Some simple animals don’t have real circulatory systems at all. Flatworms and some other invertebrate animals use simple diffusion to move digested food, gases, and waste products from one part of the organism to another.

The flat shape and system of branching internal tubes of these organisms enables their cells to receive nutrients and expel waste products over short distances.

The fluid filling the space between cells in an animal is called interstitial fluid. This fluid plays a vital role in the transport of chemicals between cells and tissues.

Why don’t all animals rely on diffusion to transport chemicals around the body? The answer lies in size and distance. The longer the distance, the longer it takes a chemical to diffuse from place to place.

Diffusion is fine for tiny animals, or for animals that are very flat. Such animals can absorb nutrients and other substances directly from the surroundings into their cells

For larger organisms, the speed of diffusion is just too slow.

Calculations show that the movement of a small molecule across a large cell can take as long as 10 seconds. To diffuse the same molecule a distance of one meter, or the height of a child, would take 32 years!

The rule of thumb is: If you can see an animal with the naked eye, it won’t rely on diffusion, and it’ll have a more complex circulatory system.

Some invertebrate animals, for example insects and most mollusks, have an open circulatory system to transport food, wastes, heat, and other materials. In this type of system, a tube-shaped heart pumps a fluid called hemolymph through arteries into the main body cavity, and from there it directly bathes the various organs. Animals with open circulatory systems do not have an extensive network of blood vessels that infiltrate the organs.

A closed circulatory system encloses the blood in a system of tubes called arteries, veins, and capillaries in vertebrates. In invertebrates they are often referred to simply as vessels. In closed systems, blood is pumped very close to the cells, but the cells are not directly bathed in the blood. Instead, the cells are surrounded by interstitial fluid.

Worms, sea urchins, and vertebrates all have closed circulatory systems. In the simplest case, shown by this earthworm, there is one dorsal and two ventral vessels running from the anterior to the posterior of the animal.

The dorsal vessel and one of the two ventral vessels are connected by pairs of transverse connecting vessels. As in insects, there is no defined heart. The dorsal vessel serves as the main ‘heart’, and the ventral vessels collect blood to be returned to the dorsal vessel.

A group of five of the connecting vessels at the anterior of the earthworm act as auxiliary hearts, forcing blood into the ventral vessels.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education