Gas exchange

• The volume of oxygen that has to be absorbed and the volume of gases that has to be removed is high in mammals because:

  They are relatively large and have a large number of cells.

  They maintain a high body temperature which is related to them having high metabolic and respiratory rates.
• Lungs in mammals are specialised surfaces which enable efficient gas exchange between air and blood.
• Lungs are located inside of the body because:

  Air is not dense enough to support the lungs.

  If the lungs were outside the body, the body would lose large amounts of water and dry out.
• To maintain a concentration difference across the alveoli, air is constantly being moved in to and out of the lungs. This process is called ventilation.
• Inspiration:

  Breathing in is an active process.

  1.       The external intercostal muscles contract and the internal intercostal muscles relax.

  2.       The ribs are pulled up and out, increasing the volume of the thorax.

  3.       The diaphragm muscles contract and flatten – also increasing the volume of the thorax.

  4.       The increased volume of the thorax results in reduced pressure in the lungs.
• Expiration:

  Largely passive process

  1.       The internal muscles contract, while the external intercostal muscles relax.

  2.       The ribs move downwards and inwards decreasing the volume of the thorax.

  3.       The diaphragm muscles relax, and it is pushed up again by the contents of the abdomen.

  4.       The decreased volume of the thorax increases the pressure in the lungs.

  5.       The pressure of the lungs is greater than that of the atmosphere, so air is forced out.
• Pulmonary ventilation rate is the total volume of air that is moved into the lungs in one minute.
• Pulmonary ventilation rate = Tidal volume x Breathing rate
• Tidal Volume (TV) - amount of air breathed in or out during inspiration/expiration
• Breathing Rate (BR) - number of breaths per minute
• The average tidal volume is about 0.5 litres
• The average breathing rate at rest is around 16 breaths per minute
• At rest, pulmonary ventilation rate is approximately 8 litres per minute
• Increase in exercise intensity leads to an increase in respiratory rate
• Alveoli are tiny, thin-walled sacs where gas exchange occurs between blood and alveolar air
• Capillaries are small vessels with very thin walls which allow gases to diffuse easily across them
• Capillaries surround the alveoli and allow diffusion of oxygen from alveolar air into capillary blood.
• The site of gas exchange in mammals is the lining of the alveoli.
• These alveoli are ideally suited for exchange because:

  -          The surface is thin.

  -          There is constant movement of air and blood.
• Diffusion of gases between alveoli and the surrounding blood is rapid. This is because:

  -          Blood vessels are slowed as they pass through the pulmonary capillaries.

  -          The distance between the air in the alveoli and the red blood cells is reduced because the red blood cells are flattened against the capillary walls.

  -          The walls of the alveoli and capillaries are very thin.

  -          The alveoli and the pulmonary capillaries have a very large total surface area.
• Breathing movements constantly ventilate the lungs.
• Blood flow throughout the capillaries maintains a concentration gradient.
• Risk factors for lung disease

  • Smoking
  • Air pollution
  • Genetics
  • Infections
  • Occupation