It is the heart that pumps fresh oxygen in the blood.
The lungs (:::::::::
The oxygen is used up.
heart
The alveoli and capillaries in the lungs pass oxygen to the blood. Both have very thin walls, which allow the oxygen to pass from the alveoli to the blood. The capillaries then connect to larger blood vessels, called veins, which bring the oxygenated blood from the lungs to the heart.
I believe its the alveoli in the lungs. These alveoli provide a large surface area for gaseous exchange.
vein
Oxygen and other gases pass through the capillaries.
capillaries
Capillaries
In the capillaries surrounding the alveoli in the lungs
Capillaries
Capillaries
Capillaries
Oxygen is lipid soluble therefore it can pass directly through the cell membranes of the capillaries.
Oxygen is transported by red blood cells and is released into tissues as they pass through capillaries in the tissue.