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Blood is made up of plasma, red blood cells, white blood cells and platelets.
Plasma
More than half of blood is comprised of plasma: a clear, yellow liquid which is mostly water. The plasma contains proteins and other substances that make the blood coagulate when the skin is cut.
During the blood's journey through the body (circulation), the plasma carries with it the nourishment that the body needs and removes any waste that the body wants to get rid of.
Red Blood Cells
There are milions of red blood cells floating around in the plasma. These carry oxygen from the lungs to all parts of the body and bring back carbon dioxide to the lungs, where it is exhaled (even the plasma carries some carbon dioxide). Red blood cells are created in bone marrow and only last for a few months, so the marrow continuously makes new cells.
White Blood Cells
White blood cells are the body's soldiers. Whenever dangerous bacteria (the enemy) enters the body, the white blood cells rush there, surround the bacteria and kill them. New white blood cells, mostly created in the bone marrow, replace those that die.
Platelets
Platelets prevent too much blood from running out, through a cut for example. Gathering at a wound, platelets bind with substances in plasma and form a layer that seals the wound. This layer dries to become a crust (a scab). Under the scab, your skin heals and after a while it falls off.
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