Blood provides a mechanism for rapid transport of nutrients, waste products, respiratory gases, and cells. It is powered by the pumping action of the heart. The average human has 5L of blood in their body (7-8% of body weight). There are about ONE BILLION red blood cells in just 2-3 drops of blood. The pH level of blood is between 7.35-7.45. The composition of blood includes red blood cells, white blood cells, platelets, and plasma.