Each part has its own job. Red blood cells carry oxygen and food to all the parts of our body. They also give blood its color. White blood cells fight infections. Plasma is the fluid that carries blood cells to all parts of our body.
Platelets make blood clots.
When we get a cut or scrape, our platelets clump together. They make a plug to cover the hole and stop the bleeding. Sometimes children bleed too much because their blood isn't clotting properly. At other times, blood clots form when they shouldn't. That's the tree trunk.