General

Oxygen intake

As you breathe in, the alveoli fill up with oxygen (O2). The oxygen is absorbed into the capillaries through the alveolar walls, which are only one cell layer thick.

Once the oxygen has entered the blood in this way, it is absorbed into the red blood cells. This changes the oxygen-depleted blood (blue) into oxygenated blood (red).

Substances like drugs that are mixed with inhaled smoke are easily absorbed into the bloodstream in this way, too.