Anatomy and Physiology: Anatomy of the Heart

Anatomy of the Heart

The four-chambered heart is the result of millions of years of evolution. The evolution of a system to circulate fluid was first, but the heart itself took much longer to reach full flower in vertebrates. Fish have a two-chambered heart: one atrium and one ventricle. An atrium (plural, atria) receives blood from the body and pumps it, using the muscular walls of the atrium, to the ventricle. The ventricle receives the blood from the atrium and pumps the blood out to the body.

Given that the ventricle must pump a greater distance than the atrium, it makes sense when you see a heart, that the walls of the ventricles are much thicker than those of the atria. The only other issue is how to guarantee that the pumping of the heart always goes in the same direction, because backflow would mean that oxygen and food would not be able to reach their target. The solution to that problem is simplicity itself: valves!

The shape of a valve allows it to open when the flow occurs in one direction, and close when the flow goes the opposite direction. In animals the valve shape is based on flaps (see Figure 11.1), and if you think about it, the free ends of the flaps point, just like an arrow, in the direction of the blood flow. As you shall see, the placement of those valves is crucial to the operation of the four-chambered heart.

In addition, the coordination of the heartbeat makes the valves function more easily. The heart contracts from top to bottom, the two upper chambers, or atria, contract (systole) first, while the lower chambers, or ventricles, relax (diastole), and vice versa.

This alternation of systole and diastole allows, for example, the relaxed ventricles to be filled by the contracting atria.

But the heart is just the start. In The Blood, you learned what the blood was for, but I never really explained to you where the blood was going. All this pumping of the heart must be sending the blood somewhere, here and there … but where?

To and Fro

In order to understand the cardiovascular system you need to grasp the vessel component of a closed system (see The Blood); there is, after all, a there there. As a closed system, despite some of the plasma leaving the vessels (see The Blood and Cardiovascular and Lymphatic Circulation) the blood never leaves the vessels unless they are damaged and you start to hemorrhage (bleed).

The pathway of the blood is called a circuit, for the blood that leaves the heart it returned to it. There are six parts to every blood circuit, in order: the pump-like heart arteries carrying blood pumped out of the heart; smaller arterioles; capillaries, where all exchanges happen; venules, which start the return to the heart; and lastly the veins, which ultimately return connect to the heart.

The blood starts when it is pumped from the heart—don't forget here that the blood flow within the vessels travels in one direction! The second part is called an artery, which is easy to remember because the blood flows away from the heart. In labeling the vessels, it is important to understand that many smaller arteries branch off from one major artery (think of a tree trunk dividing into smaller and smaller branches); each smaller artery has a specific name.

These artery names and locations are the same for everyone. On a smaller scale, the arteries continue to branch, but the branching becomes very small and starts to differ from person to person. These smaller branches, the third part of the circuit, are called arterioles. The smallest, but perhaps the most important part of every circuit, is the capillary, the fourth part of the circuit.

Capillaries are important because they are exceedingly small, with the lumen, or hollow center, of the vessel only wide enough to allow RBCs through one at a time. This small size is important, for the fluids that nourish the peripheral tissues, the wastes that are excreted, and the oxygen, water, and food that are absorbed all enter and leave the vessels here (to learn how, see Cardiovascular and Lymphatic Circulation). The fact that capillaries are so small, and that there are so many of them, means that there is an enormous surface area to volume ratio. This is an enormously important part of the circuit, for all the other portions are merely there to get the blood to and from the capillaries. Even the heart takes second stage, because, physiologically, it makes the most sense to say that all circuits start with the capillaries!

The fifth part of the circuit is analogous to the third part. Just as the arterioles are smaller versions of arteries, the venules are really just smaller versions of veins. The venules, which are once again as unique to the individual as the arterioles, drain the capillaries, and are ultimately drained into the veins (like smaller roads merging into larger highways). The sixth and last part of the circuit is analogous to the second part, for the veins are the largest vessels that take the blood to the heart. Similar to the arteries, there are many small veins that drain into the largest veins.

In the fish example, with the two-chambered heart, there is really only one circuit, called the systemic, that travels to and from the other systems of the body. Amphibians and reptiles developed a three-chambered heart to match the addition of two other circuits. Most students only know one of these circuits, the pulmonary, which is responsible for gas exchange. The third circuit, which is really a small branch that comes immediately off the systemic, is called the coronary, because its job is to nourish the heart; when a part of this circuit fails, and the blood flow stops, part of the heart might die, which is called a heart attack.