The blood which has been gathered up from the various parts of the body is dark, impure blood.

In the lungs this dark blood is spread out in very minute capillaries and exposed to the air.

While passing through the capillaries of the lungs, the blood gives up some of its impurities in exchange for oxygen from the air.

The red corpuscles absorb the oxygen and the color of the blood changes from dark purple to bright red again.

The purified blood is then carried back to the upper chamber of the left side of the heart through four large veins.