Oxygen is released into body cells primarily through capillaries, which are the smallest type of blood vessels in the circulatory system.

Role of Capillaries

Capillaries connect the arterial system to the venous system and form vast networks around body tissues. Their thin walls allow oxygen and nutrients in the blood to diffuse directly into the surrounding body cells. At the same time, waste products like carbon dioxide move from the cells into the capillaries to be carried away for removal from the body.

How Oxygen Exchange Works

• Oxygen-rich blood is pumped from the heart into arteries, which branch down to smaller arterioles and then to capillary beds.

• In the capillaries, oxygen diffuses across the vessel wall into body tissues, allowing cells to get the oxygen they need for energy and function.

• After releasing oxygen, the blood picks up waste and returns to the heart through veins.

So, it is at the capillary level—rather than in arteries or veins—that oxygen is delivered to almost all body cells.