The system of pipes in this house carries a fluid - hot water - that heats the house through a series of radiators.

The water is heated in a furnace, but it cannot move far under its own power. A powerful pump is used to move the water around the house. After the water has passed through all the radiators, it returns to the furnace to be heated again.

The analogy of the house’s heating system is very close to the way the circulatory system works in vertebrates. All vertebrates have a series of pipes, called vessels, that carry a fluid, called blood, around the body. The vessels have valves to ensure that the fluid always moves in the same direction.

The vessels are connected to a powerful pump, called the heart. The lungs enrich the blood with oxygen used in energy reactions, just as the furnace provides energy to heat the water in the house.

The various organs of the body are equivalent to the radiators of the house’s circulatory system.

Animal circulatory systems differ from very simple examples, lacking pumps and pipes, up to the complex multichambered heart of vertebrates. But circulatory systems all perform a similar range of functions.

They deliver oxygen and nutrients to cells.

They remove waste products from cells and tissues.

In many organisms, blood vessels also equalize temperature and distribute hormones.

In this activity, we’ll start by looking at the circulatory systems of animals ranging from primitive sponges and flatworms to humans. As we go along, we’ll trace the evolution of circulatory systems from their simplest beginnings to the complex circulation of mammals.

Next we’ll study the blood vessels that make up the vascular system of animals. We’ll see that different types of blood vessels have their own characteristic form, which is directly related to the function they carry out in the body. We’ll also look at the structure of blood and see just what the circulatory system moves around the body.

In the final section, we’ll consider the physiology of the heart and circulatory system in humans. We’ll conclude with a discussion of human cardiovascular disease, and how heart disease affects normal circulation.

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