Inside the Heart  Photograph by Lennart Nilsson  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.