Thread: Water
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Old 11-12-2006, 12:48 PM
EricT EricT is offline
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Since I know people won't take my word about this myth of "hoarding water" as such I thought I'd post this little thing on the function of the kidneys. I bolded the relevant parts.

The body has two kidneys, each about 4-5 inches in length and reddish in color. The kidneys are located just above the waist, behind the abdominal cavity. Each kidney contains the hilus, through which the ureters are connected. Blood vessels and lymphatic vessels also are connected at the hilus, with the blood entering through the renal artery.

Three layers of tissue surround each kidney. The renal fascia, made of a dense connective tissue, anchors the kidneys to surrounding structures.

The principle task of the kidney is to preserve the volume and composition of the extracellular fluid constant. This it must do despite a varying outside environment, and varying input. A part of this task–but only a part–is to remove from the body some of the waste by-products of metabolism which the cells cannot break down further. Thus the principal function of the kidney is not excretion, but regulation. We can move and live on dry land, even though we are three-quarters water, and survive; our cells tucked away in a carefully preserved ocean of extracellular fluid, whose composition is guarded with exquisite accuracy by the kidneys, a major part of our life-support system in this hostile environment. We can roam into deserts, and (usually) survive, or drink a six-pack of beer, or starve, or gorge, but essentially the extracellular soup remains of a constant composition, and because of this, the composition of the cells themselves is constant.

The kidney is less in control of the intracellular water, since if the kidneys do their job adequately, each cells is largely autonomous, and will extract and eject what it needs or does not need from the extracellular fluid. The kidney conserves what we need, but even more, it permits us the freedom of excess. That is, it allows us to take in more than we need of many necessities—water and salt for example—and excretes exactly what is not required. This is essential, since neither our ancestors nor we, animal or human know the composition of the foods we eat, and the only way to ensure a sufficiency of everything is to eat an excess of at least some. Finally, the kidneys preserve the volume of our body fluids as well as their composition. Given that we’re almost ¾ water, quite simply weighing oneself each day can assess the precision with which the kidney achieves this.

Despite variations in diet, exercise or fluid intake, the figures remain constant. The kidney performs its tasks, with a precision of as good as 1% and never worse than 5%, under extremely varying circumstances. If the kidneys fail suddenly, death occurs after a few days, partly because some of the accumulated metabolic waste products are toxic to the heart, which stops. More interesting, is the way in which the kidney can adapt to slow destruction by dysfunction, so that one can survive on as little as 5% of overall kidney function. The kidney has greater reserve capacity in the face of disorder than (for example) the heart or the lungs.

Why can’t we switch off urine production all together in times of drought? This is impossible because of two constraints. The first is that there is an upper limit to the degree of concentration of urine that we can achieve. This is a function of the length of the loops in the kidney tubules. The other constraint, given that there is a limit to our concentrating capacity, is that there is a minimum amount of soluble waste, which we must excrete through our kidneys each day. This is mostly nitrogen-containing compounds, principally urea; and on a normal diet we produce an amount which will not dissolve in less than about one pint of the most concentrated urine we can produce. Therefore, even on a raft in the ocean, or in the dessert, we go on passing this volume of urine. If you are dehydrated, your urine is already four times as concentrated as your blood.
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If you act sanctimonious I will just list out your logical fallacies until you get pissed off and spew blasphemous remarks.
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