Circulatory System

The circulatory system serves to move blood to a site or sites where it can be oxygenated, and where wastes can be disposed. Circulation then serves to bring newly oxygenated blood to the tissues of the body. As oxygen and other chemicals diffuse out of the blood cells and into the fluid surrounding the cells of the body's tissues, waste produces diffuse into the blood cells to be carried away. Blood circulates through organs such as the liver and kidneys where wastes are removed, and back to the lungs for a fresh dose of oxygen. And then the process repeats itself. This process of circulation is necessary for continued life of the cells, tissues and even of the whole organisms. Before we talk about the heart, we should give a brief background of the two broad types of circulation found in animals. We will also discuss the progressive complexity of the heart as one moves up the evolutionary ladder.

Many invertebrates do not have a circulatory system at all. Their cells are close enough to their environment for oxygen, other gases, nutrients, and waste products to simply diffuse out of and into their cells. In animals with multiple layers of cells, especially land animals, this will not work, as their cells are too far from the external environment for simple osmosis and diffusion to function quickly enough in exchanging cellular wastes and needed material with the environment.