Human health and physiology - Gas exchange

The need for gas exchange and ventilation in humans.
1. Cell respiration happens in the cytoplasm and mitochondria of cells and releases energy in the form of ATP for use inside the cell.
2. In humans oxygen is used in cell respiration and carbon dioxide is produced.
3. Human therefore must take in oxygen from their surrounding and release carbon dioxide. This process of swapping one gas for another is called gas exchange.
4. Gas exchange happens in the alveoli of human lungs.
  1. Oxygen diffuses from the air in the alveoli to the blood in capillaries. Carob dioxide diffuses in the opposite direction.
5. Diffusion of oxygen and carbon dioxide happens because there are concentration gradients of oxygen and carbon dioxide between the air and the blood.
  1. To maintain these concentration gradients, the air in the alveoli must be refreshed frequently. The process of bringing fresh air to the alveoli and removing stale air is called ventilation.
Adaptions of the alveolus to gas exchange
1. Although each alveolus is very small, the lungs contain hundreds of millions of alveoli in total, giving a huge overall surface area for gas exchange.
2. The wall of the elveolus consists of a single layer of very thin cells. The capillary wall also is a single layer of very thin cells, so the gasses only have to diffuse a very shot distance.
3. The alveolus is covered by a dense network of blood capillaries with low oxygen and high carbon dioxide concentrations. Oxygen therefore diffuses into the blood and carbon dioxide diffuses out.
4. Cells in the alveolus wall secrete a fluid which keeps the inner surface of the alveolus moist, allowing the gases to dissolve. The fluid also contains a natural detergent, which prevents the sides of the alveolus from sticking together.
Ventilation of the lungs
1. Air is inhaled into the lungs through the trachea, bronchi and bronchioles.
2. It is exhaled via the same route.
3. Muscles are used to lower and raise the pressure inside the lungs to cause the movements of air.
4. Inhaling
  1. 1. The external intercostal muscles contract, moving the rib cage up and out.
  2. 2. The diaphragm contracts, becoming flatter and moving down.
  3. 3. These muscle movements increase the volume of the thorax.
  4. 4. The pressure inside the thorax therefore drops below atmospheric pressure.
  5. 5. Air flows into the lungs from outside the body until the pressure inside the lungs rises to atmospheric pressure.
5. Exhaling
  1. 1. The intercostal muscles contract, moving the ribcage down and in.
  2. 2. The abdominal muscles contract, pushing the diaphragm up into a dome shape.
  3. 3. These muscle movements decrease the volume of the thorax.
  4. 4. The pressure inside the thorax therefore rises above atmospheric pressure.
  5. Air flows out from the lungs to outside the body until the pressure inside the lungs falls to atmospheric pressure.

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Human health and physiology - Gas exchange

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