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Human kidneys will clean about 1 million gallons of blood in an average lifetime.
It is believed that the Incas used anesthesia. Evidence supports the theory that shamans chewed cocoa leaves and drilled holes into the heads of patients (letting evil spirits escape), spitting into the wounds they made. The mixture of cocaine, saliva, and resin numbed the site enough to allow hours of drilling.
Intradermal injections are somewhat difficult to correctly administer because the skin layers are so thin that it is easy to accidentally punch through to the deeper subcutaneous layer.
Blood is approximately twice as thick as water because of the cells and other components found in it.
Urine turns bright yellow if larger than normal amounts of certain substances are consumed; one of these substances is asparagus.