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The human circulatory system is over 60,000 miles long

Human hearts have a big job: moving oxygen and nutrients throughout our bodies. But as much credit as the heart gets, it doesn’t work alone — the adult human circulatory system includes arteries, veins, and capillaries in a network that’s more than 60,000 miles long

In terms of distance, the blood vessels in our bodies are lengthy enough to circle the globe twice, with mileage to spare. What’s more, about 80% of that distance comes from just capillaries, the smallest blood vessels that connect veins and arteries. With each heartbeat, the circulatory system is a multifunctioning wonder, working simultaneously to oxygenate blood, remove waste from our organs, and transport hormones and nutrients to their necessary destinations. Meanwhile, this system also stabilizes our bodies by helping to fight off disease and regulate body temperature.

Not all living creatures have circulatory systems, and among those that do, they can look drastically different. Vertebrates — mammals, amphibians, reptiles, and birds — have “closed” circulatory systems, meaning that blood is transported through the body sealed within arteries and veins. Invertebrates (think snails, crabs, and octopuses) have “open” systems with no veins, where blood flows freely throughout the entire body cavity and is directly absorbed by the organs. And some animals, such as jellyfish, anemones, and corals, have no blood or circulatory systems at all, instead relying on the water they live in to supply the oxygen and nutrients they need.

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