OK, it’s time for another science-y post. Usually, I take on something very relevant to my specialty—it’s a helluva lot easier to write about stuff I already know. But some basics are just really cool, and worth exploring, even though I’ll have to step a bit outside my comfort zone. In this case, it’s the heart. Because I’m venturing a bit on the wild side, I consulted an expert, whose hot, hot science helped illuminate this topic.
If you’ve taken a basic biology course, you probably have some idea of how the human heart works, but understanding can be a bit deeper if we look at the heart through the lens of things that can go wrong.
The heart is one the most easily recognized and, yet, mysterious organs in the human body. Physicians of the Hippocratean school first described the valves of the heart in the 4th century B.C. Since, the heart has historically been described as the core of love,spirituality, intellect, and emotion. The heart was originally thought to be the source of conscious thought. The heart is a recognizable icon, source of stress for some of us who have witnessed friends and family with cardiovascualr disease, and a subject of poetry and song. However, to the physician and physiologist, the human heart is simply a pump whose primary purpose is the circulation of blood throughout the circulatory system. In fact, the majority of cardiac physiology and pathology (disease) can be easily demystified and understood by thinking of the heart as a mechanical pump.
Human heart, post-mortem obviously
The heart is a pump
The heart has one basic function — to receive oxygen poor blood from the body, pump it to the lungs where it can be replenished with oxygen, and then pump it back to the body where that oxygen is needed.
If we sliced through the human heart we would find that it has four compartments or chambers. The top chambers, or atria (singular = atrium), receive the blood. The lower chambers, or ventricles, pump the blood.
Human heart from Gray’s Anatomy showing the four chambered heart.
The following diagram illustrates how blood travels through the heart:
- Oxygen poor blood returns from the body and enters the right atrium
- Oxygen poor blood passes from the right atrium to the right ventricle. The right ventricle then pumps the oxygen poor blood into the pulmonary artery. Sometimes people are confused by the pulmonary artery because most people think of arteries as having oxygen rich blood. The true definition of “artery” is any vessel that takes blood away from the heart.
- Oxygen poor blood from the pulmonary artery travels to the lungs where the oxygen is replenished.
- Oxygen rich blood returns from the lungs and enters the left atrium
- Oxygen rich blood passes from the left atrium into the left ventricle. The left ventricle then pumps oxygen rich blood into the aorta.
- Oxygen rich blood from the aorta is then circulated through out the body. The oxygen is utilized and the blood becomes oxygen poor. Return to step 1 and begin the cycle again.
As you may see, the heart and lung essentially function as a single “super-organ” whose purpose is to gather blood, oxygenate it, and return it to the body. Here’s a schematic of the process we just described, for those of you who like simpler diagrams.
This schematic simplifies what this dual-organ does. Oxygen rich blood is pumped from the left side of the heart to be used in the body. “Used” oxygen-poor blood returns to the right side of the heart where it is pumped to the lungs for a refill. The newly-oxygenated blood returns to the left heart to continue the cycle.
To understand just how important this rather basic plumbing is, let’s look at what happens when something goes wrong.
Transposition of the Great Arteries
During development of the fetus, bad things can happen to the heart-lung’s plumbing. There are many kinds of congenital heart defects, but one which will serve as a reasonably useful example is called “transposition of the great arteries” (TGA).
Before birth, blood is shunted away from the lungs, mostly via a connection called the ductus arteriosus (DA, #3 in the sketch below), since breathing amniotic fluid is not particularly useful. The DA connects the aorta and the pulmonary artery, allowing blood to bypass the lungs. As soon as a baby is born, a set of physiologic changes occurs which closes the DA. Once the DA closes, blood flows normally, entering the right heart, being pumped to the lungs by the pulmonary artery, and back to the left heart, thence to the rest of the body again.
In TGA, this normal post-natal flow is altered. The “great arteries” of TGA are the aorta and the pulmonary artery (PA). Below is pictured an anatomical schematic, and a stripped-down version. The aorta is the red one that carries O2 rich blood to the body. The PA is the blue one that carries O2 poor blood to the lungs. In TGA, these two get switched during development, with the PA pumping blood to the body instead of the lungs, and the aorta pumping blood to the lungs instead of the rest of the body. Since the fetus doesn’t rely on the lungs, there aren’t any problems until birth, when the fetal circulation is switches to a normal post-natal one.
Schematic of TGA
You can see that rather than having the normal serial circulation, in TGA there is a parallel circulation in which blood supplying the body fails to get oxygenated. Oxygen rich blood from the lungs, rather than being pumped into the systemic circulation is pumped back to the lungs in an endless loop. Over on the right side of the heart, oxygen-poor blood from the body simply recirculates without ever making it to the oxygen-rich environment of the lungs. This is a problem. Most babies born with this problem are quite ill, and their blue hue signals a “cyanotic heart defect”. Most will die quickly without immediate intervention.
It’s often impossible to do a definitive operation immediately, so there are procedures that can temporarily save a baby’s life. Once such clever intervention is known as the Senning Procedure. In this procedure, a catheter is snaked from a blood vessel in the thigh up into the heart and a hole is created between the atria. This hole (called an atrial septal defect, which can also occur naturally) allows blood from the oxygen-rich left heart to mix with blood from the oxygen-poor right heart, and thence circulate to the rest of the body. Another common intervention is infusion of a medication to keep the ductus arteriosus open, helping to keep oxygenated and deoxygenated blood mixing. These two interventions can save the baby’s life until open heart surgery, where the two arteries will be switched back into their proper positions.
Heart-lung physiology is pretty cool stuff, and there are lots of online resources from basic to complex, so go and learn.