Chapter 8. Carriage of gases in the blood and acid/base balance

Transcription

1 Chapter 8 Carriage of gases in the blood and acid/base balance Liu Baoyi Respiratory Department of Qilu Hospital April, 2013

2 Oxygen and Carbon Dioxide Transport in Blood

3 Basic Mechanism of the Gases Transportation Two forms of the gases: physical dissolution and chemical combination. Most of oxygen and carbon dioxide in the blood is transported in chemical combination Only the gas in physical dissolution express PP and diffuse to a place with low PP. Dynamic balance between the two forms: Physical dissolution PP PP Chemical combination

4 I. Transport of Oxygen

5 Oxygen Transport Method Percentage Dissolved in Plasma 1.5 % Combined with Hemoglobin 98.5 % Bound to Hgb Dissolved

6 Hemoglobin Structure Protein made up of 4 subunits Every subunit contains a heme moiety attached to a polypeptide chain.

7 Oxyhemoglobin Formation An oxygen molecule reversibly attaches to the heme portion of hemoglobin. The heme unit contains iron ( +2 ) which provides the attractive force. O 2 + Hb HbO 2

8 In normal adults, most of the hemoglobin contains 2α and 2 β chains. Each of the 4 iron atoms can bind reversibly on O 2 molecule. The iron stays in the ferrous state, so that the reaction is an oxygenation, not an oxidation.

9 When saturated with O 2 (4 O 2 in one hemoglobin molecule), it is always written Hb 4 O 8. The reaction is rapid, requiring less than 0.01 second. The deoxygenation (reduction) of Hb 4 O 8 is also very rapid.

10 Basic Concepts: Oxygen Capacity ( 氧容量 ): The maximum quantity of oxygen that will combine chemically with the hemoglobin in a unit volume of blood; normally it amounts to 1.34 ml of O 2 per gm of Hb or 20 ml of O 2 per 100 ml of blood. Oxygen Content( 氧含量 ): how much oxygen is in the blood Oxygen Saturation ( 血氧饱和度 ): A measure of how much oxygen the blood is carrying as a percentage of the maximum it could carry

11 The oxygen-hemoglobin dissociation curve: ( 氧离曲线 ) the curve relating percentage saturation of the O -carry 2 power of hemoglobin to the PO 2.

12 The oxygen-hemoglobin dissociation curve A. Flattened upper portion B. Steep middle portion C. Lower portion

13 Factors that Shift the Oxygen- Hemoglobin Dissociation Curve

14 Shifting the Curve

15 1. ph and PCO 2 : Bohr effect

16 2. Temperature

17 3. 2,3-diphosphoglycerate, 2,3,-DPG (2, 3- 二磷酸甘油 ) A byproduct of anaerobic glycolysis. Present in especially high concentration in red blood cells because of their content of 2,3-DPG mutase. The affinity of hemoglobin for O 2 diminishes as the concentration of 2,3-DPG increase in the red blood cells.

18 Importance: The normal DPG in the blood Hypoxic condition that last longer than a few hours Disadvantage: The excess DPG also makes it more difficult for the hemoglobin to combines with O 2 in the lungs.

19 4. Effect of Carbon Monoxide (CO) CO combines Hb at the same point as does O 2, and can displace O 2 from hemoglobin. CO binds with about 250 times as much tenacity as O 2. Therefore, a P CO only a little greater than 0.4 mmhg can be lethal. In the presence of CO (low concentration), the affinity of hemoglobin for O 2 is enhanced,

20 Effect of CO & Anemia on Hb-O 2 affinity Normal blood with Hb=15 gm/dl, anemia with Hb=7.5 gm/dl, and normal blood with 50% COHb (carboxyhemoglobin).

21 5. Fetal Hemoglobin Advantage Increased O 2 release to the fetal tissues under the hypoxic condition.

22 6. Myoglobin

23 Carbon monoxide poisoning 1. Why CO is poisonous? 2. Symptoms of CO poisoning. 3. The treatment of CO poisoning

24 II Carbon Dioxide Transport Method Percentage Dissolved in Plasma 7-10 % Chemically Bound to Hemoglobin in RBC s % As Bicarbonate Ion in Plasma % Dissolved bound to Hb HCO3-

25 Carbaminohemoglobin Formation Carbon dioxide molecule reversibly attaches to an amino portion of hemoglobin. CO 2 + Hb HbCO 2

26 Carbonic Acid Formation The carbonic anhydrase stimulates water to combine quickly with carbon dioxide. CO 2 + H 2 0 H 2 CO 3

27 Bicarbonate Ion Formation Carbonic acid breaks down to release a hydrogen ion and bicarbonate. H 2 CO 3 H + + HCO - 3

28 CO 2 Transport and Cl - Movement

29 Carbon Dioxide Dissociation Curve Haldane effect For any given PCO 2, the blood will hold more CO 2 when the PO 2 has been diminished. Reflects the tendency for an increase in PO 2 to diminish the affinity of hemoglobin for CO 2.

30 Mechanism of Haldane effect Combination of oxygen with hemoglobin in the lungs cause the hemoglobin to becomes a stronger acid. Therefore: 1) The more highly acidic hemoglobin has less tendency to combine with CO 2 to form CO 2 Hb 2) The increased acidity of the hemoglobin also causes it to release an excess of hydrogen irons

31 Interaction Between CO 2 and O 2 Transportation 1. Bohr effect

32 2. Haldane effect

33 Acid/base balance

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35 Interpretation of arterial blood gases

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