23.6 Respiration: Pulmonary and Tissue Gas Exchange PDF

23.6 Respiration: Pulmonary and Tissue Gas Exchange* Gas exchange is the movement of respiratory gases between blood and either alveoli or cells of systemic tissues. The movement of these gases between blood in pulmonary capillaries and the alveoli of the lungs is called pulmonary gas exchange, wher...

23.6 Respiration: Pulmonary and Tissue Gas Exchange* Gas exchange is the movement of respiratory gases between blood and either alveoli or cells of systemic tissues. The movement of these gases between blood in pulmonary capillaries and the alveoli of the lungs is called pulmonary gas exchange, whereas the movement of respiratory gases between blood in systemic capillaries and cells of systemic tissues is called tissue gas exchange. Here we describe the general chemical principles of gas exchange; then we describe the specifics of pulmonary gas exchange in the lungs and tissue gas exchange at systemic cells. 23.6a Chemical Principles of Gas Exchange LEARNING OBJECTIVES 36. Define partial pressure and explain how its is altered with changes in total pressure and the percentage of a gas. 37. Describe a partial pressure gradient and the movement of a gas relative to its partial pressure gradient. 38. Explain the laws that govern gas solubility. Page 927 Gas exchange within the body is influenced by certain chemical principles. Three of these principles are addressed in this section: (a) partial pressure of a gas (and Dalton’s law), (b) partial pressure gradients, and (c) gas solubility (and Henry’s law). Partial Pressure and Dalton’s Law The gases in air move collectively down a (total) pressure gradient through the respiratory passageway during breathing, as described in section 23.5. In contrast, each gas moves independently down its own partial pressure gradient during gas exchange. But what is partial pressure? Partial pressure is the pressure exerted by each gas within a mixture of gases and is measured in mm Hg; it is written with a P followed by the symbol for the gas. For example, the partial pressure for oxygen is written as Po2. Partial pressure of a gas is determined by multiplying the total pressure exerted by all of the gases in an area by the percentage of the gas within the mixture of gases. We can use atmospheric pressure and the mixture of gases in air within the atmosphere to more fully explain partial pressure. We have seen that at sea level atmospheric pressure has a value of 760 mm Hg. The percentage of each of the most common gases in the atmospheric air are as follows—nitrogen (78.6%), oxygen (20.9%), carbon dioxide (0.04%), and water vapor (0.46%). Thus, the partial pressure for each of these gases is: 597 mm Hg, 159 mm Hg, 0.3 mm Hg, and 3.5 mm Hg, respectively. The calculation for the partial pressure for each of these four gases is shown in figure 23.26. Figure 23.26 Partial Pressure. The partial pressure of the major gases within the atmosphere at sea level and the partial pressure of oxygen at an elevation of 10,000 feet and at 33 feet below sea level. The partial pressure of a gas is determined by multiplying the total pressure and the percentage of the gas. Observe the difference in the partial pressure of oxygen at the three different total pressures—110 mm Hg at 10,000 feet, 159 mm Hg at sea level, and 318 mm Hg at 33 feet below sea level. *Note that for the calculation at sea level, the other 0.2 mm Hg of the 760 mm Hg is contributed by the minor gases within the atmosphere. Page 928 Notice in figure 23.26 that when these partial pressures are added together, their sum must equal the total atmospheric pressure. In this example at sea level, total atmospheric pressure is 760 mm Hg. The relationship of partial pressure to total pressure is summarized by Dalton’s law, which states that the total pressure in a mixture of gases is equal to the sum of all of the individual partial pressures (see table 23.1). Consider that the partial pressure of a gas changes with either a change in (a) the total pressure exerted by all of the gases (e.g., 760 mm Hg at sea level) or (b) the percentage of the gas in composition of the gases. If you view figure 23.26, it is possible to compare how Po2 varies with total pressure. At sea level, the Po2 is 159 mm Hg (as just described; 760 mm Hg × 20.9%). However, with increasing altitude the air “thins,” and at an altitude of 10,000 feet the total pressure at this elevation is lower, at 523 mm Hg. Thus, the Po2 is calculated as 523 mm Hg × 20.9% = 110 mm Hg. In comparison, at underwater depths, the total pressure is greater than at sea level. For example, at 33 feet below sea level, the total pressure is 2× that at sea level, with a value at 1520 mm Hg. Thus, the Po2 is 1520 mm Hg × 20.9% = 318 mm Hg. (This greater atmospheric pressure is because sea water is ∼800× more dense than air.) We will see the significance of changing atmospheric pressure and its influence on gas exchange (and gas transport) in later sections. A change in the percentage of a gas and its influence on partial pressure is described next with relevant partial pressures in the body. Relevant Partial Pressures in the Body The partial pressures that are relevant for understanding respiratory gas exchange are both the Po2 and Pco2 within alveoli in the lungs, systemic cells, and circulating blood. Refer to figure 23.27 as you read through this section. Figure 23.27 Variables that Influence Gas Exchange. (a) Partial pressures in the atmosphere, alveoli, and cells of systemic tissues. (b) Partial pressure gradient within the lungs and pulmonary capillaries showing oxygen as an example. (c) Structure of hemoglobin, which binds (and releases) O2 at the iron (Fe2+) within the heme and CO2 and H+ to globin. Concept Overview Physiology Interactives Po2 and Pco2 in the Alveoli Although air from the environment is inhaled directly into the lungs, the partial pressures of the gases within the alveoli are different from the respective atmospheric partial pressures because of the change in the percentage of each gas in the air. There are several reasons for these differences in the gas percentages: (a) Air from the environment mixes with the air remaining in the anatomic dead space in the respiratory tract; (b) oxygen diffuses out of the alveoli into the blood, and carbon dioxide diffuses from the blood into the alveoli; and (c) more water vapor is present within the alveoli because of the higher humidity there. Consequently, within the alveoli the percentage of oxygen is lower (13.7%) and the percentage of carbon dioxide is higher (5.2%) than in the atmosphere. The calculation of the partial pressure of the respiratory gases in the alveoli is as follows: Alveolar Partial Pressures (at Sea Level) The Po2 is lower in the alveoli (Po2 = 104 mm Hg) than it is in the atmosphere (Po2 = 159 mm Hg), and the Pco2 is higher in the alveoli (Pco2 = 40 mm Hg) than it is in the atmosphere (Pco2 = 0.3 mm Hg) ( figure 23.27a). Note that under normal circumstances the alveolar partial pressures remain constant because oxygen, which is diffusing from the alveoli into the blood, is being resupplied to the alveoli during inspiration, and the carbon dioxide, which is diffusing from the blood into the alveoli, is being removed from the alveoli during expiration. Po2 and Pco2 in Systemic Cells In comparison, the partial pressures of both O2 and CO2 in systemic cells reflect the activities of cellular respiration. Cells use oxygen during cellular respiration and produce carbon dioxide as a waste product. So in comparison to the percentages in alveoli, the percentage of oxygen in the systemic cells is lower and the percentage of carbon dioxide is higher. The partial pressures of the respiratory gases in systemic cells at rest usually exhibit the measured values as shown here ( figure 23.27a): Systemic Cell Partial Pressures (Resting Conditions) Page 929 Note that under normal, resting conditions, the partial pressures in systemic cells remain constant because the continuous delivery of oxygen and removal of carbon dioxide by the blood (systemic capillaries) correspond with the amounts associated with cellular respiration. Po2 and Pco2 in the Circulating Blood Both Po2 and Pco2 levels in the blood are not set values, unlike the relatively constant values in the alveoli and systemic cells. The Po2 and Pco2 in the blood change continuously as the blood flows through the pulmonary capillaries, where oxygen enters the blood and carbon dioxide leaves the blood. In addition, as blood flows through systemic capillaries, the reverse occurs: Oxygen leaves the blood and carbon dioxide enters the blood. We explain in sections 23.6b and c how the values for blood Po2 and blood Pco2 change during pulmonary and tissue gas exchange. Partial Pressure Gradients A partial pressure gradient exists when the partial pressure for a specific gas is higher in one region than in another. For example, as we see in figure 23.27b, a partial pressure gradient exists for oxygen between the alveoli (Po2 = 104 mm Hg) and the blood in pulmonary capillary (Po2 = 40 mm Hg). When a partial pressure gradient exists between two regions for a given gas, then the gas moves from the region of its higher partial pressure to the region of its lower partial pressure, and it may continue to move until the partial pressures in the two regions become equal. The exchange of respiratory gases in both pulmonary gas exchange and tissue gas exchange is dependent upon partial pressure gradients. Gas Solubility and Henry’s Law In addition to the chemical principles of partial pressure and partial pressure gradients just described, gas exchange and gas transport are also influenced by the chemical principles that govern the exchange of gas between air (a gas) and blood (a liquid). These principles are summarized by Henry’s law, which states that at a given temperature, the solubility of a gas in a liquid (i.e., how much gas can either enter or leave the liquid) is dependent upon (a) the partial pressure of the gas in the air and (b) the solubility coefficient of the gas in the liquid (see table 23.1). The partial pressure of a gas is the driving force to move it into a liquid. Recall that a given partial pressure is determined by the total pressure of all of the gases in an area and the specific percentage of the gas of interest within the mixture—if either of these changes, the amount of the gas that enters the liquid changes. Carbon dioxide gas in a soft drink is an example of forcing more gas into a liquid by increasing its partial pressure. The CO2 is forced into the soda under high pressure, and then the container is sealed. When the can is opened and the pressure released, carbon dioxide leaves the soda because of the lower Pco2 of the atmosphere; over time, the soda becomes “flat.” The solubility coefficient is the volume of gas that dissolves in a specified volume of liquid at a given temperature and pressure. This coefficient is a constant that depends upon the interactions between molecules of both the gas and the liquid. The more favorable these molecular interactions, the greater the amount of gas that dissolves in the liquid. Gases vary in their solubility in water. For example, oxygen has a very low solubility in water (solubility coefficient = 0.024), whereas carbon dioxide has a solubility about 24 times that of oxygen (solubility coefficient = 0.57). Nitrogen is the least soluble of these three major gases in the atmosphere and is about half as soluble as oxygen. The order of solubility for the three gases dissolved in water would be Because the amount of a gas that dissolves in a liquid is dependent upon both its partial pressure and its solubility coefficient, gases with low solubility coefficients require larger partial pressure gradients to “push” the gas into the liquid. Nitrogen, with its very low solubility, does not dissolve in the blood in significant amounts under conditions at or above sea level because of its very low solubility coefficient (see Clinical View 23.14: “Decompression Sickness and Hyperbaric Oxygen Chambers”). Page 930 WHAT DID YOU LEARN? 29 Given the same partial pressure for oxygen and carbon dioxide, which respiratory gas enters a water solution more readily? Explain using Henry’s law. 23.6b Pulmonary Gas Exchange LEARNING OBJECTIVES 39. Describe pulmonary gas exchange and the partial pressure gradients responsible. 40. Name the two anatomic features of the respiratory membrane that contribute to efficient pulmonary gas exchange. 41. Explain ventilation-perfusion coupling and how it maximizes pulmonary gas exchange. Pulmonary gas exchange occurs within the lungs and tissue gas exchange occurs at the cells of systemic tissues ( tissue gas exchange in figure 23.28). Here we describe pulmonary gas exchange and discuss section 23.6c. Figure 23.28 Pulmonary and Tissue Gas Exchange. (a) Gas exchange takes place both in the lungs and in systemic tissues. (b) Pulmonary gas exchange occurs between the air in the alveoli and the blood within the pulmonary capillaries, due to each gas moving down its partial pressure gradient. Each gas moves until the partial pressure for that gas in the blood is equal to the partial pressure within the alveoli. (c) Tissue gas exchange occurs between the systemic cells and the blood within the systemic capillaries, due to each gas moving down its partial pressure gradient. Each gas moves until the partial pressure for that gas in the blood is equal to the partial pressure within the systemic cells. (Values for Po2 are shown in orange; values for Pco2 are shown in blue.) APR Module 11: Respiratory: Animations: Diffusion Across Respiratory Membrane Watch Video: Changes in the Partial Pressure of Oxygen and Carbon Dioxide Pulmonary gas exchange is the movement of respiratory gases between the air within the alveoli of the lungs and the blood within the pulmonary capillaries. figure 23.28b depicts events that occur in pulmonary gas exchange. Oxygen diffuses from the alveoli into the blood and carbon dioxide diffuses from the blood into the alveoli. The driving force for the diffusion of these gases is their individual partial pressure gradients. Notice that the Po2 in the alveoli is 104 mm Hg, and the blood entering the pulmonary capillaries has a Po2 of 40 mm Hg. Oxygen diffuses across the respiratory membrane from the alveoli into the pulmonary capillaries because of the Po2 partial pressure gradient, until the Po2 in the blood is equal to that of the alveoli at 104 mm Hg. Thus, blood Po2 has increased from 40 to 104 mm Hg as blood moves through the pulmonary capillaries. However, the Po2 in the alveoli remains constant because oxygen is continuously entering the alveoli through the respiratory passageways. Page 931 Simultaneously, CO2 is diffusing in the opposite direction; the Pco2 in the alveoli is 40 mm Hg and that of the blood entering the pulmonary capillaries is 45 mm Hg. Carbon dioxide diffuses down its partial pressure gradient from the blood into the alveoli until the Pco2 in the blood is equal to that of the alveoli at 40 mm Hg. Thus, blood Pco2 has decreased from 45 to 40 mm Hg as blood moves through the pulmonary capillaries. As with oxygen, the Pco2 in the alveoli also remains constant because carbon dioxide is continuously leaving the alveoli through the respiratory passageways. INTEGRATE CLINICAL VIEW 23.14 Decompression Sickness and Hyperbaric Oxygen Chambers Decompression sickness, also known as the bends, or Caisson disease, occurs when a diver is submerged in water beyond a certain depth and returns too quickly to the surface. Divers are breathing compressed air while diving; the farther they submerge, the greater the degree of compression and the higher the total pressure. The atmospheric pressure doubles (and Pn2 doubles)—for example, at 10 meters (32.8 feet) below sea level. Nitrogen, although it has low solubility, is forced into the blood. If a diver rises to the surface too quickly, there is not enough time to expel all of the nitrogen through expiration. Instead, dissolved nitrogen comes out of solution while still in the blood and tissues, and nitrogen gas bubbles develop in these tissues, including the joints. This is similar to how carbon dioxide comes out of a soda when a can is opened. The nickname the bends comes from the fact that divers bend their joints to try to relieve the pain. Nicole Helgason/Shutterstock Hyperbaric (hĪ΄pĕr-bar΄ik; hyper = over, baros = pressure) oxygen chambers are used to treat individuals with decompression sickness. Additional oxygen is forced into the blood plasma when an individual is placed in a chamber with pressures higher than atmospheric pressure (i.e., greater than 760 mm Hg [or 1 atm]). Hyperbaric oxygen chambers also can be used to treat certain disorders such as carbon monoxide poisoning, foot ulcers associated with diabetes, traumatic crush injuries, severe anemia, and gas gangrene. The additional oxygen that enters the tissue accelerates the tissue healing rate. Efficiency of Gas Exchange at the Respiratory Membrane The efficiency of both O2 and CO2 diffusion during pulmonary gas exchange is dependent upon anatomic features of the respiratory membrane: its large surface area and its minimal thickness. The aggregate surface area of the respiratory membrane in a healthy lung measures approximately 70 square meters—a little less than half the size of a tennis court. The minimal thickness of this barrier measures approximately 0.5 micrometer ( figure 23.28b). The relative thickness of the respiratory membrane is effectively increased in pneumonia (see Clinical View 23.8: “Pneumonia”). Physiologic adjustments also contribute to maximizing gas exchange at the alveoli. Some alveoli, at any given time, are well ventilated and some are not; similarly, some regions of the lung have ample blood moving through pulmonary capillaries, and some do not. The smooth muscle of both the bronchioles that lead into the alveoli and the arterioles that carry blood to pulmonary capillaries can contract and relax to maximize gas exchange. This inherent ability of bronchioles to regulate airflow and arterioles to regulate blood flow simultaneously is called ventilation-perfusion coupling ( figure 23.29). Figure 23.29 Ventilation-Perfusion Coupling. (a) Bronchioles dilate or constrict in response to changes in Pco2 in air within the bronchioles. (b) Pulmonary arterioles dilate or constrict in response to changes in either blood Po2 or blood Pco2. Ventilation is altered by changes in bronchodilation and bronchoconstriction. Bronchioles dilate in response to an increase in Pco2 within the air of the bronchiole, whereas they constrict in response to a decrease in Pco2. Perfusion is altered by changes in pulmonary arteriole dilation and constriction. These arterioles dilate in response to either an increase in Po2 or a decrease in Pco2 in the blood, whereas they constrict in response to either a decrease in Po2 or an increase in Pco2. Note these changes in bronchioles and pulmonary arterioles occur independently of one another. WHAT DID YOU LEARN? 30 How do the partial pressures of oxygen and carbon dioxide in blood change during pulmonary gas exchange? 31 Which of the following would decrease pulmonary gas exchange: loss of alveoli, fluid accumulation in the lungs, arteriole vasoconstriction, and bronchiole dilation? Explain. 23.6c Tissue Gas Exchange LEARNING OBJECTIVES 42. Explain tissue gas exchange. 43. Compare and contrast pulmonary and tissue gas exchange. Page 932 INTEGRATE CLINICAL VIEW 23.15 Emphysema Emphysema (em-fi-zē΄mă; en = in, physema = blowing) is an irreversible loss of pulmonary gas exchange surface area due to inflammation of air passageways distal to the terminal bronchioles, in conjunction with widespread destruction of pulmonary elastic connective tissue. These combined events lead to dilation of individual alveoli, as well as merging of individual alveoli with others. The result is a decrease in the total number of alveoli, and the subsequent loss of gas exchange surface area. A person with advanced emphysema is unable to expire effectively, so that stagnant, deoxygenated air builds up within the abnormally large (but numerically diminished) alveoli. Most cases of emphysema result from damage caused by smoking. Once the tissue in the lung has been destroyed, it cannot regenerate, and thus there is no cure for emphysema. The best therapy for an emphysema patient is to stop smoking and try to get optimal use from the remaining lung tissue by using a bronchodilator, seeking prompt treatment for pulmonary infections, and taking oxygen supplementation if necessary. (a) A section of an emphysemic lung shows dilated alveoli. (b) Micrograph shows alveoli are abnormally large and nonfunctional. (a) ©CNRI/Science Source; (b) ©McGraw-Hill Education/Al Telser Tissue gas exchange is the movement of respiratory gases between the cells within systemic tissues and the blood within the systemic capillaries. figure 23.28c depicts the events that occur during tissue gas exchange. Oxygen diffuses from the blood into systemic cells, and simultaneously, carbon dioxide diffuses from the systemic cells into the blood. The driving force for the diffusion of these gases is their individual partial pressure gradients. INTEGRATE CLINICAL VIEW 23.16 Respiratory Diseases and Efficiency of Pulmonary Gas Exchange Certain diseases can decrease the efficiency of oxygen and carbon dioxide exchange due to changes in the anatomic structure of the respiratory membrane. For example, individuals with emphysema, lung cancer, or tuberculosis, or those who have survived surgical removal of a lung, have decreased numbers of alveoli and therefore decreased surface area for gas exchange. Individuals with pneumonia or those with congestive heart failure (see Clinical View 19.1: “Congestive Heart Failure”) of the left side of the heart are at risk for fluid buildup, resulting in a thickened respiratory membrane. Changes in ventilation-perfusion coupling can also decrease efficiency of gas exchange. Individuals with narrowing of air passageways from bronchitis or asthma experience decreased air reaching the alveoli, whereas those with obstructed blood flow from a pulmonary embolism have decreased blood flow into the pulmonary capillaries. These disease conditions result in a decrease in blood Po2 as less oxygen enters the blood and an increase in blood Pco2 as more carbon dioxide remains within the blood. Notice that the Po2 in the systemic cells is 40 mm Hg, whereas the blood as it enters the surrounding systemic capillaries has a Po2 of 100 mm Hg. Therefore, oxygen diffuses out of the systemic capillaries down its partial pressure gradient into the cells until the blood Po2 is equal to the partial pressure in the cells at 40 mm Hg. Thus, blood Po2 has decreased from 100 to 40 mm Hg as blood moves through the systemic capillaries. Simultaneously, carbon dioxide is diffusing in the opposite direction. The Pco2 in systemic cells is 45 mm Hg, whereas the blood entering the systemic capillaries is 40 mm Hg. Carbon dioxide diffuses down its partial pressure gradient from the cells into the blood until blood Pco2 is 45 mm Hg. Thus, blood Pco2 has increased from 40 to 45 mm Hg as blood moves through the systemic capillaries. Unless conditions change, such as when engaging in strenuous activity, the partial pressure of each gas in the systemic cells remains relatively constant because the continuous delivery of oxygen and removal of carbon dioxide correspond with the amounts associated with cellular respiration (see section 3.4). Integration of Pulmonary and Tissue Gas Exchange The changes to blood Pco2 and blood Po2 that occur during pulmonary and tissue gas exchange are integrated in figure 23.30. During pulmonary gas exchange, blood Pco2 decreases from 45 to 40 mm Hg; during tissue gas exchange, blood Pco2 increases from 40 to 45 mm Hg ( figure 23.30a). Notice that the Pco2 values reverse as the blood makes its way through the two cardiovascular circuits: 45 to 40 mm Hg in the pulmonary capillaries, 40 to 45 mm Hg in the systemic capillaries. Figure 23.30 Changes in Respiratory Gas Partial Pressures Within the Blood. Observe that (a) the blood Pco2 values reverse as the blood makes its way through the capillaries of the two cardiovascular circuits: going from 45 to 40 mm Hg in the pulmonary capillaries, and from 40 to 45 mm Hg in the systemic capillaries, and (b) the blood Po2 values essentially reverse as the blood makes its way through the two capillaries of the cardiovascular circuits: going from 40 mm Hg to 104 mm Hg in the pulmonary capillaries, and from 100 to 40 mm Hg in the systemic capillaries. WHAT DO YOU THINK? 6 What can account for the blood Po2 arriving at systemic capillaries with a lower Po2 value than when it left the pulmonary capillaries? Page 933 Blood Po2 increases from 40 to 104 mm Hg during pulmonary gas exchange, and blood Po2 decreases from 100 to 40 mm Hg during tissue gas exchange ( figure 23.30b). When we discussed pulmonary blood circulation in section 23.4b, we noted that bronchial veins drain small amounts of deoxygenated blood into the pulmonary veins prior to the blood returning to the heart, where it is subsequently pumped by the left ventricle through the systemic circulation. This input of deoxygenated blood accounts for the decrease in Po2 from 104 to 100 mm Hg. WHAT DID YOU LEARN? 22 How do the partial pressures of oxygen and carbon dioxide in blood change during tissue gas exchange?