Why is the whites of the eyes blue?

What is the reason why the whites of the eyes turn blue?

The outermost layer of normal eyeball is white sclera tissue, and the inner layer of sclera is choroid, which is brownish black. Blue eyes are an early sign of iron deficiency anemia. Because the human sclera is composed of collagen tissue, and iron in the body is an important auxiliary factor of collagen synthesis, if the body lacks iron and collagen synthesis is insufficient, the sclera will become thinner and cannot effectively cover the brown-black choroid in the eyeball, and it will look pale blue under natural light. Some diseases also show the phenomenon of blue sclera. So go to the hospital in time, and the doctor will give you relevant examinations to find out the primary disease that causes scleral cyanosis and treat it according to the cause.