Blood is a continuously circulating fluid that supplies the body with nutrients, oxygen, and waste removal. Blood is mostly liquid, with numerous cells and proteins suspended in it, making blood "thick" than pure water. The average person has about 5 liters (more than a gallon) of blood.
A liquid called plasma makes up half of the blood's contents. Plasma contains proteins that help blood clot, transport substances through the blood, and perform other functions. Blood plasma also contains glucose and other dissolved nutrients.
About half of blood volume consists of blood cells:
• Red blood cells, which carry oxygen to tissues.
• White blood cells, which fight infection.
Platelets are tiny cells that help blood clot.
Blood is carried through blood vessels (arteries and veins). Blood is prevented from clotting in blood vessels by their smoothness, and a fine balance of clotting factors.
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