Medical Definition of blood
1: the fluid that circulates in the heart, arteries, capillaries, and veins of a vertebrate animal carrying nourishment and oxygen to and bringing away waste products from all parts of the body
2: a fluid of an invertebrate comparable to blood
3: blood regarded in medieval physiology as one of the four humors and believed to be the seat of the emotions
4: descent from parents of recognized breed or pedigree
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