AJP - Regulatory, Integrative and Comparative Physiology, Vol 247, Issue 1 3-14, Copyright © 1984 by American Physiological Society
Cardiovascular shunts and oxygen transport in lower vertebrates
S. C. Wood
O2 transport in animals with normal or abnormal cardiovascular shunts is
analyzed using a two-compartment model. This analysis reveals that the O2
affinity of blood is a key factor determining the arterial, venous, and
tissue O2 partial pressure (PO2) levels. Depending on the value of
"critical" PO2, factors that decrease O2 affinity (e.g., increased
temperature and reduced pH) should increase tissue PO2 and favor increased
tissue oxygenation. Species comparisons suggest that O2 affinity is closely
linked to critical PO2. In contrast to the model for environmental hypoxia,
hypoxia due to shunting a right-shifted O2 dissociation curve is, at sea
level, always adaptive. When hypoxia due to shunting is combined with
external hypoxia, a decreasing O2 affinity causes increased blood and
tissue PO2 values up to a "breaking point," which varies with shunt
fraction, inspired PO2, lung PCO2, and respiratory properties of the blood.
External hypoxia would cause ectotherms with high preferred body
temperatures (e.g., lizards) to have blood and tissue PO2 values beyond the
breaking point. This led to the hypothesis, now confirmed in several
species, that external hypoxia would result in a reduction of preferred
body temperature.
