It is helpful to think of respiration taking place in these stages:

1. We breathe in, and air is drawn into the lungs.
2. The oxygen in the air moves from the lungs into the blood.
3. The oxygen-rich blood is circulated to cells all over the body.
4. Oxygen enters the cells across the cell membranes.
5. Carbon dioxide moves in the opposite direction, across the cell membranes into the blood.
6. Carbon dioxide is carried back to the lungs in the blood.
7. The carbon dioxide is transferred from the blood into the air in the lungs.
8. We breathe out, and the carbon dioxide is expelled.

Of course, many of these processes are taking place simultaneously - so that at any one time, oxygen and carbon dioxide are moving in opposite directions between the blood and the air in the lungs, and between the blood and the cells in the tissues.