Blood is the living fluid that all life is based on. This blood is composed of 60% liquid part and 40% solid part. The liquid part called Plasma is made up of 90% water and the remaining 10% nutrients, hormones, etc. is easily replenished by food, medicines, etc. But the solid part that contains RBC (red blood cell), WBC (white blood cell) and Platelets take valuable time to be generated.
On the other hand, blood cannot be harvested but it can be collected from a donor. This means you can save a life by donating blood in need.