Blood is a fluid that transports oxygen from the lungs to body tissue and carbon dioxide from body tissue to the lungs. Blood also transports nourishment from digestion and hormones from glands to all areas of the body. Blood also conveys disease fighting substances to the tissues and waste to the kidneys. Blood is thicker than water and has a slightly salty taste. An adult's body usually has 10 pints of blood in circulation. Blood is composed of billions of living blood cells floating in a liquid called plasma. If you took a small sample of this blood and poured it into a test tube and then put it in a machine called a centrifuge, you would be able to see the layers of this blood. This machine spins the blood around so fast that it separates the red blood cells, from the white blood cells, from the platelets. The red blood cells sink to the bottom because they are the heavier, more solid parts, but the plasma remains at the top because it is lighter. Plasma is 95% water and the other 5% is made up of dissolved substances including salts.
Origins of blood in the body
All of the cellular elements of blood, including the red blood cells that transport oxygen, the platelets that trigger blood clotting in damaged tissues, and white blood cells of the immune system, derive ultimately from the same progenitor or precursor cells, the hematopoietic stem cells in the bone marrow. As these stem cells can give rise to all of the different types of blood cells they are often known as pluripotent hematopoirtic stem cells. Initially they give rise to stem cells of more limited potential, which are the immediate progenitors of red blood cells, platelets, and the two main categories of white blood cells.
Blood in the Bible
After the Great Flood, God commanded Noah as follows:
But flesh with the life thereof, [which is] the blood thereof, shall ye not eat. And surely your blood of your lives will I require; at the hand of every beast will I require it, and at the hand of man; at the hand of every man's brother will I require the life of man. Whoso sheddeth man's blood, by man shall his blood be shed: for in the image of God made he man. Genesis 9:4-6 (KJV)
This commandment was part of the Noahide laws rather than the Mosaic law and was thus binding on all people.
The Council of Jerusalem, as recorded in the Book of Acts in the New Testament, reaffirmed the prohibition on eating blood:
But that we write unto them, that they abstain from pollutions of idols, and [from] fornication, and [from] things strangled, and [from] blood. Acts 15:20 (KJV)
For it seemed good to the Holy Ghost, and to us, to lay upon you no greater burden than these necessary things; That ye abstain from meats offered to idols, and from blood, and from things strangled, and from fornication: from which if ye keep yourselves, ye shall do well. Fare ye well. Acts 15:28-29 (KJV)
Jehovah's Witnesses believe that blood transfusions are forbidden by God in the above passages from Acts.
The shedding of blood is also necessary for the remission of sin:
And almost all things are by the law purged with blood; and without shedding of blood is no remission. Hebrews 9:22 (KJV)
In the Old Testament, this meant animal sacrifices. In the New Testament, it means the Blood of Christ.