You have probably seen blood in a film or filmstrip about blood, or even some fresh blood. But have you ever seen some preserved blood on a microscope slide?
Blood is a liquid with some small solid objects in it. The red blood cells carry oxygen. The white blood cells help the body to fight against infection and diseases. When the skin is cut, the platelets help to form a clot. This stops further bleeding.
The liquid part of the blood is called plasma. Plasma is water with lots of things dissolved in it, for example glucose.
The pump of you heart makes blood run in your blood vessels. One of the most important things that blood does is to carry things around the body. For example, it carries oxygen and glucose to the parts where they are needed. Blood also carries waste products away from the places where they are produced. This is why the blood and the blood vessels are called a transport system.