Oxygen diffuses from the alveoli into the blood capillaries during gas exchange in the lungs. Air enters the lungs through the nasal cavity and trachea, with oxygen-rich alveolar air reaching the alveoli. The blood in the surrounding capillaries is oxygen-poor and carbon dioxide-rich. Oxygen diffuses into the red blood cells where it binds to hemoglobin, while carbon dioxide diffuses from the blood into the alveoli. The oxygenated blood is then transported around the body where gas exchange occurs again, with oxygen diffusing into cells and carbon dioxide diffusing into the blood.

1. The diffusion of oxygen from the alveoli
into the blood capillaries
•During inhalation, air is taken into the lungs
through the nasal cavity, passing through the
trachea, bronchi, bronchioles and finally alveoli.
•The air that reaches the alveoli is called alveolar
air. It is rich in oxygen and poor in carbon dioxide.
•The blood that reaches the capillaries
surrounding the alveoli is poor in oxygen and rich
in carbon dioxide.

2. •Gas exchange takes place in the lungs by
diffusion.
•Oxygen diffuses from the alveoli into the
red blood cells in the capillaries.
•At the same time, the red blood cells in the
capillaries release carbon dioxide which
diffuses into the alveoli.

4. •Red blood cells contain the oxygen –
carrying pigment called hemoglobin.
•Oxygen combines with hemoglobin
to form oxyhaemoglobin.
•The blood cells are transported
around the body.

6. •Gas exchange also takes place between the
capillaries and the body cells.
•Capillaries have a higher concentration of
oxygen than the body cells.
•Body cells have a higher concentration of
carbon dioxide than the capillaries.
•Oxyhaemoglobin releases oxygen which
diffuses into the body cells.
•Carbon dioxide diffuses into the blood at the
same time.