Sex Matters: A Comprehensive Comparison of Female and Male Hearts

Figures

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  Figure 1. Sex differences in the healthy human heart across life. At birth, the female heart is +5% larger than the male heart (Altman and Dittmer, 1962); during adulthood, the female heart becomes −26% smaller than the male heart (Molina and DiMaio, 2012, 2015); in the elderly, differences become less pronounced and the female heart is −4% smaller than the male heart (Sheikhazadi et al., 2010). Mouse (Slawson et al., 1998) and whale (Race et al., 1959) hearts are shown for comparison.

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  Figure 2. Sex differences in healthy heart size. The whole-heart view compares an average male human heart (Zygote Media Group Inc., 2014), shown in red, to an isometrically scaled down female heart, which is −26% smaller in mass. Based on isometric scaling, the female ventricular wall thickness and its ventricular and atrial diameters would be −9.0% smaller than their male counterparts.

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  Figure 3. Physiological adaptation in endurance and resistance athletes. According to Morganroth's hypothesis, endurance athletes tend to develop eccentric hypertrophy associated with ventricular dilation caused by volume overload, while the wall thickness remains relatively constant. Resistance athletes tend to develop concentric hypertrophy associated with ventricular wall thickening caused by pressure overload, while the chamber size remains relatively constant.

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  Figure 4. Physiological adaptation to volume and pressure overload. Volume overload induced by endurance training causes primarily eccentric growth associated with ventricular dilation, while pressure overload induced by resistance training causes primarily concentric growth associated with ventricular wall thickening. Simulations allow us to probe the effects of endurance and resistance training on muscle fiber length and thickness and on the geometry of the heart as a whole. Blue colors indicate the baseline fiber length and thickness; yellow colors indicate a fiber lengthening and thickening of up to 40%.

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  Figure 5. Physiological adaptation to dynamic and static exercise and training duration and intensity. According to Mitchell's classifications, left, dynamic exercise is associated with an increase in cardiac output, while static exercise is associated with an increase in blood pressure. According to Beaudry's classifications, right, the degree of cardiac adaptation depends on the duration and intensity of training.

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  Figure 6. Sex differences in short-term fiber stretch and elastic response. Simulated short-term fiber stretch for male and female hearts, left and right, subject to the stroke volumes of healthy and athlete's hearts, top and bottom. In the healthy heart, simulated fiber stretches remain well within the physiological regime of 1.10 to 1.15, with a few local peaks on the order of 1.20. In the athlete's heart, simulated fiber stretches would exceed the physiological regime and reach values of 1.20 to 1.30, with almost the entire heart experiencing stretches above 1.20. Volume overload on the whole heart level induces an overstretch on the cellular level and triggers physiological adaptation in the athlete's heart. Male and female hearts respond similarly under both healthy and athletic conditions.

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  Figure 7. Sex differences in long-term fiber lengthening and adaptive growth. Simulated long-term fiber stretch for male and female hearts, left and right, subject to the stroke volumes of healthy and athlete's hearts, top and bottom. In the healthy heart, fiber stretches remain well within the physiological regime of 1.10 to 1.15, with a few local peaks on the order of 1.20. In the athlete's heart, chronically elevated fiber stretches trigger a chronic fiber lengthening of up to 1.50. Volume overload on the whole heart level induces an overstretch on the cellular level and triggers physiological adaptation in the athlete's heart. Male and female hearts respond similarly under both healthy and athletic conditions.

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  Figure 8. Differentiating physiological from pathological adaptation. Wall thickness and cavity size are diagnostic criteria for which a subset of physiologically remodeled athlete's hearts overlaps with pathologically remodeled dilated and hypertrophic cardiomyopathy hearts, left. Relative wall thickness and sex-specific left ventricular mass are criteria which differentiate normal geometry from hypertrophic, even in the case of extreme athletic adaptation, right.