is called cardiac looping.
This isn’t the end of the heart’s development by far. Next, our heart grows ears — although not ones it can hear with. Like those fluffy bunny ears that are so popular at hen’s nights, they only look similar to the real thing. Scientists are still unsure about the precise function of these heart-ears, which are in fact nothing more than appendages to the heart’s atria. What doctors do know, though, is that they are responsible for the release of a hormone that will later stimulate urinary excretion. Our heart not only pumps blood around our bodies, it also helps us to pee.
By this stage, almost a month has gone by since the egg cell was fertilised, and the embryonic heart can now be divided into recognisable sections that will become the chambers known as the atria (where blood enters the heart) and the ventricles (where blood is expelled). Precursors to the cardiac valves form, as do the early stages of the septum, or dividing wall between the right and left side of the heart. However, that wall does not form a complete partition in the embryonic heart, and will not fully close until a few days after birth.
In fact, there is an oval hole between the right and left atria, called the foramen ovale. Blood flows through this aperture from the right atrium into the left, and then on around the embryo’s body. Why is that? The reason is simple: embryos are not yet able to breathe independently, so it would make little sense to invest in the laborious process of pumping blood through the embryo’s lungs. This short-cut is all it takes to avoid that.
What eventually results from all this development is muscly on the outside and hollow on the inside (and thus could be said to bear a resemblance to a certain former governor of California).
Act Two: the newborn heart
The heart of a newborn baby is quite different from that of an adult. About the size of a walnut, it works much more quickly. It beats up to 150 times a minute — even at rest: baby doesn’t have to have been doing any sport. That’s about twice as fast as the normal adult heart rate. The reason for this is simply that a newborn’s heart is still very small and it pumps only a small amount of blood with each contraction. However, now that the heart is working entirely on its own, the foramen ovale closes during the first few days of life. With that connection blocked, the right side of the heart now pumps blood into the pulmonary circulation system of the lungs,* and the left side pumps blood round the rest of the newborn baby’s body.
In the theatre, this is the stage when the first signs of conflict usually appear. The same is true of the heart. If something has gone seriously wrong with the development process of the heart, this is when it will become known, if it hasn’t already. Although prenatal diagnostic techniques are now very advanced in the developed world, they are still not perfect, unfortunately. When doctors listen to an abnormal infant heart, they will often be able to diagnose a heart defect based on the sounds they hear.
The most common of these is what doctors call a ventricular septal defect, when the wall dividing the heart’s two ventricles has a hole in it.† In the most serious cases, a young life must begin with major heart surgery. It depends on the size of the opening. Minor defects can heal up by themselves without any medical intervention, and as long as the newborn child appears to be vigorous and thriving, there is no immediate danger to the baby’s life. The decisive factor is whether the infant’s organs are receiving enough oxygen. If this is the case, then doctors, parents, and, most importantly, junior can breathe easy.
Act Three: the strong heart
The heart of a healthy 20-year-old human contracts somewhere between 60 and 80 times a minute. If it is well trained, it can beat quite significantly more slowly when its owner is at rest. And this bundle of muscle is practically bursting with energy. The best way to gain an idea of its internal structure is to cut it open and take a look. For me, as a student of medical anatomy, this was an extremely exciting experience. But it might not be everyone’s idea of fun.
Let’s take a look at it from the point of view of a red blood cell, also known to scientists as an erythrocyte. It, and its many fellow red blood cells, gets its name from the red pigment haemoglobin, which it contains. Its main job is to transport oxygen from our lungs to the rest of our body, and, on return, to transport carbon dioxide back to our lungs.
Imagine you are an RBC (the slang term among medical types for red blood cell). You are transporting carbon dioxide — bonded to your haemoglobin — from one of the organs of the body, let’s say the brain, through a blood vessel back towards the heart. So you must be in a vein, since that’s the term for all the vessels that transport blood to the heart, while those that carry blood from the heart to the rest of the body are called arteries. After a few twists and turns, you eventually end up in the superior vena cava, a vessel that empties directly into the heart. And it is into the heart’s right atrium that you are now swept, along with your cargo of carbon dioxide. From there, you pass into the right ventricle of the heart. Hurry now, don’t dawdle, we have a mission to complete!
To get from the heart’s right atrium to the right ventricle, you pass through an atrioventricular valve known to medics as the tricuspid valve (the Latin word cuspis means ‘point’ or ‘tip’). Once you have left the right atrium via that valve, there is no going back — if you are in a healthy heart. All the heart’s valves are unidirectional: they only let blood flow one way. This is a trusty means of making sure blood does not flow in the wrong direction, from the right ventricle back into the atrium. Thus, in a healthy heart, blood always only flows in one direction, and does not, for example, slosh back and forth between the ventricle and the atrium.
Continuing your journey, you leave the right ventricle via another valve — the pulmonary valve — heading towards the lungs. Having passed through that valve, you now find yourself in the pulmonary artery, the artery of the lung. This shows, by the way, that the much-quoted rule ‘arteries transport oxygenated blood and veins transport deoxygenated blood’ is in fact nonsense. After all, you’re still carrying your cargo of carbon dioxide, making you ‘deoxygenated’, although you are currently floating through an artery, not a vein. Once more for clarity, the more accurate rule is: arteries carry blood away from the heart, veins towards it (although there are still some small exceptions to this rule, e.g. in connection with the liver*).
On arrival in the lungs, you complete the first part of your mission as an RBC by unloading your carbon dioxide and taking on a fresh cargo, this time of oxygen. With that freight on board, you now set out on a return journey through the pulmonary vein (!) back towards the heart. There, you and your many fellow erythrocytes flow into the left atrium and on, through a third valve, into the left ventricle, the last ventricle on your voyage. The valve between the left ventricle and the left atrium is known as the bicuspid† or mitral valve, so called because its shape reminded anatomists of the kind of bishop’s hat known as a mitre.
The left ventricle is the bodybuilder among the chambers of the heart. It has by far the thickest muscle wall. This isn’t surprising, since it needs to build up a great deal of pressure to keep our blood constantly flowing and to pump it to even the furthest reaches of our body. Now, on we travel, through a final valve, the aortic valve, and into the aorta, the body’s main artery. This vessel describes a graceful curve around the heart, from which vessels branch off towards the head and the arms. It then continues into the abdomen, where it splits into ever-smaller branches to provide fresh blood to all our organs and tissues, right down to the tips of our toes.
We are now approaching the climax of our drama of the heart. Everything is working fine, the heart and vascular system seem to be indestructible. But things are about to take a tragic turn.
Act Four: the ailing heart
