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In most cases, kidneys can recover from almost complete loss of function, such as in acute kidney (renal) failure.
The immune system needs 9.5 hours of sleep in total darkness to recharge completely.
After a vasectomy, it takes about 12 ejaculations to clear out sperm that were already beyond the blocked area.
Your skin wrinkles if you stay in the bathtub a long time because the outermost layer of skin (which consists of dead keratin) swells when it absorbs water. It is tightly attached to the skin below it, so it compensates for the increased area by wrinkling. This happens to the hands and feet because they have the thickest layer of dead keratin cells.
About one in five American adults and teenagers have had a genital herpes infection—and most of them don't know it. People with genital herpes have at least twice the risk of becoming infected with HIV if exposed to it than those people who do not have genital herpes.