The Circulatory System

The main function of a circulatory system is to transport important substances from place to place in the body. These include:

· Oxygen—moved from lungs, gills, body surfaces, etc. to internal tissues that require it.
· Carbon Dioxide—moved from all the body’s cells to lungs, gills, or body surfaces where it can enter the external air or water.             CO2 is a metabolic waste product from the process of respiration.
· Nutrients—moved from the digestive tract (typically) to all the cells of the body
· Wastes—ammonia, urea, CO2, and a host of others. These are transported to places where they can be removed
           from the body’s fluids such as the lungs for CO2, or the kidneys for urea, ammonia, and others.
· Heat—heat is generated by muscle action. When muscles are working hard, they generate enough heat to be damaging to the           muscle and surrounding a tissues. Fortunately the blood flowing through hard-working muscles transport the excess heat           to the entire body where the excess is radiated into the surrounding air or water by body surfaces, gill, lungs, mouth &           pharynx (panting in dogs, etc.).
· Hormones—animals produce a large number of hormones which play a variety of roles in the normal functioning of the body.           Most hormones are transported from gland to “target” organ by the bloodstream. We will mention a few of these in our           coverage of the systems.

Not all animals have a circulatory system. In fact, many of the animal phyla lack such a system (never evolved one). How would animals be able to live without a circulatory system? Many of the simpler animal groups that lack this system accomplish the functions of circulation by simple diffusion of the items mentioned above. Such animals are almost always aquatic which usually allows for diffusion of dissolved gases (O2 & CO2) across body surfaces. Most such animals are also small, flat, and/or permeated by water channels (sponges) so that no cells are very far from a body surface in contact with the water. Finally, such animals typically have low metabolic rates such that they don’t require a lot of nutrients or oxygen, don’t generate much ammonia and other wastes, and don’t generate much heat—and water is an excellent heat sink that would easily prevent overheating in a small aquatic animal.

Most animals have what is called an open circulatory system. This is found in Arthropods (insects, spiders, shrimp, etc.) and most Mollusks (snails, slugs, clams, etc.). These two phyla are the two largest of the 32 animal phyla. Together they contain at least 80% of all animal species, so clearly the incomplete circulatory system is the most common type found in animals. Open circulatory systems typically contain only a heart and a few large arteries. There are no capillaries or veins. –More in Class

The remaining animals (such as earthworms, mammals, fish, etc.) have what is called a closed circulatory system. A closed circulatory system has a heart (or hearts), arteries, veins, and capillaries (or at least some very small interconnecting vessels between the arteries and the veins). –More in Class