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Blood cells running through a capillary.

Blood is like a mailman, because it delivers important things, like oxygen and food, to the cells of our bodies, like a mailman delivers mail to houses.
A mailman also picks up mail from our houses, to be delivered at other places.  Likewise, our blood picks up carbon dioxide and other waste products from the cells, and delivers them to the kidneys, liver, and lungs.

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Blood cells on an alveolar wall.

Adults have 5-6 quarts of blood.  Blood is made up of red cells, white cells, platelets, and plasma.  White cells are like an army, fighting off enemies.  Red cells are bright red when they are full of oxygen, and red-blue when they are full of carbon dioxide.

I copied these pictures from page 22 of a book called "The Circulatory System" and page 27 of a book called "The Respiratory System," both written by Darlene R. Stille.

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