Unit 3: The digestive and respiratory systems                             1   2   3 

3.1. The exchange of gases

The exchange is the pass of O₂ from the alveolus to the blood and the pass of CO_2, from the blood to the alveolus.

It is produced by diffusion. This is the tendency that a gas has to pass from the place where it is more concentrated to the place where it is less concentrated, until the concentration in both compartments become equal. Diffusion is a physical process that does not need expend energy.

During inspiration, alveolus is full of air rich in oxygen and poor in carbon dioxide. The situation in blood capillaries is inversed. So that:

- Oxygen tends to pass from the alveolus to the blood.

- Carbon dioxide tends to pass from the blood to the alveolus.

To assure that the concentrations do not equalise, and to keep the continuous gas exchange:

   - Alveolus air is continuously renewed ([O₂] is always high and [CO₂] is always low)

   - Blood flow in the capillary is constant. ([O₂] is always low and [CO₂] is always high)

Diffusion is optimal when the air is clean, humid and warm. The preparation of air had place previously in the airways.

The exchange between the body tissues and the blood follows the same mechanism.

In this case:

    - Blood is richer in O₂ than cells, so that it tends to pass from capillaries to tissues.

    - Blood is poorer in CO₂ than cells, so that it tends to pass from tissues to capillaries.

The exchange is continuous because cells are always consuming oxygen and producing carbon dioxide and the bloodstream flows constantly.

3.4. Indicate where and how the air is prepared to assure an optimal

         exchange of gases in alveoli.

3.5. The atmospheric air contains about 78% of nitrogen,

        21% of oxygen, 0.03% of carbon dioxide and about 1% of argon

        and other gases. If the inspired air contains carbon dioxide,

        why this gas does not pass within the bloodstream?

3.6. When the action of mucus to keep clean the air which enters in lungs

        is insufficient, because the strange particle that has penetrated is

        too big, the body has other defensive mechanisms.

        Do you know what they are?

3.7. Listen and find the six mistakes in the text: