Where does the transformation of venous blood into arterial blood take place?

Arterial and venous blood flows in our body. The venous one goes from the cells to the heart, so there is little oxygen in it. Arterial blood is enriched with oxygen. Venous blood is much darker than arterial blood and flows more slowly through the vessels. Venous blood is saturated in the lungs. So it becomes arterial. Then, with the help of the heart, it is pumped further to all tissues and cells. We have two circles of blood circulation: small and large. Blood from the vessels does not flow into the body cavity. This means that our circulatory system is of a closed type.