I need to start with a disclaimer – I’m not a doctor and I don’t have medical training, so if there’s anything here in this post which you think you may want to apply to your own life or health, please make sure you check with a medical doctor first. As a runner, my heart is about something I’ve often wondered about when I’ve been out on the fells. When it’s just you, nature, and your heart pumping ten to the dozen, your mind drifts off to questions like ‘why the heart doesn’t get tired in the same way as your other muscles?’ This post is about what I found out whilst trying to answer that question.
According to the experts, the cardiac muscle is fundamentally different to skeletal muscle. There are certain contractile proteins which are similar, however, in terms of energy production and consumption, heart muscle cells (cardiomyocytes) are quite different to other muscle cells.
The main reason why cardiomyocytes are so resistant to fatigue is because they contain almost twice the amount of mitochondria (an organelle found in large numbers in most cells, in which the biochemical processes of respiration and energy production occur.)
The heart is also very metabolically flexible in terms of fuel – it can consume glucose, free fatty acid and lactate – which is especially pronounced at high exercise intensities.
Cardiomyocytes also have a better blood supply than other muscle cells, which coupled with the extra mitochondria, mean they are incredibly good at extracting oxygen from the blood and using it for aerobic respiration. Even at rest, the heart muscle extracts most usable oxygen from your blood – this means the only way it can improve oxygen delivery is by increasing blood flow as oxygen extraction is already so good.
If you’re wondering what would happen if we replaced all the muscle in the body with cardiac muscle, it probably wouldn’t turn out too well for you for several reasons. Besides the massive increase in daily energetic needs, cardiomycytes communicate through intercalated discs (unique structural formations found between the myocardial cells of the heart. They play vital roles in bonding cardiac muscle cells together and in transmitting signals between cells) to beat synchronously and can do this even without their nerve supply. The rest of the muscles in your body are isolated from each other which how they move effectively – imagine if a movement in one muscle triggered a contraction in all the others without nervous system input.
Finally, to quash a myth about hearts; when I was young, many people believed that a heart had a certain number of beats available. This explained why smaller mammals with faster heart beats such a mice had shorter life spans that larger animals with slower heart beats like elephants. This is simply not true. In fact, ‘saving your heartbeats’ may be a very bad idea for most of us who are in other ways healthy. Increasing your heart rate during exercise, over time, improves your hearts ability to pump blood around the body by increasing the amount of blood pumped per beat (eccentric hypertrophy). Regular cardiovascular exercise also increases vagal tone (relating to the the vagus nerve which is what slows your heart rate) meaning that in combination with increased stroke volume, your resting heart rate improves (slows). Low resting heart rates, in combination with good cardiorespiratory fitness (VO2max), have been shown to significantly lower your risks of heart disease and many other health related causes of mortality.