An adult human has about 4–6 liters of blood circulating in the body. Among other things, blood transports oxygen to various parts of the body.
Blood consists of several types of cells floating around in a fluid called plasma.
The red blood cells contain hemoglobin, a protein that binds oxygen. Red blood cells transport oxygen to, and remove carbon dioxide from, the body tissues.
The white blood cells fight infection.
The platelets help the blood to clot, if you get a wound for example
The plasma contains salts and various kinds of proteins.