Dermal Bone Buffers CO2-induced Acidosis

Biological Strategy

AskNature Team

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Protect From Gases

Gases are part of most living systems’ environments, especially those exposed to the earth’s atmosphere. Two gases that can act as threats to living systems are oxygen and carbon dioxide; both, paradoxically, are also necessary to life. Thus, living systems must obtain a balance in gas concentrations, selectively protecting from too much gas in some situations and not enough in others. For example, mound-building termites protect themselves by building ventilation shafts to remove excess carbon dioxide from their fungal gardens.

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Reptiles

Class Reptilia (“creeping”): Lizards, crocodiles, turtles, snakes

Reptiles retain some of the key characteristics that first enabled vertebrates to live permanently on land. They have dry skin covered in scales made of keratin that help prevent water loss. (Amphibians still need to stick close to water to keep their skin moist.) Many reptiles reproduce by laying eggs that are watertight and have a yolk, so eggs can develop on land while staying moist and nourished. Reptiles are also ectotherms, meaning they don’t produce their own body heat. This means they burn through energy much more slowly than “warm blooded” creatures of the same size. It also means that they need to keep warm to keep active, which is why you might see them spending a seemingly inordinate amount of time simply basking in the sun.

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Dermal bone in turtles reduces acidity resulting from carbon dioxide build up by releasing calcium and magnesium carbonates into the bloodstream.

“Drawing from physiological studies of extant tetrapods, where dermal bone or other calcified tissues aid in regulating acid–base balance relating to hypercapnia (excess blood carbon dioxide) and/or lactate acidosis, we propose a similar function for these sculptured dermal bones in early tetrapods. Unlike the condition in modern reptiles, which experience hypercapnia when submerged in water, these animals would have experienced hypercapnia on land, owing to likely inefficient means of eliminating carbon dioxide. The different patterns of dermal bone sculpture in these tetrapods largely correlates with levels of terrestriality: sculpture is reduced or lost in stem amniotes that likely had the more efficient lung ventilation mode of costal aspiration, and in small-sized stem amphibians that would have been able to use the skin for gas exchange.” (Janis et al. 2012: 3035)

“Dermal bone may have solved this problem. For instance, when turtles spend a long time underwater holding their breath, there’s no intake of oxygen to displace the carbon dioxide that slowly builds in their blood. To compensate, the dermal bone in their shell leaches calcium and magnesium ions into their bloodstream, replacing the acidic hydrogen ions that have built up there” (Perkins 2012).

Last Updated July 19, 2017

References

“When CO₂ was breathed there was a compensatory change in the strong-ion difference as manifest by an increase in plasma [HCO₃^–] that was approximately 10 meq/l both in the 10 and 20 degrees C turtles. The only significant associated strong-ion changes observed consistent with the ionic compensatory response were increases in total and ionized Ca²⁺ and total Mg²⁺. These results were unaffected at either temperature by surgical removal of the urinary bladder. Urine collected from cystectomized turtles showed no compensatory increase in acid excretion during hypercapnia; in fact, changes occurred in the opposite direction. Urinary excretion of HCO₃^– and urine pH increased significantly, whereas titratable acidity decreased significantly. No significant change occurred in ammonia excretion over the three days of hypercapnia. These data argue against compensatory roles for the kidneys and urinary bladder in this species and point to internal ionic exchanges involving bone and shell.” (Silver and Jackson 1986:1228)

Journal article