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Gas Exchange
Section 1
NORMAL STRUCTURE AND FUNCTION
Chapter 5
Gas Exchange Peter D. Wagner
The primary function of the lungs is to exchange gases between the blood and the external air. Mostly, of course, it is only O2 and CO2 that undergo exchange, but during gaseous anesthesia, the anesthetic gas is taken up by the lungs during induction or eliminated by the lungs during recovery. In addition, when a person is exposed to foreign gases in the air, these gases can be inhaled and may undergo exchange as well. Furthermore, gases with selected physical and chemical properties are sometimes used in cardiorespiratory research or even clinical care. For example, acetylene as a moderately soluble gas can be used to measure pulmonary blood flow; carbon monoxide (in very low concentrations) is routinely used to measure the lung diffusing capacity or transfer factor. Fortunately, all such gases behave in accordance with the same basic physical principles underlying gas transport and exchange—mass conservation—explained in some detail further on. Although different gases appear to behave differ ently, this reflects their different physicochemical properties related to how they are transported in blood, and not differ ences in conforming to the mass conservation principles of exchange. Moreover, gas uptake from air into blood obeys the same rules as for gas elimination from the blood to the air. Thus, the topic of gas exchange can be treated as a general process applicable to all gases, whether taken up or eliminated. Subse quent applications can be made for individual gases in accord with their blood transport properties. In this chapter, the focus is primarily on the respiratory gases O2 and, to a lesser extent, CO2. THE BASIS OF GAS EXCHANGE: VENTILATION, DIFFUSION, AND PERFUSION The lungs conduct gas exchange through three interacting pro cesses: ventilation, diffusion, and perfusion (or blood flow). Ventilation brings O2 from the air to the alveoli (and simultane ously eliminates CO2, transferred from the blood, to the air). Diffusion is the process by which O2 in the alveoli passes across the alveolar wall into the pulmonary capillary. Perfusion moves the blood through the pulmonary circulation and allows con tinuously flowing red cells to take on O2. Ventilation and per fusion are mostly convective processes that require energy expenditure by the organism. Ventilation is an alternating, bidi rectional process of inspiration and expiration, while perfusion is unidirectional from right ventricle to left atrium. Inspiration is accomplished by the respiratory muscles (diaphragm and external intercostal muscles mostly), which on contraction expand the thoracic cage, thus reducing the intrapleural pres sure around the lungs, resulting in passive lung expansion. Expiration generally is passive and occurs as the respiratory
muscles relax and allow the elastic recoil of the lung to expel air. Diffusion is passive and does not require the organism to expend energy. It simply reflects random molecular motion that over time tends to equalize molecular concentrations in space. RELATIONSHIPS BETWEEN LUNG STRUCTURE AND FUNCTION
The evolutionary “decision” to conduct gas exchange by passive diffusion (rather than by energy-requiring active transport) was a profound one that dictated the basic structure of the lungs. The laws of diffusion show that diffusive mass transfer rates are directly proportional to the surface area available for diffu sion and are inversely proportional to the distance the molecule must diffuse. The fundamental unit of structure in the lung is the alveolus, small and roughly spherical in shape, with an average radius of 150 micrometers (µm). There are about 300 million alveoli in the human lung. Each is supplied with air that must pass through the branching bronchial tree (conduct ing airways). The wall of each alveolus, shared by adjacent alveoli, is packed with capillaries. The tissue separating alveolar gas from the blood in the capillaries consists of the capillary endothelium, interstitial matrix, alveolar epithelium, and a thin layer of fluid. The entire wall is less than 0.5 µm in thickness. These dimensions imply a total alveolar surface area of about 80 m2, yet a gas volume of only 4 L (small enough to fit within the chest cavity). Thus, the actual lung can conduct diffusive exchange efficiently because of the large surface area and small diffusion distance. By contrast, if the lungs consisted of just a single large sphere of the same 4-L volume, its surface area would be only 18 m2 (640-fold less). Moreover, if the same mass of 0.5-µm-thick alveolar wall tissue covering all 300 million alveoli were spread around this one sphere, its thickness would be over 300 µm, also about 600 times greater than in the actual lung. Because diffusion rates depend on the ratio of area to thickness, the real lung is about 640 × 600, or 400,000 times better at diffusive transport than would be a single sphere of the same volume and mass. The message is that by dividing up the lung into a very large number of very small structures, diffusion becomes a feasible and energy-efficient method of gas exchange, circumventing the need for active transport. This picture of the lungs is similar in some ways to a bunch of grapes in which each grape is an alveolus, the skin is the alveolar wall (containing the capillaries) and the pulp inside is the alveolar air space. The stalks connecting each grape to its cluster depict the conducting airways and blood vessels. A major shortcoming to the grape analogy, however, is that each grape in a bunch is physically detached from all others in the bunch. However, all alveoli are connected, sharing common 37
NORMAL STRUCTURE AND FUNCTION
Chapter 5
Gas Exchange Peter D. Wagner
The primary function of the lungs is to exchange gases between the blood and the external air. Mostly, of course, it is only O2 and CO2 that undergo exchange, but during gaseous anesthesia, the anesthetic gas is taken up by the lungs during induction or eliminated by the lungs during recovery. In addition, when a person is exposed to foreign gases in the air, these gases can be inhaled and may undergo exchange as well. Furthermore, gases with selected physical and chemical properties are sometimes used in cardiorespiratory research or even clinical care. For example, acetylene as a moderately soluble gas can be used to measure pulmonary blood flow; carbon monoxide (in very low concentrations) is routinely used to measure the lung diffusing capacity or transfer factor. Fortunately, all such gases behave in accordance with the same basic physical principles underlying gas transport and exchange—mass conservation—explained in some detail further on. Although different gases appear to behave differ ently, this reflects their different physicochemical properties related to how they are transported in blood, and not differ ences in conforming to the mass conservation principles of exchange. Moreover, gas uptake from air into blood obeys the same rules as for gas elimination from the blood to the air. Thus, the topic of gas exchange can be treated as a general process applicable to all gases, whether taken up or eliminated. Subse quent applications can be made for individual gases in accord with their blood transport properties. In this chapter, the focus is primarily on the respiratory gases O2 and, to a lesser extent, CO2. THE BASIS OF GAS EXCHANGE: VENTILATION, DIFFUSION, AND PERFUSION The lungs conduct gas exchange through three interacting pro cesses: ventilation, diffusion, and perfusion (or blood flow). Ventilation brings O2 from the air to the alveoli (and simultane ously eliminates CO2, transferred from the blood, to the air). Diffusion is the process by which O2 in the alveoli passes across the alveolar wall into the pulmonary capillary. Perfusion moves the blood through the pulmonary circulation and allows con tinuously flowing red cells to take on O2. Ventilation and per fusion are mostly convective processes that require energy expenditure by the organism. Ventilation is an alternating, bidi rectional process of inspiration and expiration, while perfusion is unidirectional from right ventricle to left atrium. Inspiration is accomplished by the respiratory muscles (diaphragm and external intercostal muscles mostly), which on contraction expand the thoracic cage, thus reducing the intrapleural pres sure around the lungs, resulting in passive lung expansion. Expiration generally is passive and occurs as the respiratory
muscles relax and allow the elastic recoil of the lung to expel air. Diffusion is passive and does not require the organism to expend energy. It simply reflects random molecular motion that over time tends to equalize molecular concentrations in space. RELATIONSHIPS BETWEEN LUNG STRUCTURE AND FUNCTION
The evolutionary “decision” to conduct gas exchange by passive diffusion (rather than by energy-requiring active transport) was a profound one that dictated the basic structure of the lungs. The laws of diffusion show that diffusive mass transfer rates are directly proportional to the surface area available for diffu sion and are inversely proportional to the distance the molecule must diffuse. The fundamental unit of structure in the lung is the alveolus, small and roughly spherical in shape, with an average radius of 150 micrometers (µm). There are about 300 million alveoli in the human lung. Each is supplied with air that must pass through the branching bronchial tree (conduct ing airways). The wall of each alveolus, shared by adjacent alveoli, is packed with capillaries. The tissue separating alveolar gas from the blood in the capillaries consists of the capillary endothelium, interstitial matrix, alveolar epithelium, and a thin layer of fluid. The entire wall is less than 0.5 µm in thickness. These dimensions imply a total alveolar surface area of about 80 m2, yet a gas volume of only 4 L (small enough to fit within the chest cavity). Thus, the actual lung can conduct diffusive exchange efficiently because of the large surface area and small diffusion distance. By contrast, if the lungs consisted of just a single large sphere of the same 4-L volume, its surface area would be only 18 m2 (640-fold less). Moreover, if the same mass of 0.5-µm-thick alveolar wall tissue covering all 300 million alveoli were spread around this one sphere, its thickness would be over 300 µm, also about 600 times greater than in the actual lung. Because diffusion rates depend on the ratio of area to thickness, the real lung is about 640 × 600, or 400,000 times better at diffusive transport than would be a single sphere of the same volume and mass. The message is that by dividing up the lung into a very large number of very small structures, diffusion becomes a feasible and energy-efficient method of gas exchange, circumventing the need for active transport. This picture of the lungs is similar in some ways to a bunch of grapes in which each grape is an alveolus, the skin is the alveolar wall (containing the capillaries) and the pulp inside is the alveolar air space. The stalks connecting each grape to its cluster depict the conducting airways and blood vessels. A major shortcoming to the grape analogy, however, is that each grape in a bunch is physically detached from all others in the bunch. However, all alveoli are connected, sharing common 37