THE MAMMALIAN CIRCULATORY SYSTEM
To sustain metabolic activity, all organisms must exhange materials with their environment. In particular, animals must exchange the gases O₂ and CO₂ as part of aerobic cellular respiration across the membrane of every cell. But most of a large animal's cells are much too far away from a respiratory surface to employ simple diffusion. Consequently, in large animals, specialized circulatory systems transfer oxygen, nutrients, hormones, and other materials to the cells and remove metabolic wastes.

Circulatory systems consist of three parts:

1. blood, composed of connective tissue cells dispersed in a fluid called plasma
2. blood vessels that carry the blood
3. a pumping organ, usually a heart, that circulates the blood. This tutorial examines a typical mammalian circulatory system — that of humans.

Mammal circulatory systems are divided into two circuits: pulmonary and systemic. The pulmonary circuit carries deoxygenated blood from the heart to the respiratory surface in the lungs, where it is reoxygenated, and then back to the heart. The systemic circuit carries oxygenated blood to all the body's cells via arteries, and deoxygenated blood back to the heart via veins. The mammalian double circulatory system is efficient because it uses a separate pump (the two ventricles) to power each circuit.