Instead, cells are constantly bringing nutrients and other useful molecules circulating in the bloodstream in through their walls for use as fuel or building blocks, converting those molecules to something else, and then pushing the stuff they’ve built out of their walls to be used elsewhere in the body. Cells in the ovaries pull cholesterol molecules inside, build estrogen out of them, and then push the estrogen—a hormone with effects all over the body—out into the bloodstream. Nerves and neurons are constantly pumping ions (positively or negatively charged molecules) in and out to maintain a
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