Deep within the kidney lies a multicellular anatomical structure called the nephron. Each nephron—first identified by cellular anatomists in the late 1600s—can be imagined as a mini-kidney. The nephron is the site where the blood and kidney cells meet, and the first drops of urine are generated. The circulation of blood carries the excess salt, dissolved in plasma, to the kidneys. The blood vessels split and split further to form finer and finer-walled arteries. Finally, the thinnest arteries whirl around themselves to form a thin-walled nest of cells—so delicate and porous that the liquid,
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