The image below illustrates the main function of the cardiovascular system. The capillaries allow oxygen to move out through pores in their walls into the cells. At the same time, carbon dioxide moves from the cells into the capillaries.
In addition to the exchange of gases, other products of metabolism move from the cells into the capillaries. The cardiovascular system will transport these waste products to the liver, where they will be detoxified, or to the kidneys, where they will be excreted.
In the image, the red tubules indicate arterial vessels carrying oxygenated blood whereas the blue indicate venous vessels carrying oxygen-depleted blood.
The green tubules are not part of the cardiovascular system. They are part of the lymphatic system and carry lymph, or tissue fluid.