells into capillaries

  • Other waste substances include urea, excess ions and excess water

• Once in the blood, carbon dioxide is mostly carried dissolved in plasma

• At the lungs, carbon dioxide diffuses from the blood into the alveoli and is exhaled during breathing

  • Ventilation maintains a low concentration of carbon dioxide in the alveoli to ensure that diffusion continues

Capillary networks

• Capillary networks surround tissues, so substances can move between the blood and cells efficiently

  • Substances that move into the cells include:

    • oxygen

    • glucose

    • amino acids

    • water

    • minerals

  • Substances that move out of the cells include:

    • carbon dioxide

    • urea

    • some water

• Capillary networks are adapted to maximise the efficiency of exchange because they have:

  • very thin walls: the capillary endothelium is one cell thick endothelium, which provides a short diffusion distance

  • a narrow lumen: the capillary lumen is narrower than the diameter of a red blood cell, slowing blood flow and giving more time for exchange

  • a large total surface area: there are many capillaries, so that the total surface area of the capillary network is very large, allowing for fast exchange of materials