Respiratory pigments: interactions between oxygen and carbon dioxide transport

Abstract

Oxygen and carbon dioxide are transported in vertebrates and invertebrates by a wide range of respiratory pigments. These respiratory gases are not transported independently of one another, and this review considers the influence of carbon dioxide on oxygen transport and vice versa. A specific effect of carbon dioxide or bicarbonate, decreasing oxygen affinity, is found in many haemoglobins, but the effect is often reduced in the presence of organic phosphates. Clear experimental data are available for mammalian haemoglobins but in birds and lower vertebrates more data are required to verify the presence and magnitude of the CO2 effect. In erythrocruorins and haemocyanins CO2 increases O2 affinity, whereas in haemerythrins, as in haemoglobin, CO2 again decreases oxygen affinity. Much of our knowledge of invertebrate respiratory pigments is based, however, on data from one or two species. A specific effect of CO2 on O2 affinity has also often been found only at high CO2 partial pressures, which may be outside the physiological range for these species. More in vivo experimental data on CO2 values are required for these species, and further studies on other species may help to explain this discrepancy. The interaction of O2 and CO2 transport is mainly through the Haldane effect, i.e., deoxygenated blood having a greater capacity for CO2 than oxygenated blood. This is due directly to the formation of carbamino groups (carbamate) and also to the fact that deoxygenated blood binds relatively more protons than oxygenated blood. This forms the basis for the linkage between the Bohr and Haldane effects. In some species in which the Bohr coefficient is below −1.0, an akalosis in the tissues may be induced. Large Haldane effects may be particularly effective in promoting CO2 unloading when the partial pressure difference of CO2 between arterial and venous blood is small. Carbamate formation may account for 10–20% of the CO2 transported in mammals, but its role in lower vertebrates and invertebrates has only recently been considered. Carbon dioxide transport is modulated by those factors that influence O2 affinity as these in turn influence the Haldane effect.