So, I’m intending that this blog should be a basic course in the heart and how it adapts to exercise. It’s fair to say that this is something of a complex topic, and there is much controversy and debate. We have not got it all worked out by a long way, and I reserve the right to completely change my mind about things over time. That is the nature of medicine.
The heart is only one part of the cardiovascular system which helps you exercise. It includes the heart, the lungs, the blood, the muscles and of course the brain. My PhD touched on how the nerves and brain react to exercise. It was 3 years of pure frustration, as it turned out to be quite hard.
The first thing to understand are some basics of the heart itself. You can skip this if you already know all about it, but I will assume some sort of basic knowledge in future posts. You will have covered this in GCSE biology, or for those of us who are old and decrepit enough, O level biology.
Have a look at this picture.
Blood comes back from the body having been “used” into the right atrium via the superior vena cava (from the arms, chest and head) and the inferior vena cava (from the rest of the body). It’s important to realise that there is vast variation in the precise distribution. The right atrium is a thin walled chamber which discharges its blood into the right ventricle via the tricuspid valve. Valves are thin-walled structures that are elegantly complex pieces of engineering that keep blood going in one direction through the heart. People write entire books on valve structure and function. Valve disease can be important when considering exercise. 1-2% of the population have a “bicuspid” aortic valve, and exercise can accelerate it narrowing – it then usually requires replacement which is a major operation.
The right ventricle is the pumping chamber that forces blood into the lungs. The system is at low pressure, and the muscles of the right ventricle aren’t as well developed as those on the left side of the heart – the right ventricle is a bit thinner, and probably a bit more vulnerable to stress. This is thought to be important in the development of conditions such as ARVC (arrhythmogenic right ventricular cardiomyopathy) – more on this later. Blood flows out of the right ventricle and into the pulmonary arteries and the lungs themselves.
Once all the business of gas exchange has been done in the lungs (oxygen in, carbon dioxide out) then the blood comes back via the pulmonary veins into the left atrium. During most of my training the pulmonary veins were possibly one of the least exciting anatomical structures you had to learn about, but then it was discovered that AF starts there quite often, and now many people spend entire lives studying them.
The left ventricle is considered to be the main structure of the heart. Blood enters this from the left atrium via the mitral valve. It leaves the left ventricle via the aortic valve and passes into the aorta and from there around the body. Cardiology has traditionally focused on the left ventricle and its function.
One of the key concepts when understanding how the left ventricle works is the “ejection fraction”. This is the proportion of blood ejected from the ventricle each time it beats. Cardiologists, and patients, get very hung up on this measure, and it is important, but there are important limitations. The normal volume of the left ventricle is about 140ml in a typically sized person. Typically, around 2/3rds of the blood is ejected each time the heart beats (about 90ml). Anything over 55% or so is considered normal. But really, what the body needs is not for the left ventricle to have a particular ejection fraction, what the body needs is enough oxygen delivered to the tissues. That depends on how much blood is pumped around the system each minute and how much oxygen it is carrying. Doctors often forget this. I’ll talk more about this in another post. Disease of the heart muscle is termed cardiomyopathy. When people talk about “heart failure” they are usually referring to disease affecting the muscle of the left ventricle. Exercise, particularly endurance exercise can affect the heart muscle in both good and bad ways.
The aorta branches many times when passing blood around the body. The first are the coronary arteries. In most people, there is a right coronary artery and a left coronary artery – the first part of the left coronary artery (the left main stem) branches early into the left anterior descending artery and the circumflex. Narrowings in these arteries cause angina – a pressure or discomfort on exertion (usually) felt across the chest. A sudden blockage can cause a heart attack. There is a lot of debate about whether or not exercise causes changes in the arteries, and whether or not those changes are harmful. Again, a subject for a future post.
So that is the whirlwind tour of the heart structure. In future posts I will touch on valve disease, diseases of the heart muscle and coronary artery disease. Then having finished with the plumbing I will move onto the electrics. It will become clear that in fact as doctors we probably know less than you think. The world of sports medicine is in its infancy really.