journey of blood around your body takes about fifty seconds to complete. Curiously, the blood passing through the chambers of the heart does nothing for the heart itself. The oxygen that nourishes it arrives via the coronary arteries, in exactly the way oxygen reaches other organs. The two phases of a heartbeat are known as the systole (when the heart contracts and pushes blood out into the body) and diastole (when it relaxes and refills). The difference between these two is your blood pressure. The two numbers in a blood pressure reading—let’s say 120/80, or “120 over 80” when spoken—simply
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