Abstract
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.
Access this chapter
Tax calculation will be finalised at checkout
Purchases are for personal use only
Selected References
Bachofen H, Schurch S, Urbinelli M, Weibel ER. Relations among alveolar surface tension, surface area, volume, and recoil pressure. J Appl Physiol. 1987;62:1878–87.
Butler JP, Tsuda A. Transport of gases between the environment and alveoli – theoretical foundations. Compr Physiol. 2011;1(3):1301–16.
Chan ED, Chan MM, Chan MM. Pulse oximetry: understanding its basic principles facilitates appreciation of its limitations. Respir Med. 2013;107:789–99.
Cotes JE, Chinn DL, Miller MR. Lung function. 6th ed. Oxford: Blackwell Scientific Publications; 2006.
Culver BH, Graham BL, Coates AL, et al. Recommendations for a standardized pulmonary function report. An official American Thoracic Society technical statement. Am J Respir Crit Care Med. 2017;196:1463–72.
Engel LA. Gas mixing within the acinus of the lung. J Appl Physiol. 1983;54:609–18.
Forster RE. Chapter 5. Diffusion of gases across the alveolar membrane. In: Farhi LE, Tenney SM, editors. Handbook of physiology. Section 5.3. The respiratory system. Vol IV. Gas exchange. Bethesda, MD: American Physiological Society; 1987. p. 71–88.
Graham BL, Brusasco V, Burgos F, et al. 2017 ERS/ATS standards for single-breath carbon monoxide uptake in the lung. Eur Respir J. 2017;49:1600016.
Graham BL, Mink JT, Cotton DJ. Effects of increasing carboxyhemoglobin on the single breath carbon monoxide diffusing capacity. Am J Respir Crit Care Med. 2002;165:1504–10.
Graham BL, Mink JT, Cotton DJ. Implementing the three equation method of measuring single breath carbon monoxide diffusing capacity. Can Respir J. 1996;3:247–57.
Hollowell J, Van Assendelft O, Gunter E, et al. Hematological and iron-related analytes—reference data for persons aged 1 year and over: United States, 1988–94. National Center for Health Statistics. Vital Health Stat. 2005;11:1–156.
Huang YC, O’Brien SR, MacIntyre NR. Intrabreath diffusing capacity of the lung in healthy individuals at rest and during exercise. Chest. 2002;122(1):177–85.
Huang YC, MacIntyre NR. Real-time gas analysis improves the measurement of single-breath diffusing capacity. Am Rev Respir Dis. 1992;146(4):946–50.
Hughes JM, Bates DV. Historical review: the carbon monoxide diffusing capacity (DLCO) and its membrane (DM) and red cell (Theta.Vc) components. Respir Physiol Neurobiol. 2003;138(2–3):115–42.
Jones RS, Meade F. A theoretical and experimental analysis of anomalies in the estimation of pulmonary diffusing capacity by the single breath method. Q J Exp Physiol. 1961;46:131–43.
Kaminsky DA, Whitman T, Callas PW. DLCO versus DLCO/VA as predictors of pulmonary gas exchange. Respir Med. 2007;101(5):989–94.
Kohn HN. Zur Histologie der indurierenden fibrinösen Pneumonie. Münch Med Wschr. 1893;40:42–5.
Krogh M. The diffusion of gases through the lungs of man. J Physiol. 1915;49:271–300.
Lambert MW. Accessory bronchiole-alveolar communications. J Pathol Bacteriol. 1955;70:311–4.
MacIntyre N, Crapo R, Viegi G, et al. Standardisation of the single-breath determination of carbon monoxide uptake in the lung. Eur Respir J. 2005;26:720–35.
McCormack MC. Facing the noise: addressing the endemic variability in D(LCO) testing. Respir Care. 2012;57(1):17–23.
Ogilvie CM, Forster RE, Blakemore WS, Morton JW. A standardized breath holding technique for the clinical measurement of the diffusing capacity of the lung for carbon monoxide. J Clin Invest. 1957;36:1–17.
Paiva M, Engel LA. Gas mixing in the lung periphery. In: Chang HK, Paiva M, editors. Respiratory physiology. An analytic approach. Lung biology in health and disease, vol. 40. New York: Marcel Dekker, Inc; 1989. p. 245–76.
Piiper J, Scheid P. Chapter 4. Diffusion and convection in intrapulmonary gas mixing. In: Farhi LE, Tenney SM, editors. Handbook of physiology. Section 5.3. The respiratory system. Vol IV. Gas exchange. Bethesda, MD: American Physiological Society; 1984. p. 51–69.
Roughton FJW, Forster RE. Relative importance of diffusion and chemical reaction rates in determining rate of exchange of gases in the human lung, with special reference to true diffusing capacity of pulmonary membrane and volume of blood in the lung capillaries. J Appl Physiol. 1957;11:290–302.
Sikand RS, Magnussen H, Scheid P, Piiper J. Convective and diffusive gas mixing in human lungs: experiments and model analysis. J Appl Physiol. 1976;40:362–71.
Smith TC, Rankin J. Pulmonary diffusing capacity and the capillary bed during Valsalva and Muller maneuvers. J Appl Physiol. 1969;27:826–33.
Stanojevic S, Graham BL, Cooper BG, et al. Official ERS technical standards: Global Lung Function Initiative reference values for the carbon monoxide transfer factor for Caucasians. Eur Respir J. 2017;50:1700010.
Wanger J. ATS Pulmonary Function Laboratory Management and Procedure Manual. 3rd ed: American Thoracic Society, New York, NY, USA; 2016. https://www.thoracic.org/professionals/education/pulmonary-function-testing/
West JB, Dollery CT, Naimark A. Distribution of blood flow in isolated lung; relation to vascular and alveolar pressures. J Appl Physiol. 1964;19:713–24.
West JB, Wagner PD. Pulmonary gas exchange. Am J Respir Crit Care Med. 1998;157:S82–7.
West JB. Respiratory physiology: the essentials. 9th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2012.
Author information
Authors and Affiliations
Corresponding author
Editor information
Editors and Affiliations
Appendix
Appendix
The MIGET is based on the physical principles governing inert gas elimination by the lungs. When an inert gas in solution is infused into systemic veins, the proportion of gas eliminated by ventilation from a lung unit depends only on the solubility of the gas and the V̇A/Q̇ ratio of that unit. The relationship is given by the following equation:
where Pc′ and \( \mathrm{P}\overline{\mathrm{v}} \) are the partial pressures of the gas in end-capillary blood and mixed venous blood, respectively, and λ is the blood-gas partition coefficient. The ratio of Pc′ over \( \mathrm{P}\overline{\mathrm{v}} \) is known as the retention.
To obtain the V̇A/Q̇ distribution of the lung, a saline solution containing low concentrations of six inert gases of different solubility (sulfur hexafluoride [SF6], ethane, cyclopropane, isoflurane, diethyl ether, and acetone) is infused slowly into a peripheral vein until a steady state is reached. The inert gas concentrations in the arterial, mixed venous, and expired gas samples are collected and analyzed. Retention and excretion values for the inert gases are graphed against their solubility in blood. With a 50-compartment model, the retention-solubility plots can be transformed to obtain the distribution of V̇A/Q̇ ratios in the lung. A lung containing shunt units (V̇A/Q̇ = 0) shows increased retention of the least-soluble gas, SF6. Conversely, a lung having large amounts of ventilation-to-lung units with very high V̇A/Q̇ ratios and dead space (V̇A/Q̇ = infinity) shows increased retention of the high-solubility gases (such as ether and acetone).
In healthy subjects, the distributions for both ventilation and blood flow (dispersion) are narrow and span only one log of V̇A/Q̇ ratios. Essentially, no ventilation or blood flow occurs outside the range of approximately 0.3–3.0 on the V̇A/Q̇ ratio scale, and no significant intrapulmonary shunt is detected. With aging, the dispersion of ventilation and perfusion increases. In older subjects, as much as 10% of the total blood flow may go to lung units with V̇A/Q̇ values of less than 0.1, but still no shunt is detected. The increased low V̇A/Q̇ regions adequately explain the decreased Pao2 and increased \( {\mathrm{P}}_{\left(\mathrm{A}-\mathrm{a}\right){\mathrm{O}}_2} \) difference with aging. The cause of such age-related V̇A/Q̇ mismatch often is attributed to degenerative processes in the small airways with aging.
Various abnormal patterns of V̇A/Q̇ distributions measured by the MIGET method adequately explain gas exchange abnormalities in diseased lungs. For example, Fig. 5.6 shows the distribution of V̇A/Q̇ ratios from an individual with chronic obstructive lung disease. The V̇A/Q̇ distribution is bimodal, and large amounts of ventilation go to lung units with extremely high V̇A/Q̇ ratios. This V̇A/Q̇ pattern can be seen in individuals with predominant emphysema (Fig. 5.6, top). Presumably the high V̇A/Q̇ regions represent lung units in which many capillaries have been destroyed by the emphysematous process. In some patients, there are regions of low V̇A/Q̇ (Fig. 5.6, middle), as is commonly seen in patients with predominant chronic bronchitis. Finally, some patients have combinations of both high and low V̇A/Q̇ units (Fig. 5.6, bottom). Note that the main modes of V̇A and Q̇ in the middle and the bottom graphs center on units with V̇A/Q̇ ratio greater than 1 (high V̇A/Q̇ units).
Rights and permissions
Copyright information
© 2018 Springer International Publishing AG, part of Springer Nature
About this chapter
Cite this chapter
Graham, B.L., MacIntyre, N., Huang, Y.C. (2018). Gas Exchange. In: Kaminsky, D., Irvin, C. (eds) Pulmonary Function Testing. Respiratory Medicine. Humana Press, Cham. https://doi.org/10.1007/978-3-319-94159-2_5
Download citation
DOI: https://doi.org/10.1007/978-3-319-94159-2_5
Published:
Publisher Name: Humana Press, Cham
Print ISBN: 978-3-319-94158-5
Online ISBN: 978-3-319-94159-2
eBook Packages: MedicineMedicine (R0)