Tissue-paper thin but tough, the valves of
the human heart open and close to pump 6 quarts (0.9 liters) of blood a day
through 60,000 miles (97,000 kilometers) of vessels. That’s equivalent to 20
treks across the United States from coast to coast. The heart is a magnificent
machine when it’s in good working order. But coronary heart disease is the
number one killer of American men as well as women, resulting in 500,000 deaths
in the United States and killing 7.2 million people worldwide each year.