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Growth and Renewal
Humans are born with a set number of heart muscle cells, or cardiomyocytes, which increase in size as our heart grows larger during childhood development. Recent evidence suggests that there these cells actually are slowly turned over as we age, but that less than 50% of the cardiomyocytes we are born with are replaced during a normal life span. The growth of individual cardiomyocytes not only occurs during normal heart development, it also occurs in response to extensive exercise (athlete’s heart), heart disease, or heart muscle injury such as after a myocardial infarct. A healthy adult cardiomyocyte has a cylindrical shape and is approximately 100 μm long and 10-25μm in diameter. Cardiomyocyte hypertrophy occurs through sarcomerogensis, the creation of new sarcomere units in the cell. During heart volume overload, cardiomyocytes grow through eccentric hypertrophy where they extend lengthwise but have the same diameter resulting in ventricular dilation. During heart pressure overload, cardiomyocytes grow through concentric hypertrophy where they grow larger in diameter but have the same length resulting in heart wall thickening.