Question

how is blood generated in our body

Answer 1

The bone marrow produces stem cells, the building blocks that the body uses to make the different blood cells – red cells, white cells and platelets

Answer 2

◆Hey There◆


1)Red blood cells,Platelets and White Blood cells are produced by Bone marrow.
The blood cells when young are called stem cells.Bone marrow continuously produces three types of blood cells(Red blood cells,Platelets,White Blood cells).

2)Plasma is made up of water and salts that we absorb through our digestive tracts everyday and it is a product of several organs of our body.
i)Albumin in the plasma is produced by the liver
ii)Antibodies in the plasma are a product of the immune system

So plasma is not produced by any specific organ.

99% of the blood cells floating in plasma are red blood cells,which carry oxygen from the lungs to the rest of the body and give blood its red color.

Note
Average life Span of Red blood cells is 4 months. 
Average life Span of White blood cells is 3-4 days.
Average life Span of Circulating platelets 8-9 days.