We have two bean shaped kidneys about the size of a fist, that are located near the lower back. As oxygenated blood enters our kidneys through the renal artery, it branches into finer arteries, then passes into a filtration system called nephrons. There are about 1 million nephrons in each kidney - which are composed of a glomerulus and a 3-part tubule (proximal convoluted tubule, loop of Henle and distal convoluted tubule). These remove nutrients, toxins and water from the blood to so-call "measure the levels". Then the nephrons send nutrients and water back into the blood (to be reabsorbed) in amounts that the body needs at any given time. The toxins and other compounds that our bodies don't need, like urea and extra water, become urine and are sent through peristalsis to tubes called ureters out to the
bladder. When we have enough water in our blood stream excess water in our urine will make it pale colored to clear. When we, however are dehydrated, our kidneys recirculate the water back into the blood stream and there will be less water in our urine - making it yellow.