The presence of a circulatory system reduces the distance a substance must diffuse to enter or leave a cell. The distance is reduced because the circulatory system connects the aqueous environment of the cell with organs specialized for exchange. Oxygen diffuses from air in the lungs across the thin lung epithelium into the blood. This oxygenated blood is carried by the circulatory system to all parts of the body. As the blood passes through capillaries in the tissues, oxygen diffuses from the blood into the cells across the cells' plasma membranes.
Carbon dioxide is produced by the cells and moves in the opposite direction through the same system.
The circulatory system does more than move gases, it is a critical component to maintaining homeostasis of the body. The chemical and physical properties of the immediate surroundings of the cells can be controlled using the carrying capacity of the blood.