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Bio-bullets

What is the real value of a kidney?

Remembering that even the heartbeat was misunderstood for centuries, could the kidney yet prove to be . . . an oxido-transformer?

professor caricatureProf. Mumblebard claims: "Renal function is no mystery: it is simply the homeostatic regulation of body fluids, the main mechanism of which is urination of excess water, salts, acids and nitrogen. Although other organs such as the skin and intestines{njaccess 2, 3, 4, 5, 6, 7, 8} also contribute to the elimination of potentially toxic wastes from the blood, the kidney is the only organ shared by all vertebrates that is dedicated to excretion."{!njaccess}… See the hidden half of Prof. Mumblebard’s claim by subscribing here{/njaccess}

logoRobin and the Honey Badger respond: "The true electrochemical function of the kidney is deceptive. ‘Homeostatic regulation’, although valid, is too vague to give an understanding of renal physiology. ‘Urination’ also falls short, because it is a secondary rather than primary function of the kidney. Clues to the real specialisation of the kidney – the balancing and reformulating of pro-oxidants and anti-oxidants throughout the body – lie in four observations. Firstly, the intricacy and energetic cost of the kidney suggest that its true function is more fundamental than any aspect of excretion. Indeed, the paired kidneys are so powerful that they reformulate, molecule by molecule, all the blood plasma of the whole human body every 22 minutes. Secondly, the kidney provides a uniquely neutral environment because oxygen is excluded and the active transport of ions across renal cell membranes does not generate action potentials. Thirdly, nearly all substances filtered from the blood into the renal tubules are reabsorbed into the blood, with only a small fraction being excreted. This is partly because{njaccess 2, 3, 4, 5, 6, 7, 8} urea and uric acid, although often assumed to be mere wastes, are costly to synthesise and are useful anti-oxidants. And lastly, most of the true excreta subsequently pass directly from the blood into the urinary tract. Based on these observations, renal physiology transcends the mere maintenance of ionic balance in the blood via excretion and acid/alkaline buffering. Instead, diverse molecules can routinely be quarantined – separate from blood and lymph – in a body compartment where electrons can be traded among the intercepted pro-oxidants, anti-oxidants and ions. In the resulting reformulation, radicals can be processed to optimise the overall effects of redox reactions throughout the body, with the various pro-oxidant and anti-oxidant substances being mixed and matched appropriately. In this way homeostasis can be achieved in the precise sense of controlling ‘free radicals’ yet retaining enough pro-oxidants of appropriate types to kill cancerous cells and current pathogens. In summary, the anatomy and function of the kidney are consistent with a dedication as an ‘oxido-transformer’ for the whole body, balancing the overall costs and benefits of respiration in all organs. This new interpretation better explains the complexity and power of the kidney because management of the chain reactions of oxidation is the ultimate challenge for aerobic metabolism."{!njaccess}… Reveal the hidden half of this response by Robin and the Honey Badger by subscribing here{/njaccess}

 

Please join us here at the Bio-edge with your own comments. In the discussion below we encourage links to any evidence supporting either Prof. Mumblebard or Robin and the Honey Badger. Illustrations are welcome but please cite all sources or we may be forced under copyright to delete your comment.


What is the real value of a kidney?

[1] the synthesis of which, although using superfluous nitrogen, is too expensive to be explained in terms of excretion alone

 

Last modified on 13 July 2015

Comments   

0 #1 Selenius 2014-06-20 14:42
You make the point that there are cheaper or more direct ways of excreting than via kidneys. Did you know that this applies even to uric acid? It’s known that in humans uric acid can be excreted from the blood into the intestines, so that it emerges in the feces. Up to a third of total elimination of uric acid can be got rid of via this extra-renal pathway in mammals. So I guess it’s true that two aspects of the behaviour of uric acid cast doubt on the excretory role of kidneys: these organs reabsorb much of the uric acid back from the nephron into the blood instead of simply dumping it in the urine, and the uric acid can be excreted into the gut anyway (whence some of it can be re-absorbed back into the blood). Since uric acid is almost synonymous with urine in birds and reptiles, this does seem to give a certain ambivalence to kidneys, not so?
0 #2 Taklamakan 2014-07-08 08:42
One quarter of your blood supply passes through your kidneys every minute. Given how hardworking these organs are I find it amazing that people can survive with only one kidney for decades. After reading this bio-bullet I do see that it’s rather puzzling that so much goes on in the kidneys and yet there’s a lot of leeway in our dependence on this organ. Does seem to suggest that the main function is something longer-term than the short-term needs for excretion. So I suppose the paradox of the kidneys is that they work so intensively and yet not necessarily for immediate effect? bbc.co.uk/.../kidneys.shtml
0 #3 Johns 2014-07-16 11:36
If your interpretation of kidney function is right, we’d predict that dialysis patients would tend to die of oxidant damage, rather than uremia/toxicity , etc. We know that dialysis patients do tend to die 10-15 years younger than kidney transplant recipients. Some studies seem to suggest that the longer a patient is on dialysis before the transplant, the less time the new kidney will last. It’s not clear why this occurs and maybe it supports your interpretation. See Wolfe et al. (1999) NEJM 341, 1725ff
0 #4 Freerad 2014-07-16 14:45
Part 1/2 Excretion? Funnily enough, the kidneys themselves make some of the substances they excrete! Glutathione peroxidase is such an important antioxidant that its activity is 7 times more than the antioxidant activity of catalase in the case of the brain. And it’s not in the liver but in the proximal convoluted tubule of the kidneys that glutathione peroxidase is made in the first place! Creatine has an electron to spare so it’s also an antioxidant, serving to neutralise superoxide.
0 #5 Freerad 2014-07-16 14:46
Part 2/2: Creatinine is formed non-enzymatical ly from creatine (i.e. it’s an anhydride of creatine); there’s hardly any tubular reabsorption of creatinine, which is both filtered and secreted, and exits in the urine. Point is, creatine is made in not only the liver but also the kidney. Vitamin C is excreted mainly in urine, and in reptiles and some birds (but not mammals) vitamin C is synthesised in the kidneys. And in dogs the kidneys themselves synthesise urea de novo. The renal cortex concentrates iodine (although not as much as thyroid) and I is also antioxidant and excreted mainly in urine.
0 #6 Johns 2014-08-06 08:36
One clue to the priorities of kidney function: glomerular filtration rate is unaffected by whether you’ve just drunk water or not. When you drink, this does stimulate your body to pass the excess water in urine. However, it’s not because of an increased rate of filtration of blood plasma into the nephrons. Instead, it’s because of reduced reabsorption of water from the renal tubules. This implies that the kidneys are beavering away even when excess water is unavailable for flushing waste materials in urine. The futility of this is puzzling, if after all this hard work the solutes in the filtrate just get put back into the bloodstream. So thanks for coming up with a good question.

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