Editing Bohr effect (section)

== Special cases ==
[[File:Humpback whale noaa.jpg|thumb|Though they are one of the largest animals on the planet, humpback whales have a Bohr effect magnitude similar to that of a guinea pig.]]

=== Marine mammals ===
An exception to the otherwise well-supported link between animal body size and the sensitivity of its haemoglobin to changes in pH was discovered in 1961.<ref name=":0">{{Cite journal|last=Riggs|first=Austen|date=1961-04-01|title=Bohr Effect in the Hæmoglobins of Marine Mammals|journal=Nature|language=en|volume=190|issue=4770|pages=94–95|doi=10.1038/190094a0|pmid=13741621|bibcode=1961Natur.190...94R|s2cid=26899569}}</ref> Based on their size and weight, many [[marine mammal]]s were hypothesized to have a very low, almost negligible Bohr effect.<ref name=":1" /> However, when their blood was examined, this was not the case. [[Humpback whale]]s weighing 41,000 kilograms had an observed <math display="inline">{\scriptstyle \Delta \log (P_{50}) \over \Delta \text{pH}}</math> value of 0.82, which is roughly equivalent to the Bohr effect magnitude in a 0.57&nbsp;kg [[guinea pig]].<ref name=":1">{{Cite journal|last=Riggs|first=Austen|date=1960-03-01|title=The Nature and Significance of the Bohr Effect in Mammalian Hemoglobins|journal=The Journal of General Physiology|language=en|volume=43|issue=4|pages=737–752|doi=10.1085/jgp.43.4.737|issn=0022-1295|pmid=19873527|pmc=2195025}}</ref> This extremely strong Bohr effect is hypothesized to be one of marine mammals' many adaptations for deep, long dives, as it allows for virtually all of the bound oxygen on haemoglobin to dissociate and supply the whale's body while it is underwater.<ref name=":0" /> Examination of other marine mammal species supports this. In [[pilot whale]]s and [[porpoise]]s, which are primarily surface feeders and seldom dive for more than a few minutes, the <math display="inline">{\scriptstyle \Delta \log (P_{50}) \over \Delta \text{pH}}</math>was 0.52, comparable to a [[Cattle|cow]],<ref name=":1" /> which is much closer to the expected Bohr effect magnitude for animals of their size.<ref name=":0" />

=== Carbon monoxide ===
Another special case of the Bohr effect occurs when [[carbon monoxide]] is present. This molecule serves as a [[Competitive inhibition|competitive inhibitor]] for oxygen, and binds to haemoglobin to form [[carboxyhemoglobin|carboxyhaemoglobin]].<ref name=":4" /> Haemoglobin's affinity for CO is about 210 times stronger than its affinity for O<sub>2</sub>,<ref>{{Cite book|title=Guyton and Hall Textbook of Medical Physiology (12th ed.)|last=Hall|first=John E.|publisher=Saunders/Elsevier|year=2010|isbn=978-1416045748|location=Philadelphia, Pa|pages=502}}</ref> meaning that it is very unlikely to dissociate, and once bound, it blocks the binding of O<sub>2</sub> to that subunit. At the same time, CO is structurally similar enough to O<sub>2</sub> to cause carboxyhemoglobin to favor the R state, raising the oxygen affinity of the remaining unoccupied subunits. This combination significantly reduces the delivery of oxygen to the tissues of the body, which is what makes carbon monoxide so [[Carbon monoxide poisoning|toxic]]. This toxicity is reduced slightly by an increase in the strength of the Bohr effect in the presence of carboxyhemoglobin. This increase is ultimately due to differences in interactions between heme groups in carboxyhemoglobin relative to oxygenated hemoglobin. It is most pronounced when the oxygen concentration is extremely low, as a last-ditch effort when the need for oxygen delivery becomes critical. However, the physiological implications of this phenomenon remain unclear.<ref name=":4">{{Cite journal|last1=Hlastala|first1=M. P.|last2=McKenna|first2=H. P.|last3=Franada|first3=R. L.|last4=Detter|first4=J. C.|date=1976-12-01|title=Influence of carbon monoxide on hemoglobin-oxygen binding|journal=Journal of Applied Physiology|volume=41|issue=6|pages=893–899|issn=0021-8987|pmid=12132|doi=10.1152/jappl.1976.41.6.893}}</ref>