The blood is moved throughout the body via the circulatory system. The circulatory system is composed of a propelling organ, the heart, and a closed system of vessels - arteries, veins, capillaries, arterioles, and venules. Blood is pumped from the left side of the heart through the arteries, which are large, elastic vessels. The arteries gradually diminish in size, as they divide to carry blood throughout the body, into arterioles. As the arterioles enter a tissue, they branch into countless microscopic vessels called capillaries.
Capillaries are so small that only one cell can pass through at one time.
It is at the capillary level that the exchange of nutrients and wastes takes place. Before leaving the tissue the capillaries merge to form venules which merge to form progressively larger vessels, called veins, which carry blood back to the right side of the heart. The blood is then pushed into the lungs where it becomes oxygenated and is them pushed into the left side of the heart for re-circulation. It is literally thicker than water and more adhesive. Water has a viscosity of 1.0.