There's enough blood in the human body to fill a bit more than a 1-gallon milk jug.
The average adult has about 1.2 to 1.5 gallons (4.5 to 5.5 liters) of blood circulating inside their body, said Dr. Daniel Landau, a hematologist and oncologist at the University of Florida Cancer Center – Orlando Health.
If you had no blood, you'd weigh 8 to 10 percent less. (Of course, you wouldn't be alive, either.) So, for example, in a person weighing 120 lbs. (54 kilograms), blood takes up about 9.6 to 12 lbs. (4.4 to 5.4 kg). [Top 10 Amazing Facts About Your Heart]
By the time they're 5 or 6 years old, children have about the same amount of blood as adults do. But because children are smaller and their bones, muscles and organs don't weigh as much, their blood makes up a larger percentage of their body weight than it does in adults, Landau said.
In comparison, newborn babies have barely any blood. A newborn baby weighing between 5 and 8 lbs. (2.3 to 3.6 kg) has only about 1 cup (0.2 liters) of blood in their body, he added.
That's about the same amount of blood as a 10-lb. (4.5 kg) cat has in its body, said Dr. Greg Nelson, a veterinarian with Central Veterinary Associates in Valley Stream, New York. Dogs have slightly more blood (about 86 milliliters per kilogram, compared with about 55 milliliters per kilogram in cats), meaning that an 80-lb. (36 kg) dog has 0.8 gallons (3 liters) of blood, Nelson said.
When adults donate blood, health care workers take 1 pint (about half a liter), Landau said. Blood cells have a life span of about 120 days, and the body constantly makes new red blood cells in the bone marrow. But it still takes time to regenerate these cells, so you can't donate blood every day.
"That's the reason you can only donate so often — because you're waiting for the blood to recover, [which] typically [takes] about four to six weeks," Landau told Live Science.
In adults, blood contains about 0.8 gallons (3 liters) of plasma, red blood cells, white bloods cells and platelets. Vitamins, electrolytes and other nutrients are dissolved in the blood, and are carried to the body's cells and organs.
For instance, gold makes up about 0.02 percent of human blood.
"The joke is, if you're trying to sell the gold in your blood, you actually need about 40,000 people's blood in order to have enough gold to sell," Landau said.
But those 40,000 people would yield only about 8 ounces (28 grams) of gold — not enough to make anyone rich, he said.
Iron is far more plentiful in the blood. This element helps red blood cells keep their circular shape, explaining why adults have about 0.11 to 0.14 ounces (3 to 4 grams) of iron floating around in their blood, he said.
Additional reporting by Andrea Thompson. Follow Life's Little Mysteries on Twitter @llmysteries. We're also on Facebook & Google+.