The human heart does not look much like those Valentine's Day candies or the pictures we drew on our love notes when we were in elementary school. The current human heart is a large muscular organ with four chambers, a septum, several valves, and other various parts necessary for pumping blood all around the human body. However, this amazing organ is a product of evolution and has spent millions of years perfecting itself in order to keep humans alive.
Invertebrate animals have very simple circulatory systems. Many do not have a heart or blood because they are not complex enough to need a way to get nutrients to their body cells. Their cells are able to just absorb nutrients through their skin or from other cells. As the invertebrates become a little more complex, they use an open circulatory system. This type of circulatory system does not have any blood vessels, or has very few. The blood is pumped throughout the tissues and filters back to the pumping mechanism. Like in earthworms, this type of circulatory system does not use an actual heart. It has one or more small muscular areas capable of contracting and pushing the blood and then reabsorbing it as it filters back. However, these muscular regions were the precursors to our complex human heart.
Of the vertebrates, fish have the simplest type of heart. While it is a closed circulatory system, it has only two chambers. The top is called the atrium and the bottom chamber is called the ventricle. It has only one large vessel that feeds the blood into the gills to get oxygen and then transports it around the fish's body.
It is thought that while fish only lived in the oceans, amphibians like the frog were the link between water dwelling animals and the newer land animals that evolved. Logically, it follows that frogs would therefore have a more complex heart than fish since they are higher on the evolutionary chain. In fact, frogs have a three chambered heart. Frogs evolved to have two atria instead of one, but still only have one ventricle. The separation of the atria allows frogs to keep the oxygenated and deoxygenated blood separate as they come into the heart. The single ventricle is very large and very muscular so it can pump the oxygenated blood throughout the various blood vessels in the body.
The next step up on the evolutionary ladder is the reptiles. It was recently discovered that some reptiles, like turtles, actually have a heart that has a sort of a three and a half chambered heart. There is a small septum that goes about halfway down the ventricle. The blood is still able to mix in the ventricle, but the timing of the pumping of the ventricle minimizes that mixing of the blood.
The human heart, along with the rest of the mammals, is the most complex having four chambers. The human heart has a fully formed septum that separates both the atria and the ventricles. The atria sit on top of the ventricles. The right atrium receives deoxygenated blood coming back from various parts of the body. That blood is then let into the right ventricle which pumps the blood to the lungs through the pulmonary artery. The blood gets oxygenated and then returns to the left atrium through the pulmonary veins. The oxygenated blood then goes into the left ventricle and is pumped out to the body through the largest artery in the body, the aorta.
This complex, but efficient, way of getting oxygen and nutrients to body tissues to billions of years to evolve and perfect.