The word “blood” comes from Old English blod, derived from Proto-Germanic blodam.

Your blood makes up about 7–8% of your body weight. Men typically have 5–6 liters in their bodies, while women have 4–5 liters. Your blood plasma, however, is composed mostly of water.

At any given time, you also have about 25 trillion red blood cells.

Blood is technically always red; its shade (darker or lighter) depends on oxygen levels, with highly oxygenated blood appearing bright red.

Dr. James Blundell performed the first successful human blood transfusion in 1818.

If your blood vessels were laid end to end, they would wrap around the Earth 2.5 times.

Red blood cells are extremely flexible discs, able to squeeze into spaces narrower than a human hair.

Red blood cells only live for about four months.

If you have blood type O, mosquitoes are about 80% more likely to bite you. They do not feed on blood for themselves; females require it to produce their eggs.

Pregnant women experience about a 50% increase in blood plasma volume.

Roughly 118 million blood donations are collected each year, saving millions of lives. However, stored donations last only 42 days.

The first official blood bank was established in Chicago in 1937.

A single drop of blood contains about 5 million red blood cells.Your blood is not actually thicker during cold months; rather, your vessels become constricted in colder weather, creating that sensation.

Antarctic icefish actually have clear blood because they lack hemoglobin.

Blood smells metallic because of its high iron content. Some animals, such as octopuses and horseshoe crabs, have blood rich in copper.

Rh-null is the rarest blood type in the world. Fewer than 100 people are known to have it globally, and it is called “Golden Blood.”

Leeches are still used in modern medicine to improve blood flow after surgery.

Karl Landsteiner discovered blood types in 1901.

The American military once shamefully and incorrectly segregated blood by race when collecting and distributing donations.

Over an average lifetime of 70 years, a human heart pumps about 1.5 million barrels of blood.

People do not usually faint from the pain or fear of blood donation, but from a reflex called vasovagal syncope—essentially the body “shutting down” temporarily. Sugary snacks, like cookies, help tremendously.

Warm-blooded and cold-blooded animals are somewhat misnamed. Cold-blooded creatures adopt the temperature of their environment, while warm-blooded creatures maintain a stable internal temperature regardless of climate. Cold-blooded species, however, require far less food than their warm-blooded counterparts.