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1、2019/4/29,1,5.3. Gas transport,Oxygen is transported by the blood in both physically dissolved and hemoglobin-bond forms. Carbon monoxide binds the same site on hemoglobin as oxygen does and is dangerous to life. Carbon dioxide (CO2) is transported in three forms: physically dissolved, bicarbonate,
2、and carbaminohemoglobin,2019/4/29,2,Oxygen Transport in the Blood,2019/4/29,3,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 h
3、eme.,2019/4/29,4,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): Hemoglo
4、bin that does not bind with oxygen.,2019/4/29,5,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 o
5、f hemoglobin can bind up 1.341.39 ml of O2 (depending on methemoglobin levels).,2019/4/29,6,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 for
6、th. 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.,2019/4/29,7,Quantitative Description of O2 Binding to Hb,Oxygen capacity is the maximum of oxygen that hemoglobin can carry per 10
7、0 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
8、oxygen content) is oxyhemoglobin equilibrium curve (oxygen dissociation curve).,2019/4/29,8,Oxygen Dissociation Curve,2019/4/29,9,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 dissocia
9、tion 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 si
10、gnificance of this portion of the curve is that a large amount of O2 is released from hemoglobin with a small change in PO2.,2019/4/29,10,Changes in Blood Chemistry Alter Hemoglobins Affinity for O2,To remember Just think that an exercising muscle is hot and acidic and produces large amount of carbo
11、n dioxide, all of which favor the unloading of more oxygen to the metabolic needs of muscles,DPG = 2,3-diphosphoglycerate,2019/4/29,11,Bohr Effect,Bohr effect (after the Danish physiologist, Christin Bohr) refers to the effect of CO2 on the affinity of hemoglobin on oxygen. Increased CO2 shifts the
12、oxyhemoglobin dissociation curve to the right, promoting oxygen release. Bohr effect is caused in part by the change in pH that occurs as CO2 increases.,2019/4/29,12,P50 Indicates the Binding Activity of Hemoglobin for O2,The PO2 value at which 50% of hemoglobin is saturated. The normal range is bet
13、ween 26 and 28 mmHg in arterial blood. High P50 allows more O2 to be released.,2019/4/29,13,Carbon Monoxide Poisoning,2019/4/29,14,Properties of Carbon Dioxide,CO is an odorless, colorless, and non-irritating gas; it is virtually undetectable CO is produced by incomplete combustion With enough O2, C
14、O2 is generated With less O2, CO is generated,2019/4/29,15,Carbon Monoxide Binds to the Site of Hb as Oxygen Does,It binds to Hb at the same site as oxygen does, forming carboxyhemoglobin (HgbCO) and preventing O2 binding. The binding affinity for CO is 200-250 times as that for oxygen.,2019/4/29,16
15、,Carbon Monoxide-Hemoglobin Dissociation Curve,2019/4/29,17,Carbon Monoxide Reduces O2 Content,2019/4/29,17,2019/4/29,18,Higher Concentration of Carboxyhemoglobin in Cigarette Smokers,In healthy individuals, carboxyhemoglobin occupies 12% of the Hb binding sites In people who smoke cigarettes, occup
16、ation of Hb binding sites can be increased up to 10% Stop smoking,2019/4/29,19,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,2019/4/29,20,Treatments of Carbon Monoxide Poisoning,The appropriate treatment is the administration of pure oxygen High CO2 (5%) is helpful in stimulating respiration and consequently promoting the removal of CO fro
