When it relaxes the chest and lungs contract and the air is expelled from the lungs.
Now, before considering what happens to the air in the lungs, let us look a little into the matter of the circulation of the blood.

The blood, as you know, is driven by the heart, through the arteries, into the capillaries, thus reaching every part of the body, which it vitalizes, nourishes and strengthens.

It then returns by means of the capillaries by another route, the veins, to the heart, from whence it is drawn to the lungs.
The blood starts on its arterial journey, bright red and rich, laden with life-giving qualities and properties.

It returns by the venous route, poor, blue and dull, being laden down with the waste matter of the system.

It goes out like a fresh stream from the mountains; it returns as a stream of sewer water.