The only muscle in our body that never rests. What an extraordinary phenomenon when you think about it. It beats for the majority of people, more than once per second on average, throughout our lives. At 1.16 beats on average per second, more than 100,000 per day, this is equivalent to almost 40 million beats per year and 3 billion beats a lifetime. Just try to vigorously close your hand 100 times for a minute. Now think about doing it for 85 years. You would have a beautiful forearm.

We have already talked about the importance of the different facets of healthy lifestyle habits on cardiovascular health (February is heart month – let’s talk about health!) And the possible benefits of supplements (For love of the heart). Here I will tell you about my passion, the biology of life.

The heart is used to circulate nutrients, breathing gases, hormones, waste, dissipate and regulate temperature, as well as to carry the defenses of the organism to the places that need it most. The first heart would have appeared, from an evolutionary point of view, 500 million years ago. It was simply a contractile tube which later evolved into an organ with four closed chambers (for mammals). Amphibians, like frogs, still have a heart with a single ventricle, but two atria. The hearts of fish are also different.

The heart and its functions have evolved to become very specific to each living species. The great blue whale, the largest of the mammals, has a heart rate of about 20 beats per minute. In mice, this can reach 500 to 600 beats per minute. An elephant, 30 beats per minute. So yes, the bigger the animal, the less quickly it will pump. There are some exceptions for hibernating animals. Their hearts can go as low as between 10 and one beat per minute during hibernation. The record goes to a Canadian frog that has developed a remarkable adaptation to the cold. The cruciferous tree frog (Pseudacris crucifer for scientific friends) is able to survive below zero during a period when its heart completely stops beating. This is possible thanks to two adaptations: its liver produces glycogen which serves as an antifreeze (up to -7 degrees Celsius) and these cells then use an anaerobic metabolism which does not require oxygen.

In humans, the heart is approximately 13 centimeters long by 8 centimeters wide. The two sides of the heart work independently of each other. The right side receives the blood loaded with CO2 and sends it to the lungs to evacuate and be re-oxygenated. The left side then receives this freshly oxygenated blood to send it to the rest of the body. Since the heart is close to the lungs, the work of the right ventricle is less demanding than that of the left ventricle which must push blood from the feet to the head. Thus, the walls of the left ventricle are three times thicker than that of the right ventricle. Here is a link that allows you to see an animation of the working heart on the Heart and Stroke Foundation website: functioning of the heart.

As for aging, unfortunately, it ages on the same principles as other organs of the human body. As we age, some cells become dysfunctional and are not replaced as they should have been. They become senescent and they malfunction. Other cells die and are not replaced either. These spaces left without cells will then be occupied by cells producing collagen fibers (scars): fibroblasts. Calcification (hardening) and fatty deposits also appear in certain parts of the heart. During this process, the number of muscle cells, myocytes, decreases and more markedly in men than in women. The heart therefore works less and less well, it tires more easily and the valves are less functional.

There is still good news! Recent studies have clearly shown that the aging processes of the heart evolve at the same speed as the aging of the rest of the body. They are not related to chronological age, but rather to biological age. Biological age is dependent on how we live, not on the year of our birth. Thus, the age of our heart would be representative of the way we live and will encompass all of our lifestyle habits. It is therefore possible to keep our heart healthier longer, to allow it to continue its hard work which consists in keeping us alive.

 

References:

  • Keller KM, Howlett SE. Sex Differences in the Biology and Pathology of the Aging Heart. Can J Cardiol. 2016;32(9):1065–1073.
  • Website of Heart and Stroke Foundation of Canada : https://www.heartandstroke.ca/
  • Stephenson A, Adams JW, Vaccarezza M. 2017. The vertebrate heart: an evolutionary perspective. J Anat. 2017;231(6):787–797.