Physiology of Respiration • Exchange of Oxygen and Carbon Dioxide

Class: B.Sc. HONOURS ZOOLOGY, IV Sem Paper: Animal Physiology: LSS (Theory Class) Teacher’s name: Meenakshi Rana Date: 21st March, 2020 Time: 12:30 – 2:30 PM Physiology of Respiration Exchange of oxygen and carbon dioxide The purpose of breathing is to provide a continual supply of oxygen and continuous removal of carbon dioxide. In pulmonary and systemic gas exchange, oxygen and carbon dioxide diffuse from areas of higher partial pressures to areas of lower partial pressures. The partial pressure of a gas is the pressure exerted by that gas in a mixture of gases. External respiration (pulmonary gas exchange): exchange of gases occurs between alveoli of the lungs and pulmonary blood capillaries by the process of simple diffusion. Internal respiration (systemic gas exchange): exchange of gases occurs between systemic blood capillaries and tissue cells. Exchange of these two gases is governed by two gas laws – Dalton’s law and Henry’s law. According to Dalton's law of partial pressure, each gas in a mixture of gases exerts a pressure, known as its partial pressure, which is equal to the pressure the gas would exert if it were the only gas present. The total pressure of the mixture is the sum of the partial pressures of all the gases present. According to Henry’s law, the quantity of a gas that will dissolve in a liquid is proportional to the partial pressure of the gas and its solubility (given that the temperature remains constant). Because solubility is a constant and the temperature of the blood does not vary significantly, the concentration of a gas dissolved in a fluid (such as plasma) depends directly on its partial pressure in the gas mixture. Atmospheric pressure at sea level is 760 mm Hg. Since there is 21% oxygen by volume in the atmosphere, the partial pressure of oxygen is 0.21 × 760 = about 160 mm Hg. This value is called the partial pressure of oxygen (PO2) because it is the portion of atmospheric pressure 1 contributed by O2. Similarly, nitrogen constitutes about 78% of the atmosphere, so its partial pressure is equal to 0.78 × 760 = 593 mm Hg. The partial pressure of CO2, PCO2, is much less, only 0.2 mm Hg at sea level. Transport of oxygen and carbon dioxide Oxygen transport Oxygen does not dissolve easily in water, so only about 1.5% of inhaled O2 is dissolved in blood plasma, which is mostly water. About 98.5% of blood O2 is bound to hemoglobin in red blood cells. Each 100 mL of oxygenated blood contains the equivalent of 20 mL of gaseous O2. 2 The heme portion of hemoglobin contains four atoms of iron, each capable of binding to a molecule of O2. Oxygen and hemoglobin bind in an easily reversible reaction to form oxyhemoglobin. The 98.5% of the O2 that is bound to hemoglobin is trapped inside RBCs, so only the dissolved O2 (1.5%) can diffuse out of tissue capillaries into tissue cells. Thus, it is important to understand the factors that promote O2 binding to and dissociation (separation) from hemoglobin. The relationship between hemoglobin and oxygen partial pressure (Oxygen– hemoglobin dissociation curve) The most important factor that determines how much O2 binds to hemoglobin is the PO2; the higher the PO2, the more O2 combines with Hb. When reduced hemoglobin (Hb) is completely converted to oxyhemoglobin (Hb–O2), the hemoglobin is said to be fully saturated; when hemoglobin consists of a mixture of Hb and Hb–O2, it is partially saturated. The percent saturation of hemoglobin expresses the average saturation of hemoglobin with oxygen. For instance, if each hemoglobin molecule has bound two O2 molecules, then the hemoglobin is 50% saturated because each Hb can bind a maximum of four O2. The relationship between the percent saturation of haemoglobin and PO2 is illustrated in the oxygen–hemoglobin dissociation curve. A plot of percent oxygen saturation of hemoglobin versus partial pressure of O2 for hemoglobin is S-shaped or sigmoidal. 3 Factors affecting the affinity of hemoglobin for oxygen Although PO2 is the most important factor that determines the percent O2 saturation of hemoglobin, several other factors influence the tightness or affinity with which hemoglobin binds O2. In effect, these factors shift the entire curve either to the left (higher affinity) or to the right (lower affinity). The following four factors affect the affinity of hemoglobin for O2: 1. Acidity (pH): As acidity increases (pH decreases), the affinity of hemoglobin for O2 decreases, and O2 dissociates more readily from hemoglobin. In other words, increasing acidity enhances the unloading of oxygen from hemoglobin. When pH decreases, the entire oxygen– hemoglobin dissociation curve shifts to the right; at any given PO2, Hb is less saturated with + O2, a change termed the Bohr effect. The Bohr effect works both ways: An increase in H in blood causes O2 to unload from hemoglobin, and the binding of O2 to hemoglobin causes unloading of H+ from hemoglobin. The explanation for the Bohr effect is that haemoglobin can act as a buffer for hydrogen ions (H+). But when H+ ions bind to amino acids in hemoglobin, they alter its structure slightly, decreasing its oxygen-carrying capacity. Thus, lowered pH drives O2 off hemoglobin, making more O2 available for tissue cells. By contrast, elevated pH increases the affinity of hemoglobin for O2 and shifts the oxygen–hemoglobin dissociation curve to the left. 2. Partial pressure of carbon dioxide: CO2 also can bind to hemoglobin, and the effect is + similar to that of H (shifting the curve to the right). As PCO2 rises, hemoglobin releases O2 more readily. PCO2 and pH are related factors because low blood pH (acidity) results from high PCO2. As CO2 enters the blood, much of it is temporarily converted to carbonic acid (H2CO3), a reaction catalyzed by an enzyme in red blood cells called carbonic anhydrase. The carbonic acid thus formed in red blood cells dissociates into hydrogen ions and bicarbonate ions. As the + H concentration increases, pH decreases. Thus, an increased PCO2 produces a more acidic 4 environment, which helps release O2 from hemoglobin. Decreased PCO2 (and elevated pH) shifts the saturation curve to the left. 3. Temperature: Within limits, as temperature increases, so does the amount of O2 released from hemoglobin. Heat is a byproduct of the metabolic reactions of all cells, and the heat released by contracting muscle fibers tends to raise body temperature. Metabolically active cells require more O2 and liberate more acids and heat. The acids and heat in turn promote release of O2 from oxyhemoglobin. Fever produces a similar result. In contrast, during hypothermia (lowered body temperature) cellular metabolism slows, the need for O2 is reduced, and more O2 remains bound to hemoglobin (a shift to the left in the saturation curve). 5 4. BPG: A substance found in red blood cells called 2, 3-bisphosphoglycerate (BPG), previously called diphosphoglycerate (DPG), decreases the affinity of hemoglobin for O2 and thus helps unload O2 from hemoglobin. BPG is formed in red blood cells when they break down glucose to produce ATP in a process called glycolysis. When BPG combines with hemoglobin by binding to the terminal amino groups of the two beta globin chains, the hemoglobin binds O2 less tightly at the heme group sites. The greater the level of BPG, the more O2 is unloaded from hemoglobin. Certain hormones, such as thyroxine, human growth hormone, epinephrine, norepinephrine, and testosterone, increase the formation of BPG. The level of BPG also is higher in people living at higher altitudes. 6 Oxygen Affinity of Fetal and Adult Hemoglobin Fetal hemoglobin (Hb-F) differs from adult haemoglobin (Hb-A) in structure and in its affinity for O2. Hb-F has a higher affinity for O2 because it binds BPG less strongly. Thus, when PO2 is low, Hb-F can carry up to 30% more O2 than maternal Hb-A (Figure 23.22). As the maternal blood enters the placenta, O2 is readily transferred to fetal blood. This is very important because the O2 saturation in maternal blood in the placenta is quite low, and the fetus might suffer hypoxia were it not for the greater affinity of fetal haemoglobin for O2. Please watch: Gaseous exchange: 1. https://www.youtube.com/watch?v=qDrV33rZlyA 2. https://www.youtube.com/watch?v=6qnSsV2syUE Transport of oxygen 1. https://www.youtube.com/watch?v=BYGPkRFvzOc Reference: PRINCIPLES OF ANATOMY AND PHYSIOLOGY (Tortora and Derrickson) 7 .

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