The heart is the hardest working muscle in the human body, pumping out oxygenated, nutrient-rich blood with every beat. But no matter how hard this muscle works each second, it still needs exercise to stay strong.
SciGirls Connect
The heart is the hardest working muscle in the human body, pumping out oxygenated, nutrient-rich blood with every beat. But no matter how hard this muscle works each second, it still needs exercise to stay strong.