The development of gill breathing in fish and lung breathing in land animals needed a more efficient circulatory system.

The development of the heart as a muscular organ capable of pumping blood around the body was the next main evolutionary advance.

A simple heart consists of two chambers. One, the atrium, receives incoming blood and acts as a reservoir. When the atrium is full, it pumps blood to a second chamber, the ventricle.

The simplest vertebrate heart is seen in fish, as this diagram shows.

The atrium is a relatively thin-walled chamber in which blood collects before passing to the muscular ventricle.

The heart pumps blood out to the gills, where oxygen diffuses into the blood. Blood then passes from the gills to the body tissues. From there, it is collected and returned to the heart.

The problem with this system is that the blood pressure drops as it passes through the gill capillaries, and it drops yet again as it passes through capillaries in the body tissues.

Fish partly get over this problem by using their body’s motion through the water to help the passage of blood through the circulatory system.

Amphibians, like this frog, have one ventricle just like in fish, but they have two atria.

This modification allows blood pumped from the ventricle to the lungs and skin to pick up oxygen, then return to the heart to have its pressure boosted before passing to the body tissues. More pressure equals more oxygenated blood passing to the tissues in a reduced amount of time.

In amphibians, the blood returning to the left atrium from the lungs is oxygenated and is ready for the body tissues. But as it passes into the ventricle, it is only partially separated from the deoxygenated blood returning from the tissues.

Some of the freshly oxygenated blood, therefore, goes back to the lungs again.

This is a design problem that seemed to be waiting for evolution to solve.

This circulatory system belongs to a reptile. It is almost the same as the amphibian system, but note the wall, or septum, down the center of the ventricle. The septum helps prevent the oxygenated and deoxygenated blood from mixing, thus increasing the efficiency of the circulation.

You can also see that the septum does not completely separate the ventricle into two chambers, so some mixing of oxygenated and deoxygenated blood could still take place.

Now look at this circulatory system. The septum is now complete, and there are two distinct ventricles. This efficient system is found in birds and mammals. It ensures that the oxygenated blood from the lungs passes to the body organs with no mixing in the ventricles.

Now that we’ve explored the different types of hearts found in the different vertebrate groups, see if you can match the hearts on the screen with the animals that have them.

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All: The simplest vertebrate heart is seen in fish. There is only one atrium and one ventricle in these animals.

Frogs and other amphibians have two atria and a single ventricle.

Most reptiles have a septum that partially separates the ventricle into two separate ventricles. Birds and mammals have two distinct ventricles.

Now that we’ve looked at the different kinds of circulatory systems we’ll move on to examine the blood vessels themselves.

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