02 January 2011

Biological safety factors

I have been reading up on the safety factors built into animal bodies. The underlying idea, borrowed from engineering, is simple. Its most general expression may be as follows. If the maximum possible output of an animal for a particular function is M, while the average required output is A, then the safety factor (SF) for that particular function is given by:

SF = M / A

So, if SF is about 1.0, then there really is no safety factor. If the required output happens to exceed the average output, the animal will not be able to keep up with demand and its survival will be jeopardized. On the other hand, safety factors greater than 1.0 assure that if the required output ever exceeds the average, the animal will still survive.

Of course, there is a cost to keeping a safety factor much larger than 1.0. Therefore, evolution settles not for the best, but for an optimum somewhere between just enough and the best.

A good review on this topic was written by Jared Diamond*. According to Diamond, pancreas has one of the highest known biological safety factors, which is 10. From an evolutionary stand point, this is obviously good; one can survive long enough to reproduce even when one's pancreas is seriously damaged. But in the case of modern humans who are usually living far beyond their allocated lifespans, a damaged, but functioning pancreas is not necessarily a good thing. Diamond explains:

...malabsorption, due to decreased absorption of ingested food by pancreatic proteases and lipases, is not observed until pancreatic enzyme output has dropped to only 10% of normal peak values. That is why pancreatic cancer is so insidiously difficult to detect: patients show no telltale symptoms of malabsorption until 90% of pancreatic function has been destroyed, by which time the cancer has usually metastasized to other organs.
I will write more about this subject in the future.

*Diamond, J. 2002. Quantitative evolutionary design. Journal of Physiology 542:337–345.

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