In this chapter, we describe how the various cells and organs of the body are connected. This occurs via the Circulation Tree, a system of tubes in which the blood leaves and returns to the heart. Circulation is not only as a system of tubes but also the vessel walls are important in the interaction with different components in the blood itself. Circulation of body fluids not only involves the transport of oxygen, carbon dioxide, nutrients, waste, cells, and proteins that defend our body but also maintains the balance between the various fluid compartments and ultimately ensures the integrity of the vascular tree. Moreover, hormones produced by the various endocrine glands reach their target organs via circulation. In this same chapter, the lymphatic circulation is also described. Fluids pass out of the blood in the capillaries to become interstitial fluids containing nutrients, hormones and oxygen and return to the blood with waste products and CO2 via the lymphatic system of vessels.