Thursday, March 4, 2010

Classical Gas

In which the blogger ponders breaking wind, benefits of a toxic gas, mass extinctions (ah, the paleontological link), and taking blood pressure in mice – yes, the usual mélange.

A Passing Gas

Pediatrician Howard Bennett contributes a column to the kids’ page of the Washington Post and his little identifying blurb at the bottom of each column says that “he writes about gross things” – very true. His columns are direct, informative, fun, and never condescending to his (supposed) young audience. A quintessential example is the column that ran on February 22, 2010, with the daring title (daring for the Post and, yes, it generated complaints) of :

“Ever Wondered Why People Fart?”

Thank you for asking. Yes, I have.

According to Bennett, it’s a combination of the air we swallow (some of which reemerges as belches – topic of his previous month’s column, of course) and the job done on undigested food by the bacteria that inhabit our gut. These bacteria add gases, including oxygen, nitrogen, hydrogen, carbon dioxide, methane, and hydrogen sulfide, to the swallowed air and that gaseous mixture has to get out some how.

As for that smell, credit goes to the hydrogen sulfide some of the bacteria produce as their own waste product from chowing down. Hydrogen sulfide (H2S) – that’s the paleontological link here. More on that in a moment.

I’m still lingering on the flatulence and the smell. Biologist Betsey Dexter Dyer knows bacteria and she wants us to know them, too. In her book, A Field Guide to Bacteria (2003), Dyer identifies the macroscopic field marks of bacteria, those signs around us of the presence of different species of bacteria that we can see (without a microscope), smell, taste (think I’ll pass), and touch. As for the bacteria in our intestinal tract, it’s not surprising they’re there, because, as Dyer writes, “Guts are safe, nutrient-rich places, extremely popular as habitats . . . .” (p. 16-17) The gut-dwelling bacteria work at fermenting and processing food our own enzymes cannot. Dyer ascribes the smell of flatulence to sulfate-reducing bacteria, and some fermenter bacteria, particularly when they process foods with abundant sulfur compounds (e.g., broccoli). These sulfate-reducing bacteria in our gut produce hydrogen sulfide as a waste product.

But, wait, isn’t hydrogen sulfide, even in relatively slight concentrations, highly toxic? Yup, it kills. Farting, clearly, is no joking matter.

“Yet, paradoxically, we need H2S to survive,” asserts biologist Rui Wang in a recent Scientific American article on the positive and potentially lifesaving attributes of hydrogen sulfide. His primary focus is not on the gas being generated in our intestines, but rather on that produced by the human body in blood vessels. (Toxic Gas, Lifesaver, by Rui Wang, Scientific American, March 2010) It turns out that an enzyme found in blood vessels combines with a specific amino acid to produce hydrogen sulfide, among other compounds. Working with rats, Wang found that H2S helps to regulate blood pressure by causing smooth muscle cells to relax, thereby dilating blood vessels. (The effects of another gas, nitric oxide, on reducing blood pressure are well established.) Wang notes that the potentially positive reach of hydrogen sulfide in the human body may extend beyond the cardiovascular system; it may have positive effects in the nervous system. Regulation of metabolism may also be one of its effects, leading Wang to some dramatic speculation about hydrogen sulfide hibernation to stabilize trauma victims at disaster sites. Pretty heady stuff.

Smelly Extinctions – An Upside?

So, a little hydrogen sulfide, good; more, bad. Each instance of hydrogen sulfide production in the human body appears to have an upside, despite the toxicity of the compound. Wang suggests that this capacity of humans to utilize hydrogen sulfide has its roots in our long, long, long ago ancestors’ survival of the massive Permian extinction. The theory of the causes of this extinction, described by Wang, features enormous amounts of hydrogen sulfide generated by bacteria thriving in oxygen-depleted oceans. When that gas, so the theory goes, bubbled to the surface of the ocean, it made the atmosphere toxic. Wang writes:

The importance of H2S in human physiological processes is probably a holdover from that long-ago time. The creatures that survived this catastrophe were the ones able to tolerate and, in certain cases, even consume hydrogen sulfide, and we humans have retained some of that affinity for the gas.

If true, I wonder if our tolerance for hydrogen sulfide produced by sulfate-reducing bacteria in our intestines is another legacy of that survival.

With regard to the Permian extinction and hydrogen sulfide, Wang cites an article by paleontologist Peter D. Ward (Impact from the Deep, Scientific American, October, 2006). To say that Ward is prolific is a gross understatement. He’s written many informative and entertaining books on this and related topics, and there are various videos out there of him lecturing on these issues, including a succinct and amusing one posted on dotSub from about a year ago.

Ward, as I understand him, posits that possibly only the extinction at the end of the Cretaceous was the direct outcome of an impact with Earth of a large extraterrestrial object. He labels others, such as the Permian and the one marking the divide between the Triassic and Jurassic, as “greenhouse extinctions” and a central villain in each is hydrogen sulfide. Ward asserts that the greenhouse extinctions follow a generally similar sequence of steps (a few are cited in this post – for more detail, see Under A Green Sky (2007), particularly p. 137-138): temperatures on the planet rise relatively suddenly due to the release of carbon dioxide and methane from vast areas of volcanic action called blood basalts; the warming of the world changes ocean circulation patterns leading to a growing presence of warm low-oxygen water at the ocean bottom; over time, this anoxic water pushes up toward the top, reaching sunlight which sparks massive growth of bacteria in this water that produce hydrogen sulfide in enormous quantities; this gas bubbles from the ocean into the atmosphere killing some life as it goes and subsequently destroys the ozone layer, delivering the coup de grâce.

Now, that’s what my family, invariably in a conversation around the dinner table, would certainly call “TF” or terminal flatulence.

Postscript – Working with Mice

To refine his research further, Wang and others developed a line of mice lacking the enzyme that enables production of hydrogen sulfide in blood vessels. They analyzed the impact on blood pressure of the absence of this compound in these mice and then its reintroduction.

So, how is blood pressure measured in mice?

With little blood pressure cuffs, attached to their tails.

Not sure I’d trust those readings. Surely, a lab mouse’s blood pressure must soar as a lab worker approaches with the little blood pressure cuff (“oh, no, not the tail again”) – yes, a classic case of “white coat hypertension.”

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