呼吸科呼吸系统3课件

5.3. Gas transport lOxygen is transported by the blood in both physically dissolved and hemoglobin- bond forms. lCarbon monoxide binds the same site on hemoglobin as oxygen does and is dangerous to life. lCarbon dioxide (CO2) is transported in three forms: physically dissolved, bicarbonate, and carbaminohemoglobin *1 Oxygen Transport in the Blood *2 *3 Hemoglobin (Hb) Each Hb molecule consists of four globular protein (globin, 珠蛋白) subunits. Each subunit is composed of a protein chain tightly associated with a non-protein heme（血红素） group. Oxygen binds to iron (Fe2+) on heme. Two Forms of Oxygen Exist in the Blood Physically dissolved: 2% Dissolved O2 (ml/dL) = 0.003 (ml/dL/mmHg) X Pa O2 (mmHg) Combined with Hb: 98% Both physically dissolved and chemically bond are important. Oxyhemoglobin (HbO2): Hemoglobin binds to oxygen. Deoxyhemoglobin (Hb): Hemoglobin that does not bind with oxygen. *4 Oxygen Binds to Hemoglobin The binding of oxygen to hemoglobin is reversible and very quick. The reaction of this biding is oxygenation but not oxidation (ferrous remains as ferrous). Each hemoglobin molecule can bind four oxygen molecules. Each gram of hemoglobin can bind up 1.341.39 ml of O2 (depending on methemoglobin levels). *5 Positive Cooperativity Changes in the affinity of hemoglobin as each successive O2 molecule binds to a heme site. Binding of the first O2 molecule increases the affinity for the second O2 molecule, and so forth. The affinity for the forth O2 molecule is the highest. This change in affinity facilitates the loading of O2 in the lungs and the unloading of the O2 at the tissues. *6 Quantitative Description of O2 Binding to Hb Oxygen capacity is the maximum of oxygen that hemoglobin can carry per 100 ml blood . Hb-bound oxygen content is the amount of oxygen actually carried by hemoglobin per100 ml blood. Oxygen saturation (SO2) is the percentage saturation of hemoglobin with oxygen that is calculated from the ratio of oxygen content over capacity. The relationship between and PO2 and SO2 (or oxygen content) is oxyhemoglobin equilibrium curve (oxygen dissociation curve). *7 Oxygen Dissociation Curve *8 Analysis of Oxygen Dissociation Curve When PaO2 is increased from 60 and 100 mmHg, SaO2 rises only by 7%. The clinical significance of the flat portion of oxygen dissociation curve is that a drop PO2 from 100 to 60 mmHg still results in hemoglobin saturation of 90%. Rising PaO2 above 100 mmHg barely affects oxygen content. In the steep portion of this curve, blood oxygen content and thus oxygen delivery to tissues are compromised upon PO2 falls below 60 mmHg. The significance of this portion of the curve is that a large amount of O2 is released from hemoglobin with a small change in PO2. *9 Changes in Blood Chemistry Alter Hemoglobins Affinity for O2 *10 To remember Just think that an exercising muscle is hot and acidic and produces large amount of carbon dioxide, all of which favor the unloading of more oxygen to the metabolic needs of muscles DPG = 2,3-diphosphoglycerate Bohr Effect lBohr effect (after the Danish physiologist, Christin Bohr) refers to the effect of CO2 on the affinity of hemoglobin on oxygen. lIncreased CO2 shifts the oxyhemoglobin dissociation curve to the right, promoting oxygen release. lBohr effect is caused in part by the change in pH that occurs as CO2 increases. *11 P50 Indicates the Binding Activity of Hemoglobin for O2 lThe PO2 value at which 50% of hemoglobin is saturated. lThe normal range is between 26 and 28 mmHg in arterial blood. lHigh P50 allows more O2 to be released. *12 Carbon Monoxide Poisoning *13 Properties of Carbon Dioxide *14 lCO is an odorless, colorless, and non-irritating gas; it is virtually undetectable lCO is produced by incomplete combustion lWith enough O2, CO2 is generated lWith less O2, CO is generated Carbon Monoxide Binds to the Site of Hb as Oxygen Does *15 lIt binds to Hb at the same site as oxygen does, forming carboxyhemoglobin (HgbCO) and preventing O2 binding. lThe binding affinity for CO is 200-250 times as that for oxygen. Carbon Monoxide-Hemoglobin Dissociation Curve *16 Carbon Monoxide Reduces O2 Content *17 *17 Higher Concentration of Carboxyhemoglobin in Cigarette Smokers In healthy individuals, carboxyhemoglobin occupies 12% of the Hb binding sites In people who smoke cigarettes, occupation of Hb binding sites can be increased up to 10% Stop smoking *18 No Hyperventilation Response to Carbon Monoxide Poisoning Because arterial PO2 is normal, no feedback mechanism to warn that the oxygen content is low, so people with CO poisoning will die peacefully *19 Treatments of Carbon Monoxide Poisoning lThe appropriate treatment is the administration of pure oxygen lHigh CO2 (5%) is helpful in stimulating respiration and consequently promoting the removal of CO from the blood *20 Carbon Dioxide Transport *21 The Source and Forms of Carbon Dioxide *22 Carbon Dioxide Transport Carbon dioxide is carried in the blood in three distinct forms: Physically dissolved in the plasma (10%) As bicarbonate ions in the plasma and in the red cells (60%) As carbamino proteins (30%) *23 Formation of Bicarbonate and Bicarbonate Ions *24 HCO3- moves out of RBC in exchange for chloride Chloride shift Carbaminohemoglobin Formation lCarbon dioxide molecule reversibly attaches to an amino portion of hemoglobin to form carbaminohemoglobin（氨基甲酰血红蛋白） *25 CO2 + HbHbCO2 CO2 Dissociation Curve and the Haldane Effect *26 The Relationship among O2, CO2 and H+ Bohr Effect The effect of PCO2 and H+ on the affinity of hemoglobin on oxygen (or the oxygen-hemoglobin equilibrium curve). Haldane Effect For any given PCO2, the blood will hold more CO2 when the PO2 is being reduced. *27 The Haldane Effect lFor any given PCO2, the blood will hold more CO2 when the PO2 is being diminished. lThe effect is reversed in the lung when O2 is transported from the alveoli to the red blood cells. *28 Summaries on Gas Exchange and Gas Transport The uptake of oxygen is determined by the diffusion properties of the alveolar-capillary membrane, the oxygen partial pressure gradient, and pulmonary capillary blood flow. Oxygen is transferred by the blood in two forms: dissolved and oxyhemoglobin. Oxygen dissociation curve is the functional relationship between oxygen partial pressure and the percentage of hemoglobin saturated with oxygen. P50 is a measure of Hb affinity to bind with oxygen. Hb affinity is inversely related to P50. Changes in blood chemistry (pH, PaCO2, and temperature) and 2.3, DPG alter the oxygen dissociation curve. *29 Summaries on Gas Exchange and Gas Transport-Continued lAs a product of non-complete combustion, CO is dangerous to life. l It binds to Hb in a competitive manner. lThe binding affinity for Hb is 250 times higher than that of oxygens. lPaO2 is normal, so there is lack of feedback mechanism to indicate that oxygen content is low. lThe appropriate treatment is the administration of high concentration of oxygen; high CO2 may be helpful in the case of spontaneous breathing is still existed. *30 Summary on Oxygen Transport lOxygen is transported by the blood in two forms: oxyhemoglobin and physically dissolved. lThe relationship between oxygen partial pressure and the saturation of hemoglobin is the oxygen dissociation curve. lChanges in blood chemistry (pH, PaCO2, temperature and 2.3-DPG) alter oxyhemoglobin equilibrium curve. lP50 is a measure of Hb