 This video will cover the following objective from respiratory physiology, outline the process of gas exchange and summarize the process of oxygen and carbon dioxide transport within the respiratory system, define and contrast the process of external and internal respiration, define the Bohr effect, describe how pH and temperature affect the oxygen hemoglobin dissociation curve. Dalton's law states that the total pressure of a mixture of gases is equal to the sum of the individual partial pressures for each gas. For example, if the total pressure of the atmosphere at sea level is 752 millimeters mercury and the concentration of nitrogen in the air is 79 percent, the partial pressure of nitrogen is 593 millimeters of mercury. You could calculate that by multiplying the total pressure of 752 millimeters mercury by 0.79. Similarly, if we wanted to calculate the partial pressure of oxygen, well, we could, because if nitrogen and oxygen are the only gases in that mixture, we could just take the total pressure and subtract 593 to get 159 or we could multiply the 752 by 0.21 because 21 percent of the air is oxygen and so the air is normally around 79 percent nitrogen and 21 percent oxygen. Other gases are in much lower concentrations. Understanding how the partial pressures of gases will contribute to the diffusion of gases is important to help us understand the forces driving gas exchange and to understand how gas exchange will be influenced by changes in the atmospheric pressure. For example, at high altitude, such as on top of Mount Everest, the atmospheric pressure is only 253 millimeters of mercury. The concentration of oxygen and nitrogen in the air is not different. There's still 21 percent oxygen and 79 percent nitrogen. However, the partial pressures are much lower and so the partial pressure of oxygen would only be around 53 millimeters of mercury on top of Mount Everest and this would greatly decrease the rate of diffusion of oxygen into the blood. External respiration is gas exchange between the blood in the pulmonary capillaries and the air in the lungs within the alveoli and the partial pressure gradients are driving simple diffusion of oxygen and carbon dioxide across the respiratory membrane. The respiratory membrane is the wall of the alveolis and the wall of the pulmonary capillary, which are held together by a fused basement membrane of loose connective tissue. Oxygen moves from a relatively high partial pressure in the alveolis where the partial pressure is around 100 millimeters of mercury in the normal circumstance at sea level and it's moving into the blood of the pulmonary capillary where the partial pressure of oxygen is normally around 40 millimeters mercury. In contrast, carbon dioxide is moving out of the blood in the pulmonary capillaries where there's a relatively high partial pressure of carbon dioxide. The partial pressure of carbon dioxide in the pulmonary capillaries is around 46 millimeters of mercury and the partial pressure of carbon dioxide in the air in the alveoli is around 40 millimeters of mercury. So after external respiration the partial pressure of oxygen in the blood that's flowing back in pulmonary veins is up around 100 millimeters of mercury and the partial pressure of carbon dioxide is decreased down to around 40 millimeters of mercury. So when oxygen dissolves in the plasma it can then diffuse across the plasma membrane of erythrocytes and enter into the cytoplasm of the red blood cell where it can bind to the protein hemoglobin. In this way hemoglobin drastically increases the oxygen carrying capacity of the blood by removing oxygen from the plasma. This enables continued diffusion of oxygen from the air of the alveoli into the blood plasma as hemoglobin can become saturated with oxygen. Similarly there's a mechanism to increase the efficiency of carbon dioxide transport in the blood. Some carbon dioxide can bind to hemoglobin also some carbon dioxide will react with water in order to form carbonic acid that dissociates to form bicarbonate and bicarbonate is the primary source of transport for carbon dioxide in the blood. Then when the blood reaches the pulmonary capillaries bicarbonate can react with hydrogen ions to form carbonic acid and carbonic anhydrase can inter-convert the carbonic acid into produce carbon dioxide in water and this allows more carbon dioxide to move out of the blood into the alveoli as carbonic anhydrase is continually converting carbonic acid into carbon dioxide the carbon dioxide is then diffusing out of the red blood cell into the plasma and then carbon dioxide diffuses from the blood plasma into the air in the alveoli across the respiratory membrane. Internal respiration refers to gas exchange between the blood within the systemic capillaries and the interstitial fluid of tissues throughout the body as cells throughout the body use oxygen for aerobic cellular respiration the partial pressure of oxygen in the interstitial fluid decreases and as these cells produce ATP from aerobic cellular respiration they also produce the metabolic waste product carbon dioxide and so carbon dioxide concentration of the interstitial fluid increases and the partial pressure of carbon dioxide in the interstitial fluid increases so normally the partial pressure of carbon dioxide increases to around 46 millimeters of mercury in the interstitial fluid and the partial pressure of carbon dioxide in the pulmonary or in the systemic capillaries is initially around 40 millimeters of mercury coming in from the arteries but as carbon dioxide diffuses into the blood the blood carbon dioxide partial pressure rises to around 46 millimeters mercury and the the blood oxygen partial pressure is normally around 100 millimeters mercury in the blood coming from the arteries into the systemic capillaries then the partial pressure of oxygen in the interstitial fluid is usually below 40 millimeters of mercury and oxygen will diffuse out of the blood into the interstitial fluid until the partial pressure of oxygen in the blood arrives at around 40 millimeters of mercury in the blood that's deoxygenated blood draining into the venules to return back to the to the right atrium of the heart through veins carbon dioxide is transported in the blood some of that carbon dioxide is dissolved in plasma but the only around 40 millimeters of mercury partial pressure of carbon dioxide can dissolve in the plasma and that's only a small percentage of the total carbon dioxide that's transported in the blood so around 7% of the carbon dioxide transported in the blood is just dissolved in the plasma but carbon dioxide can diffuse into erythrocytes and within the erythrocyte it can bind to hemoglobin and around 23% of the carbon dioxide transported in the blood is bound to hemoglobin the carbon dioxide binds to a different site on hemoglobin than oxygen binds to but we'll see that carbon dioxide binding to hemoglobin will have an important influence on the affinity of hemoglobin for oxygen then the majority of carbon dioxide will react with water within the red blood cell and a reaction catalyzed by the enzyme carbonic anhydrase forming carbonic acid then carbonic acid will lose a hydrogen ion forming carbonate and this will have the effect of lowering the pH of the blood and the bicarbonate ion ends up being the primary transport form of carbon dioxide around 70% of the carbon dioxide that's being transported in the blood has been converted into bicarbonate the majority of oxygen transport in the blood is being carried bound to hemoglobin there are four heme groups one within each of the globin polypeptide subunits of the hemoglobin protein so one hemoglobin protein can carry four oxygen molecules here we can see a graph of the oxygen hemoglobin dissociation curve on the x-axis we can see the partial pressure of oxygen within a tissue and on the y-axis we can see the oxygen saturation of hemoglobin represented in percentage and so in the lungs where there's a high partial pressure of oxygen the hemoglobin in the blood of the pulmonary capillaries becomes a hundred percent saturated where four molecules of oxygen bind to each hemoglobin protein then as that blood is delivered out through the systemic circuit in the systemic capillaries oxygen diffuses out of the blood and then oxygen is released from hemoglobin we can see at a partial pressure of 40 millimeters of mercury about 75 percent of the hemoglobin is bound to oxygen meaning there's about three molecules of oxygen for every hemoglobin protein and so about 25 percent of the oxygen that was being transported by hemoglobin was released into the tissues and this means that there's still 75 percent of the oxygen carrying capacity after oxygen has been released to tissues under normal circumstances and this helps provide some security against hypoxia so that there's a reserve of oxygen available for strenuous activity that requires an increase in the metabolic activity and increased rate of oxygen consumption here we see the effect of pH on the oxygen hemoglobin dissociation curve and so the pH of the blood influences the affinity of hemoglobin for oxygen as we see the shift between a pH of 7.4 which is shown in the pink line if the pH falls down to 7.2 you can see the the curve shifts down into the right as we can see with the blue line this represents a decrease in the affinity of hemoglobin for oxygen because at the same partial pressure there's a lower oxygen saturation of hemoglobin if we look at 40 millimeters of mercury oxygen concentration or partial pressure of oxygen that's typical of the systemic capillaries we can see that at a pH of 7.2 there would only be 60 percent oxygen saturation of hemoglobin in comparison with at a pH of 7.4 there would be 75 percent oxygen saturation of hemoglobin then we can see at a higher pH at 7.6 the curve shifts up into the left as we can see represented with the orange line at the same partial pressure of 40 millimeters of mercury now at 7.6 at a pH of 7.6 the oxygen saturation of hemoglobin would be increased to around 80 percent and so this effective pH on the affinity of hemoglobin for oxygen is also known as the Bohr effect so the Bohr effect is a decrease affinity of hemoglobin for oxygen in response to a low pH or the opposite would be an increase in the affinity of hemoglobin for oxygen in response to an elevated blood pH similarly carbon dioxide causes a decrease in the affinity of oxygen for hemoglobin which helps to stimulate the delivery of oxygen in tissues where the carbon dioxide concentration is increasing and the pH is decreasing the affinity of hemoglobin for oxygen decreases helping to release oxygen into those tissues that have the highest demand because they have a high metabolic rate producing large amounts of carbon dioxide similar to the effect of pH and carbon dioxide temperature also has an influence on the affinity of hemoglobin for oxygen we can see that increasingly temperature has the effect of decreasing the affinity of hemoglobin for oxygen and this should make sense that as a tissue is working hard with a high metabolic rate has a high demand for oxygen there's heat being produced by that increased metabolic rate and that heat then further stimulates a decrease in the affinity of hemoglobin for oxygen enabling more efficient delivery of oxygen to that tissue that has a high metabolic rate two three bisphosphoglycerate commonly just abbreviated two three BPG is a molecule that has the influence of decreasing the hemoglobin affinity for oxygen two three BPG is a side product of glycolysis and the normal intermediate of glycolysis one three bisphosphoglycerate would be converted to three phosphoglycerate by the enzyme phosphoglycerate kinase but if the rate of glycolysis is very high one three bisphosphoglycerate can accumulate faster than phosphoglycerate kinase can convert it to three phosphoglycerate and the enzyme bisphosphoglycerate mutase converts some of the extra one three BPG into two three BPG and then two three BPG it can be converted by a two three BPG phosphatase into three phosphoglycerate providing another pathway to get through glycolysis although with a less efficient pathway because we would not get the ATP that's generated by phosphoglycerate kinase if we took this alternate pathway however the the benefit of producing two three BPG is that two three BPG will then bind to hemoglobin in erythrocytes and decrease the affinity of hemoglobin for oxygen and so as two three BPG is accumulating it stimulates the release of oxygen in a tissue that has a high metabolic demand