Blood Candy
Here's a "sweet" way to learn about blood.
 
Objective:
To learn the four components of blood, and their relative amounts, by creating "blood" candy
 
Materials:
Candy red hots
Corn syrup
Marshmallows
Red candy sprinkles
Plastic spoons
Plastic bowls
Red and blue food coloring
 
Procedure:
Explain that blood has four components.  Plasma (55 percent of blood volume) is a thick, clear, yellowish liquid that carries dissolved food and wastes. Red blood cells (44 percent of blood volume) carry oxygen and carbon dioxide around the body.  Red blood cells are produced in the bone marrow.  White blood cells (0.5 percent of blood volume) are bigger than red blood cells and attack germs.  Platelets (0.5 percent of blood volume) are bits of cells that help blood clot.  
 
Create a blood model in the plastic bowl using red hots for red blood cells, corn syrup for plasma, marshmallows for white blood cells, and red candy sprinkles for platelets.  Emphasize the relative amounts of the blood components.  Let children experiment with different amounts to create their model.  This is also a great math lesson for older students.
 
Conclusion:
You have created a tasty blood model.  Use the spoons to sample your "blood."  There are five million red blood cells, ten thousand white blood cells, and two hundred fifty thousand platelets in a pinhead size drop of blood.  There are eight different blood types: A+, A-, B+, B-, AB+, AB-, O+, and O-.  O+ is the most common blood type.