Circulation: A Comparison of Reptiles, Mammals, and Birds

  • F. N. White
Part of the Proceedings in Life Sciences book series (LIFE SCIENCES)


The evolution of endothermy has required structural elaboration of the airways and respiratory exchange surfaces. The presence of a completely septated heart in endotherms precludes the use of predominantly neurogenic control of pulmonary vascular resistance, as seen in noncrocodilian reptiles. The occurrence of ventilation-perfusion inequities in the finely subdivided airways of endotherms has required local vascular control in response to prevailing alveolar \({\text{P}}_{{\text{O}}_{\text{2}} }\) with potential for intrapulmonary shunting from poorly ventilated to more adequately ventilated respiratory units. No evidence for this mechanism is seen in the pulmonary circulation of turtles. Although mean air convection requirements are similar in reptiles and endotherms at similar body temperatures and mean blood flow requirements are similar when adjusted for differences in the O2 capacity of blood, many noncrocodilian reptiles exhibit temporal variation in pulmonary blood flow during a given respiratory cycle. Right-to-left intracardiac shunting is intensified with the length of the apneic period. Such shunts are not possible in endotherms because of complete cardiac septation. An hypothesis is presented to the effect that intracardiac shunting may aid in depletion of available O2 stores during protracted apnea. The shunt may profitably be viewed as a CO2 shunt past the alveolar gas phase. The resulting influence on alveolar and systemic P \({\text{P}}_{{\text{CO}}_{\text{2}} }\) may augment O2 binding to Hb in the lungs while intensifying unloading through the Bohr effect at the tissue level. From this perspective, the reptilian blood flow distribution patterns may be viewed as adaptive to the protracted apnea and lower respiratory frequency characterizing the group. These differences may be related to differences in metabolic intensity, respiratory frequency, and the consequent adaptive value of differing control mechanisms for pulmonary vascular resistance in ectotherms and endotherms.


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Copyright information

© Springer-Verlag Berlin Heidelberg 1978

Authors and Affiliations

  • F. N. White
    • 1
  1. 1.Physiological Research Laboratory, Scripps Institution of OceanographyUniversity of CaliforniaSan Diego, La JollaUSA

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