Quoting from "How We Die" (1) by the late Sherwin B. Nuland:
the muscle cell of the heart is one of those that cannot reproduce — as it gets older, it simply wears out and dies. The biological processes that throughout life have been making replacement parts for dying structures within each cell can no longer do their job. The mechanism by which a newly generated piece of cell membrane or intracellular structure can take the place of a section dead of too much use finally becomes inoperative. After a lifetime of regenerating spare parts, the nerve and muscle cells' capacity of rejuvenation gradually shuts down. The tactic of continuous renewal within each heart-muscle cell is then defeated by the overwhelming strategy through which aging is achieving its ultimate objective of destruction.
There is a gradual decrease in the cardiac output while at rest, and when the heart is stressed by exercise or emotion, its ability to increase is less than required by the needs of arms, lungs, and every other structure of the body. The maximal rate attainable by a perfectly healthy heart falls by one beat every year, a figure' so reliable that it can be determined by subtracting age from 220. If you are fifty years old, it is unlikely that your heart can manage much more than 170 beats per minute, even under the most extreme conditions of emotion or exercise. These are only some of the ways in which the aging and stiffening myocardium loses its ability to adapt to the challenges presented to it by everyday life.
the muscle cell of the heart is one of those that cannot reproduce — as it gets older, it simply wears out and dies. The biological processes that throughout life have been making replacement parts for dying structures within each cell can no longer do their job. The mechanism by which a newly generated piece of cell membrane or intracellular structure can take the place of a section dead of too much use finally becomes inoperative. After a lifetime of regenerating spare parts, the nerve and muscle cells' capacity of rejuvenation gradually shuts down. The tactic of continuous renewal within each heart-muscle cell is then defeated by the overwhelming strategy through which aging is achieving its ultimate objective of destruction.
There is a gradual decrease in the cardiac output while at rest, and when the heart is stressed by exercise or emotion, its ability to increase is less than required by the needs of arms, lungs, and every other structure of the body. The maximal rate attainable by a perfectly healthy heart falls by one beat every year, a figure' so reliable that it can be determined by subtracting age from 220. If you are fifty years old, it is unlikely that your heart can manage much more than 170 beats per minute, even under the most extreme conditions of emotion or exercise. These are only some of the ways in which the aging and stiffening myocardium loses its ability to adapt to the challenges presented to it by everyday life.
1. https://archive.org/stream/HowWeDie/How%20We%20Die_djvu.txt