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.
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.