The Bible speaks of the importance of blood to the body
"For the life of a creature is in the blood, and I have given it to you to make atonement for yourselves on the altar; it is the blood that makes atonement for one's life." Leviticus 17: 11
The Bible is reliable in its understanding of the importance of blood to the life of the body - something modern science has only appreciated for a few centuries.
Up until the early 1800's, science believed that bloodletting was the best way to treat many illnesses. This draining of blood from the body was part of the "humoral system" of medicine, and it was believed that blood could stagnate in the body (and not circulate as we know it does today). This stagnant blood was believed to throw off the so-called balances in the body. This approach to medicine held sway in much of the ancient world including the Islamic nations, ancient Greece, and other regions of the world as well.
In fact, historians believe it was blood letting (through the use of leeches) that ultimately took George Washington's life in 1799, as his attending physicians drained as much as 4 lbs. of blood from his body. Ultimately, George Washington's doctors bled him to death not understanding how critical blood is to the body's health.
They were really without excuse however because William Harvey (who was the personal doctor of both King James I and King Charles I) had discovered in 1616 AD the blood circulatory system and its essential nature to the life of our bodies.
What does Scripture have to say about blood (remember, this passage was wriiten 3400 years ago)?
"For the life of the flesh is in the blood" Leviticus 17: 11a
(This illustration comes from Alcohol Health & Research World, Vol. 14, No. 4, 1990. Note that the heart has been split into two halves to better illustrate each side's functions)