There are about 5,000 ml of blood in a person’s body, and blood plays a very important role in the human body. The blood contains red blood cells, white blood cells, platelets and other substances. The body’s red blood cells carry oxygen to the brain and all cells of the body. In addition, the blood contains nutrients that the body absorbs from the digestive tract and transports to various locations, parts and cells of the body through the blood circulation. A person’s blood supplies the needs of the body, and a person’s red blood cells are renewed once every three months. If you donate about 200-400ml of blood, it basically has no effect on your body, because human cells are constantly being renewed.