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The respiratory process consists of three components. Ventilation, diffusion and perfusion. Ventilation consists of two parts:
  1. Inspiration, which is the expansion of the chest with a negative intrapulmonary pressure when air flows into the thorax; and
  2. Expiration, when the intrapulmonary pressure is higher than the atmosphere, air will flow out of the lungs.
Diffusion is the process whereby gases move from an area of high pressure to low pressure. This includes during - Internal respiration - this is the movement in the internal tissues between cells and capillaries, and - External respiration - when gas is exchanged between the alveoli and lung capillaries. Perfusion refers to the blood flow to tissues and organs. Alveoli are perfused by capillaries so the diffusion of oxygen and carbon dioxide can take place.