There are five liters of blood in the adult human body, which is about 7 percent of your total body weight. The entire blood supply makes a complete circuit of the heart, lungs, blood vessels, and back to the heart every 60 seconds.

There are four major blood types, O, A, B and AB. Nearly half the population is type O, and they can donate blood to people with all other blood types but can only receive type O blood. The rarest blood type, AB, is present in about 4 percent of the population. These people can receive all types of blood.

There are about 75 to 100 trillion cells in the body, about 25 trillion of which are red blood cells. An individual red blood cell contains about 250 million molecules of the iron-containing protein called hemoglobin, which is capable of picking up four molecules of oxygen. As a result, a single red blood cell can deliver up to 1 billion molecules of oxygen.

Laid end-to-end, the arteries, capillaries and veins in the average child would stretch for about 60,000 miles and, in the average adult, would be about 100,000 miles — enough to wrap around the world at least twice.