National Geographic Daily News

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.

Share