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SAI International School
Subject- Biology, Class-VII
Chapter- Respiration in Organisms
Subtopic- Exchange of Gases, Types of Respiration
Module- 15
Lesson Notes

EXCHANGE OF GASES

Exchange of gases takes place in the lungs between inhaled air and the blood. It occurs
in the alveoli of the lungs.

 Alveoli (singular - alveolus) are grape-like clusters surrounded by the network of
thin-walled blood capillaries.

Fig -1 Alveoli surrounded by blood capillaries.

 After we inhale oxygen-rich air, there is a greater concentration of oxygen in the
alveoli than in the blood present in the blood capillaries, so oxygen diffuses from
the alveoli into the blood across the capillaries (see figure -2 below).

 Carbon dioxide, in contrast, is more concentrated in the blood present in the
blood capillaries than in the alveoli, so it diffuses in the opposite direction. The
process, which involves in the exchange of gases, is called Diffusion.

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