The volume of blood in a normal person is 7-8% of body weight. The blood volume of an average person is about 4000-5000ml. a person weighing 60kg has a blood volume of about 4200-4800ml. most of the blood is circulated in the cardiovascular system, which is called circulating blood, and some of the blood is retained in the liver, lungs, and subcutaneous area, which flows slowly. this blood is called stored blood. Blood is made up of two parts: plasma and blood cells, which are divided into three categories: red blood cells, white blood cells and platelets. Blood is red because red blood cells contain hemoglobin, which can carry oxygen, so blood can transport oxygen.