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(1) The circulating fluid (plasma) and suspended formed elements, such as red blood cells, white blood cells and platelets in the vascular system of humans and other vertebrates.

(2) A functionally similar fluid flowing in the bodies of many animals (e.g. hemolymph in insects).


Blood is the circulating fluid in the body of eukaryotic animals. In humans and other vertebrates, the blood is pumped through the body by the heart, and appears bright-red when hemoglobin is oxygenated. Its major functions include the following:

A mammalian blood is comprised typically of 55% plasma and 45% cellular elements (blood cells and platelets). The plasma contains water, ions, plasma proteins, nutrients, respiratory gases, hormones, and metabolic byproducts. The cellular elements are comprised of red blood cells, leukocytes, and platelets.

Word origin: Old English blōd, Proto-Germanic *blodam, of uncertain origin

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