Circulatory System III: The Heart

The Heart

What is a heart? This might seem like a silly question, but have you ever thought about what a heart really is?

If we really think about it, the heart has only one job, and that is to pump blood through the circulatory system. Now, since the heart is really just a pump and doesn't have any other functions, you might think that all animal hearts look and function the same way, but that's not how nature works! Instead, nature creates new forms of organs as animals evolve into new forms with changing needs.

As a result there are a wide variety of hearts in the animal kingdom in terms of shape and structure, but they all have the same job and that is to pump circulatory fluid through the circulatory system. Let's take a look at the different types of vertebrate hearts and how they evolved.

Two-Chambered Heart

All vertebrates have a closed circulatory system with one central heart. The oldest type of vertebrate heart is the two-chambered heart that is still used by all modern-day fish, like our little friend here. The two-chambered heart is a very muscular organ consisting of one atrium , which is a heart chamber that receives blood returning to the heart and one ventricle, which is a heart chamber that pumps blood out of the heart.

The two chambers are separated by a single one-way heart valve which ensures that blood travels in only one direction, out of the ventricle and into the blood vessels, where the blood makes a single loop through the circulatory system. First, the blood travels to the gills, the respiratory organ in fish that transfers oxygen from the surrounding water to the blood. The oxygen-rich blood then flows through the tissues, and finally, returns back to the heart.

Three-Chambered Heart

The two-chambered heart has served fish quite well for a very long time, but when amphibians evolved and crawled out onto land, a major evolutionary change took place in their circulatory system; they developed double circulation which basically means that they have two separate circuits of blood flow .

One circuit, called the pulmonary circuit, leads to the respiratory organs to oxygenate the blood. The second circuit, called the systemic circuit, carries oxygenated blood to the various body tissues. As a result of the double circulation, amphibians have a three-chambered heart consisting of two atria and one ventricle.

Blood is pumped first through the pulmonary circuit where it is oxygenated and then returns to the heart through the left atrium. It then enters the left side of the common ventricle, and from there, most of the oxygen-rich blood is pumped through the systemic circuit to distribute the oxygen to the tissues before it is returned to the right atrium of the heart. From there the blood flows into the right side of the common ventricle before it is pumped back out into the pulmonary circuit.

Because the ventricle is shared by both circuits, some mixing of oxygen-rich and oxygen-poor blood does occur. However, mixing is reduced by the presence of a ridge in the center of the ventricle that somewhat separates the left and right side of the ventricle.

Four-Chambered Heart

Once the three-chambered heart evolved, the logical next step in the evolution of the heart was to completely separate the ventricle into two distinct chambers to ensure that oxygen-rich and oxygen-poor blood from the two circuits do not mix. This evolutionary progression between three- and four-chambered hearts can be seen in various species of reptiles.

Reptiles generally have three-chambered hearts, but different species of reptiles have walls of varying sizes that partially separate the ventricle. The lone exceptions are the crocodile species, which have a complete septum, creating a four-chambered heart that is very similar to the four-chambered heart found in birds and mammals, including humans.

The Human Heart

The flow of blood through the human circulatory system is very similar to that of amphibians. In humans, oxygen-poor blood enters the heart in the right atrium before flowing into the right ventricle. From there, it is pumped into the pulmonary arteries, which lead to the lungs.

Blood is oxygenated in the lungs and returns to the heart via the pulmonary veins before entering the left atrium. The oxygen-rich blood then flows into the left ventricle where it is pumped out into the aorta and then to every tissue of the body, where it unloads its oxygen and picks up carbon dioxide. This oxygen-poor blood is then brought back to the right atrium of the heart and the cycle starts again.

So now we know the path that the blood takes through the circulatory system, but how does the human heart actually work? Being mammals, humans have a four-chambered heart that has two atria and two ventricles. The human heart also has four one-way valves: two atrioventricular valves, that separate the atria and ventricles, and two more valves called the aortic and pulmonic valves that are located where the aorta and pulmonary arteries exit the ventricles.

These valves increase the efficiency of the heart by preventing backflow when blood isn't being pumped through them. The atria of the human heart are much smaller than the ventricles. This is because the ventricles serve as the primary pumping chambers and the atria function mainly to ensure that the ventricles are completely filled before the blood is pumped out, kind of like a 'topping off' mechanism.

Here is how it works: during each heartbeat, the heart goes through a sequence of events that result in a coordinated contraction of the heart. For about 50% of the time, blood is passively flowing from the atria, through the open atrioventricular valves and into the ventricles. When the ventricles are almost full, the atria quickly contract in unison and load a last burst of blood into the ventricles. After the atria have contracted and the ventricles are completely full, the atrioventricular valves close to prevent backflow and the ventricles contract in a longer, more deliberate contraction that ejects the blood into the arteries. The heart then relaxes and begins to fill again for the next heartbeat

In order for the heart to function properly, the muscle contractions must be coordinated. This coordination is controlled in the human heart by an electrical signal that is sent by the heart muscle to the cells around it as it contracts. The signal begins in the atria as they contract in unison. When the signal reaches the ventricles, it is delayed for a tenth of a second, and then the ventricles contract. The delay between the atrial and ventricular contractions allows time for the atria to completely fill the ventricles before they themselves contract, and pump their contents into the arteries.

Lesson Summary

OK, let's review what we learned today: the heart has one function, which is to pump circulatory fluid through the circulatory system. Even though all hearts perform the same basic function, the size and complexity of each circulatory system determines the complexity and size of the heart. Vertebrates can have two, three, or four-chambered hearts.

Fish have a two-chambered heart that pumps blood through a single circulatory loop. Blood is pumped from the heart, out to the gills, the respiratory organ in fish that transfers oxygen from the surrounding water to the blood. The oxygenated blood then flows through the tissues and, finally, returns back to the heart.

Humans have a four-chambered heart that pumps blood through the following double circulation scheme. Oxygen-poor blood enters the heart in the right atrium before flowing into the right ventricle . From there it is pumped into the pulmonary arteries which lead to the lungs. Blood is oxygenated in the lungs and returns to the heart via the pulmonary veins before entering the left atrium. The oxygen-rich blood then flows into the left ventricle where it is pumped out into the aorta and then to every tissue of the body where it unloads its oxygen and picks up carbon dioxide. This oxygen-poor blood is then brought back to the right atrium of the heart to complete the loop.