Blood is a constantly circulating fluid providing the body with
nutrition, oxygen, and waste removal. Blood is mostly liquid,
with numerous cells and proteins suspended in it, making blood
"thicker" than pure water. The average person has about 5 liters
(more than a gallon) of blood. A liquid called plasma makes up
about half of the content of blood. Plasma contains proteins
that help blood to clot, transport substances through the blood,
and perform other functions. Blood plasma also contains glucose
and other dissolved nutrients. About half of blood volume is
composed of blood cells: • Red blood cells, which carry oxygen
to the tissues • White blood cells, which fight infections •
Platelets, smaller cells that help blood to clot Blood is
conducted through blood vessels (arteries and veins). Blood is
prevented from clotting in the blood vessels by their
smoothness, and the finely tuned balance of clotting factors.