Among different groups of animals, hearts vary greatly in size and complexity. In insects, the heart is a hollow bulb with muscular walls that contract to push blood into an artery. Many insects have several such hearts arranged along the length of the artery. When the artery ends, blood percolates among the cells of the insect?s body, eventually making its way back to the heart. In an insect, blood may take as long as an hour to complete a trip around the body.

In earthworms and other segmented worms, or annelids, blood flows toward the back of the body through the ventral blood vessel and toward the front of the body through the dorsal blood vessel. Five pairs of hearts, or aortic arches, help pump blood. The hearts are actually segments of the dorsal blood vessel and are similar in structure to those of insects.

In vertebrates, or animals with a backbone, the heart is a separate, specialized organ rather than simply a segment of a blood vessel. In fish, the heart has two chambers: an atrium (receiving chamber) and a ventricle (pumping chamber). Oxygen-depleted blood returning from the fish?s body empties into the atrium, which pumps blood into the ventricle. The ventricle then pumps the blood to the gills, the respiratory organs of fish. In the gills, the blood picks up oxygen from the water and gets rid of carbon dioxide. Leaving the gills, the freshly oxygenated blood travels to various parts of the body. In fish as in humans, blood passes through the respiratory organs before it is distributed to the body. Unlike in humans, the blood does not return to the heart between visiting the respiratory organs and being distributed to the tissues. Without the added force from a second trip through the heart, blood flows relatively slowly in fish compared to humans and other mammals. However, this sluggish flow is enough to supply the fish?s relatively low oxygen demand.

As vertebrates moved from life in the sea to life on land, they evolved lungs as new respiratory organs for breathing. At the same time, they became more active and developed greater energy requirements. Animals use oxygen to release energy from food molecules in a process called cellular respiration, so land-dwelling vertebrates also developed a greater requirement for oxygen. These changes, in turn, led to changes in the structure of the heart and circulatory system. Amphibians and most reptiles have a heart with three chambers?two atria and a single ventricle. These animals also have separate circuits of blood vessels for oxygenating blood and delivering it to the body. Deoxygenated blood returning from the body empties into the right atrium. From there, blood is conducted to the ventricle and is then pumped to the lungs. After picking up oxygen and getting rid of carbon dioxide in the lungs, blood returns to the heart and empties into the left atrium. The blood then enters the ventricle a second time and is pumped out to the body. The second trip through the heart keeps blood pressure strong and blood flow rapid as blood is pumped to the tissues, helping the blood deliver oxygen more efficiently.

The three-chambered heart of amphibians and reptiles also creates an opportunity for blood to mix in the ventricle, which pumps both oxygenated and deoxygenated blood with each beat. While in birds and mammals this would be deadly, scientists now understand that a three-chambered heart is actually advantageous for amphibians and reptiles. These animals do not breathe constantly?for example, amphibians get oxygen through their skin when they are underwater?and the three-chambered heart enables them to adjust the proportions of blood flowing to the body and the lungs depending on whether the animal is breathing or not. The three-chambered heart actually results in more efficient oxygen delivery for amphibians and reptiles.

Birds and mammals have high energy requirements even by vertebrate standards, and a corresponding high demand for oxygen. Their hearts have four chambers?two atria and two ventricles?resulting in a complete separation of oxygenated and deoxygenated blood and highly efficient delivery of oxygen to the tissues. Small mammals have more rapid heart rates than large mammals because they have the highest energy needs. The resting heart rate of a mouse is 500 to 600 beats per minute, while that of an elephant is 30 beats per minute. Blood pressure also varies among different mammal species. Blood pressure in a giraffe?s aorta is about 220 mm of mercury when the animal is standing. This pressure would be dangerously high in a human, but is necessary in a giraffe to lift blood up the animal?s long neck to its brain.

Although other groups of vertebrates have hearts with a different structure than those of humans, they are still sufficiently similar that scientists can learn about the human heart from other animals. Scientists use a transparent fish, the Zebra fish, to learn how the heart and the blood vessels that connect to it form before birth. Fish embryos are exposed to chemicals known to cause congenital heart defects, and scientists look for resulting genetic changes. Researchers hope that these studies will help us understand why congenital heart malformations occur, and perhaps one day prevent these birth defects.