Oxygen from the alveolus dissolves into the bloodstream and binds to hemoglobin in red blood cells. This oxygen-rich blood is then circulated throughout the body. In tissues where oxygen levels are lower, the oxyhemoglobin releases oxygen to enter cells and undergo cellular respiration, while carbon dioxide diffuses out of cells into the bloodstream to be expelled through exhalation.

3. • The concentration of oxygen in the
alveolus is higher than that in the
blood.
• The oxygen dissolves in moist lining
of the alveolus
• Then oxygen diffuses into the blood
capillary
• In the blood, oxygen combines with
haemoglobin in the RBCs to form
oxyhaemoglobin

4. • This oxygen-rich blood is then
transported to the heart and pumped
to all part of the body
• In the body cells, the concentration of
oxygen in low.
• Thus, oxyhaemoglobin which is an
unstable compound in the blood
capillary decomposes to release
oxygen which diffuses into the body
cells for cellular respiration.
• At the same time, carbon dioxide
diffuses out of the body cells into the
blood capillaries and is transported to
the alveolus to be removed.