Gas exchange is the primary function of the lung. The basic process is the transfer of oxygen (O2) from the inspired air to the bloodstream and the transport of carbon dioxide (CO2) produced by metabolism out of the body via the expired gas. The three main structural considerations at play to facilitate this function are (1) the system of airways and the mechanical actions of the chest wall and respiratory muscles to move gas in and out of the lung; (2) the provision of a blood-gas interface which is very thin and has a very large surface area to promote the passive flow of O2 and CO2, driven only by partial pressure differences between alveolar gas and pulmonary capillary blood; and (3) the provision of a pulmonary vasculature sufficient both to perfuse the very large surface area of the blood-gas interface and to accommodate the full cardiac output with relatively low resistance.
Gas exchange also occurs in the tissues throughout the body by passive transfer. Oxygen is transported to tissues by passive transfer from arterial blood, and CO2 is transported from tissues to venous blood by passive transfer to venous blood. The main transport mechanism for O2 in the blood is by binding of O2 with hemoglobin (Hb) in the red blood cells. There is minimal O2 dissolved in plasma. The three mechanisms for transport of CO2 in the blood are (1) dissolving in plasma, (2) forming dissociated bicarbonate ions (HCO3−), and (3) binding with Hb.
Several factors affect gas exchange including the rates of lung ventilation and perfusion, the matching of ventilation to perfusion within the lung, Hb levels, alveolar O2 partial pressure, metabolic demand, exercise, and pathological changes.
The most common tests for assessing gas exchange are the single-breath uptake of carbon monoxide (CO) by the lung (called DLCO or diffusing capacity in North America and called TLCO or transfer factor in Europe), the analysis of arterial blood gases (ABG), and the measurement of oxygen saturation of Hb using pulse oximetry (SpO2). These measurements are very useful in the diagnosis and management of various lung diseases.
This chapter will describe the pathways for gas exchange, factors affecting gas exchange, measurements of gas exchange, and the interpretation of such measurements.
KeywordsGas exchange Diffusing capacity Ventilation/perfusion Arterial blood gases Pulse oximetry
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