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Biology 2nd Baccalaureate

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4.1.3. The gaseous exchange

Gas exchange

The exchange of gases (O2 and CO2) between air and blood occurs in the pulmonary alveoli. The walls of the pulmonary alveoli are very thin and are surrounded by a network of  blood capillaries.

Gas exchange is done through a physical process called diffusion, in which molecules move from where there is more concentration to where there is less until they equalize.

Intercambio de gases por difusión. El oxígeno pasa de donde hay más concentración (el interior del alvéolo) a la sangre, y el dióxido de carbono pasa de donde hay más concentración (la sangre) a donde hay menos (los alvéolos).

The red blood cells are responsible for transporting oxygen in the blood. Oxygen binds to iron in a protein, called hemoglobin, found in red blood cells, and this is how it is transported. Instead, carbon dioxide is transported dissolved in the blood plasma (the liquid part of the blood).

How does gas exchange occur?

  1. The air enters through the nasal passagespharynxlarynx, continues through the tracheabronchi and bronchioles. The bronchioles end in the pulmonary alveoli, small sacs that inflate on inspiration and deflate on expiration.
  2. In the pulmonary alveoli, gas exchange occurs, since the walls of the alveoli contain numerous capillaries.
  3. The blood reaches the alveoli poor in oxygen and rich in carbon dioxide.
  4. The air that reaches the alveolus is rich in oxygen and poor in carbon dioxide.
  5. By diffusion, gases move from where there is the highest concentration to where the concentration is lowest, until they equalize.
  6. Oxygen molecules bind to red blood cells of the blood that lead to the heart, while the carbon dioxide from the pulmonary alveoli leave the body in the next exhalation.


Composición del aire atmosférico

Composición del aire alveolar


 78 %


 21 %

 Argón y helio

 0,92 %

 Dióxido de carbono

 0,03 %

 Vapor de agua

 0,04 %


 75 %


 14 %

 Dióxido de carbono

 5 %

 Vapor de agua

 6 %

Animation: Gas exchange in the pulmonary alveoli.

Activity: Name the parts of the respiratory system.

Activity: Where do these actions occur?.

Activity: Path followed by an oxygen molecule in the respiratory system.

Activity: Functions of the parts of the respiratory system.

Gamification: Challenge yourself with these questions about the respiratory system.

Answer in your notebook

4.5.- How is oxygen transported in the blood?

Answer in your notebook

4.6.- What do we mean when we talk about gas exchange?

Answer in your notebook

4.7.- Why does oxygen pass from inside the pulmonary alveoli to the blood capillaries? And why does CO 2 do the opposite?


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Biology and Geology teaching materials for Compulsory Secondary Education (ESO) and Baccalaureate students.