A level Biology exam revision resources written by A level Examiners





 

The ventilation of the lungs is the process of breathing. At rest humans breathe 10-14 times per minute and the tidal volume is approximately 500ml. The process of breathing has two parts to it, breathing in (inspiration) and breathing out (expiration).

The process of inspiration is brought about by the following sequence of events:

  • The diaphragm muscles contract and the diaphragm flattens and moves down.
  • The intercostal muscles contract, which makes the ribs move up and out.
  • Both these activities cause the volume of the thorax to increase. As the volume increases the pressure decreases below atmospheric pressure.
  • Atmospheric air (rich in oxygen) flows into large air passages i.e.
    TracheaBronchi → largest Bronchioles down a pressure gradient.
  • Final pathway - oxygen diffuses into alveoli along the concentration gradient. In the alveoli, oxygen dissolves into a film of liquid, which then diffuses the short distance into the blood capillaries.

The process of expiration is brought about by the following sequence of events:

  • The diaphragm muscles and the intercostal muscles relax.
  • Elastic recoil of the lung tissue takes place.
  • These processes move the ribs in and down and the diaphragm upwards.
  • Which causes the volume of the thorax to decrease. As the volume decreases the pressure increases above atmospheric pressure.
  • Air moves out of the lungs and out of the body (usually through the nostrils) down a pressure gradient.


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