Oxygen Transport by Hemoglobin

Hemoglobin (Hb) constitutes a vital link between ambient O-2 availability and aerobic metabolism by transporting oxygen (O-2) from the respiratory surfaces of the lungs or gills to the O-2-consuming tissues. The amount of O-2 available to tissues depends on the blood-perfusion rate, as well as the arterio-venous difference in blood O-2 contents, which is determined by the respective loading and unloading O-2 tensions and Hb-O-2-affinity. Short-term adjustments in tissue oxygen delivery in response to decreased O-2 supply or increased O-2 demand (under exercise, hypoxia at high altitude, cardiovascular disease, and ischemia) are mediated by metabolically induced changes in the red cell levels of allosteric effectors such as protons (H+), carbon dioxide (CO2), organic phosphates, and chloride (Cl-) that modulate Hb-O-2 affinity. The long-term, genetically coded adaptations in oxygen transport encountered in animals that permanently are subjected to low environmental O-2 tensions commonly result from changes in the molecular structure of Hb, notably amino acid exchanges that alter Hb's intrinsic O-2 affinity or its sensitivity to allosteric effectors. Structure-function studies of animal Hbs and human Hb mutants illustrate the different strategies for adjusting Hb-O-2 affinity and optimizing tissue oxygen supply. (C) 2012 American Physiological Society. Compr Physiol 2:1463-1489, 2012.