Wednesday, 8 April 2009

iGCSE Biology - Breathing and Gas Exchange

Breathing and Gas Exchange


  • Lungs enclosed in the thorax (chest) by ribs and diaphragm.
  • Ribs are joined to each other by intercostal muscles.
  • Process of breathing in:
    1. Air enters nose or mouth and passes down the trachea.
    2. Trachea splits into two bronchi, one leading to each lung.
    3. Each bronchus divides into smaller and smaller bronchioles.
    4. The bronchioles each end at microscopic air sacs called alveoli.
  • The walls of the trachea and bronchi contain rings of cartilage to keep them open.
  • The thorax is separated from the lungs by the pleural cavity, which is filled with pleural fluid.
  • The pleural fluid acts as lubrication for the lungs.
  • The pleural cavity is formed by two sealed membranes around the lungs called pleural membranes.
  • The walls of the trachea produce mucus which traps particles that are breathed in.
  • Small hairs called cilia are also found on the walls of the trachea.
  • The cilia “sweep” the mucus containing dirt/bacteria particles to the mouth.
  • Ventilation is the process moving air in and out of lungs, using pressure differences.

1.       External intercostals contract, internal intercostals relax, and diaphragm contracts, increasing volume of the chest, and thus lowering pressure. Air then enters the lungs.

2.     Internal intercostals contract, external intercostals relax, and diaphragm relaxes, decreasing volume of chest, and thus decreasing pressure. Air then leaves the lungs.

  • Alveoli take carbon dioxide out of the blood and put oxygen into the blood.
  • Alveoli have large surface areas and are covered by capillaries. There are only two cells between the blood and the oxygen in the alveoli.
  • Oxygen diffuses from the air into the blood, and carbon dioxide diffuses from the blood into the air.
  • Bronchitis affects smokers. Smoking destroys cilia, resulting in a build-up of mucus. This blocks normal air flow.
  • Emphysema is a disease in which smoke damages the alveoli, causing them to fuse together and lose surface area. This results in low oxygen levels in the blood.
  • Cigarettes contain carcinogens, and thus raise the chance of getting cancer.
  • Cigarette smoke contains carbon monoxide, which bonds with haemoglobin to form carboxyhaemoglobin. This results in the blood carrying less oxygen.

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