Adaptation of Red Blood Cell Function to Hypoxia and Temperature in Ectothermic Vertebrates

Abstract

function in adaptations of ectothermic ver? tebrates to hypoxia and temperature. The term adaptation, as used in this ar? ticle, means an alteration of physiological function which compensates for a stressful environmental or organismic factor. Two such factors of paramount importance to ectothermic vertebrates are temperature and availability. The physiological function to be examined in terms of adapt? ability is transport by blood from the gas exchange surface to the intracel? lular sites of 02 utilization. Hypoxia is best defined as inadequate uptake of 02 by tissues. The cause may be environmental (reduced 02 content and/or partial pressure in the respiratory medi? um) or organismic. In the latter category there is a hierarchy of sub-causes proceeding down the oxygen cascade, i.e., impaired gas exchange, shunt, ventilation/ perfusion mismatch, hypoperfusion, or inappropriate binding properties of blood. The appropriateness of the oxy? gen binding properties of blood depends, in turn, on environmental conditions, i.e., the partial pressure of 02 (Po2) in the res? piratory medium, and on the efficacy of gas exchange and degree of shunt. The position of the oxyhemoglobin dis? sociation curve (ODC) even with full knowledge about the environmental oxy? gen availability, is inadequate information for conclusions about adaptability of propriateness for tissue oxygenation. Such conclusions are valid only if infor? mation is available on in vivo values of temperature, blood gases (arterial and mixed venous Po2 and Pco2), pH and ca? pacity. Furthermore, the position of the ODC may cause, as well as be affected by, hypoxia. For example, in vertebrates with incompletely separated pulmonary and systemic circulations (lungfishes, amphibi? ans, and reptiles), the arterial hypoxemia due to right-to-left shunting may lead to severe arterial desaturation and hypoxia if the ODC is positioned too far to the right. Conversely, hypoxia caused by environ? mental factors can, by influencing the aerobic metabolism of nucleated red cells (or anaerobic metabolism of mammalian red cells), alter (in opposite directions!) the level of allosteric effectors of hemoglobin function and shift the position of the ODC. Thus, although it is often irresistably tempting (as evidenced below) to assess physiological meaning from shapes and positions of ODCs, Bohr factors, and ap? parent enthalpy (AH) values, such specu? lations should always be recognized and identified as just that.