I believe its the alveoli in the lungs. These alveoli provide a large surface area for gaseous exchange.
It is the heart that pumps fresh oxygen in the blood.
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.
vein
capillaries
Oxygen and other gases pass through the capillaries.
Capillaries
In the capillaries surrounding the alveoli in the lungs
Capillaries
Capillaries
Capillaries
Capillaries are very thin blood vessels. Oxygen and nutrients and hormones can pass through the walls of the capillaries and reach the body's cells, while red blood cells remain in the capillaries.
Oxygen is transported by red blood cells and is released into tissues as they pass through capillaries in the tissue.