Pulse Oximetry and Oxygen Transport

Abstract

Oxygen is the most acutely necessary substrate of aerobic life. The oxygen delivery system must provide oxygen at adequate rates to all the body tissues to support their oxygen consumption for survival. In large animals, two physical constraints to transport have been solved physiologically. First, the rate of gas diffusion in liquids is extremely slow. It takes hours for oxygen to diffuse through a centimetre of water. Even though animals have increased dramatically in size from single- celled to large mammals, the diffusion distance from the oxygen source to the mitochondrion has not increased much. The vast microvascular circulatory system has solved this diffusion problem by supplying an oxygen tension source near all cells. The tissue capillary density is roughly matched to the metabolic oxygen demands of the tissues. The second physical problem is that oxygen is relatively insoluble in water, the medium of cellular life (≃ 0.003 ml O₂/100ml water/ mmHg). The haemoglobin molecule has effectively increased the blood’s oxygen “solubility” by reversibly binding oxygen. The addition of haemoglobin to the water-based oxygen transport fluid (blood) increases the carried oxygen by approximately 60-fold. This chapter will review the oxygen transport system, the theory behind oximetry and the clinical applications of pulse oximetry.