Describe how oxygen enters the blood in lungs with the help of a block diagram.
1) Gaseous exchange takes place within the lungs by diffusion from the alveoli to blood capillaries and vice versa. Alveoli in lungs are numerous and only one cell thick.
2) Alveoli are surrounded by capillaries that are also one cell thick.
3) Blood, dark red in colour flows from the heart through these capillaries and collects oxygen from the alveoli.
4) At the same time, carbon diox¬ide passes out of the capillaries and into the alveoli.
5) When we breathe out, we get rid of carbon dioxide.
6) The bright red, oxygen rich blood is returned to the heart and pumped out to all parts of the body.