Platelets
Plateletsare very small, but they are very important in blood clotting. Platelets are not
cells they are sticky little pieces of larger cells. They bud off large cells that stay in the
bone marrow. A platelet sits between a RBC and a WBC inFigure 18.18. Platelets
carry chemicals that are important for proper blood clotting. When a blood vessel gets cut,
platelets stick to the injured areas. They release chemicals calledclotting factorswhich cause
a web of protein fibers to form. This web catches RBCs and forms a clot. This clot stops
more blood from leaving the body through the cut blood vessel. The clot also stops bacteria
from entering the body.Platelets survive in the blood for 10 days before they are removed
by the liver and spleen.
Figure 18.18: A platelet lies between a RBC, at left, and a WBC at right; platelets are little
pieces of larger cells, calledmegakaryocytes,which are found in the bone marrow. ( 20 )
Transport of Chemical Messages
The blood also acts as a messenger delivery service. Chemical messages calledhormonesare
carried and delivered by the blood to cells around the body. Hormones are released into the
blood by the cells that make them and are delivered by the blood to the cells the hormones
are made for. An example of a hormone transported in the blood is insulin, which regulates
the concentration of glucose in the blood.
Control of Body Temperature
Your blood system does more than deliver oxygen and nutrients to your body cells. Your
blood also moves heat (thermal energy) around your body. When your brain senses that