Not your type?

The four basic blood groups or types, in order of frequency from most common to rarest, are O, A, B, and AB. Blood type is determined by "alleles," or possible types of a gene, that we inherit from our parents.

The different blood types reflect the possible combinations of protein molecules called antigens, which are found on the surface of the red blood cells, and antibodies, which are in the plasma.

Just as when a disease invades the body, antibodies in the blood will attack certain antigens. This means that not all human blood is compatible.

If someone were to be given a transfusion with blood that contains antibodies that are hostile to antigens in his own blood, for example, he might die from a reaction that causes red blood cells to "clump," clogging blood vessels, or to "crack," leaking hemoglobin into the body with toxic effects.

O-negative blood lacks antigens, so people with O-negative blood have been considered "universal donors," whose blood would work harmlessly in anyone's body. It turns out that even some O-negative blood can react with some rare blood types, so the concept of the "universal donor" is now a conditional one, even though O-negative blood will still be given in an emergency if a patient's blood type is not known.

Conversely, people with AB-positive blood in general can receive any type of blood because that type does not contain antibodies that attack the A or the B antigens. (Type O blood lacks those antigens.)

The "plus" and "minus" in blood types refers to a particular antigen called "the Rh factor." Anyone can receive blood without the Rh factor, but only people with the Rh factor can safely receive blood that contains it.

If a pregnant woman needs a blood transfusion during or after labor — rare but possible — she will receive only blood that is compatible with her own — ideally her own specific blood type.

Red river

Blood is actually connective tissue, the only liquid type in the human body.

How much blood we have depends on how big we are — blood accounts for about 8 percent of body weight, on average about five quarts (roughly five liters). More than half of blood is plasma, a yellowish fluid that itself is mostly water.Blood splatter

Plasma carries all the things the cells need when it begins its journey out from the heart. The bulk of its cargo is the 25 trillion red blood cells filled with oxygen, but it also carries infection-fighting white blood cells and platelets that will clot the blood when needed, as well as vitamins, electrolytes, hormones and other materials.

Animals that use a protein called hemoglobin to store oxygen have bright red blood when it's fully oxygenated, because hemoglobin contains iron. (Spider blood, for example, contains copper-rich hemocyanin, and is blue when oxygenated.)

Red blood cells are manufactured in bone marrow, and circulate in the blood for about four months. They look like fat plates, flat but curved, a shape that allows them to squeeze into capillaries. They have no nucleus, devoting as much space as they can to hemoglobin.

One entire circulation of the blood through the body of the average resting adult, given that five quarts of blood, takes about one minute. Without the life-giving oxygen it carries, the most vulnerable cells, including those in the brain, would begin to die within about five minutes, and organs would start shutting down just a few minutes later.

Image from Wikimedia Commons