How does the composition of the blood of the small circle change?

The human body has a closed system of vessels that form two circles of blood circulation. A small circle is necessary to saturate the blood with oxygen. This is due to the large capillary network within the lungs. When blood passes through the small circle, erythrocytes lose carbon dioxide. Oxygen molecules are taken in place of carbon dioxide. This is how the venous blood becomes arterial. Then the blood flows into the systemic circulation, where it spreads throughout the body and gradually becomes venous.