Capillaries

Capillaries are the small branching vessels of the circulatory system. They are the smallest blood vessel in the body and have walls that are only a single endothelial cell thick. 

The main function of the capillaries is the exchange of oxygen, waste and nutrients from cell to blood and blood to cell. The capillaries form a bridge between the arterioles and veins of the circulatory system, they recieve oxygenated blood from the arterioles and return deoxygenated blood to the veins.