基础医学各论：22.呼吸系统生理2

1、Respiration 2 Gas exchange Tissue capillaries Tissue cells CO2 CO2 O2 O2 Pulmonary capillary CO2 O2 CO2 CO2 O2 O2 Pulmonary gas exchangeTissue gas exchange Physical principles of gas exchange Laws governing gas diffusion Henrys law The amount of dissolved gas is directly proportional to the partial

2、pressure of the gas Boyles law states that the pressure of a fixed number of gas molecules is inversely proportional to the volume of the container. Laws governing gas diffusion Grahams Law When gases are dissolved in liquids, the relative rate of diffusion of a given gas is proportional to its solu

3、bility in the liquid and inversely proportional to the square root of its molecular mass Laws governing gas diffusion Ficks law The net diffusion rate of a gas across a fluid membrane is proportional to the difference in partial pressure, proportional to the area of the membrane and inversely propor

4、tional to the thickness of the membrane D: Rate of gas diffusion T: Absolute temperature A: Area of diffusion S: Solubility of the gas P: Difference of partial pressure d: Distance of diffusion MW: Molecular weight MWd ATSP D Factors affecting gas exchange Changes in the concentration of dissolved g

5、ases are indicated as the blood circulates in the body. Oxygen is converted to water in cells; cells release carbon dioxide as a byproduct of fuel catabolism. In lungs Oxygen diffusion along the length of the pulmonary capillaries quickly achieves diffusional equilibrium, unless disease processes in

6、 the lungs reduce the rate of diffusion. In tissue Factors that affect pulmonary gas exchange Thickness of respiratory membrane Surface area of respiratory membrane Ventilation-perfusion ratio (V/Q) Respiratory membrane surfactant epithelial cell interstitial space alveoluscapillary red blood cell e

7、ndothelial cell Ventilation-perfusion ratio Alveolar ventilation (V) = 4.2 L Pulmonary blood flow (Q) = 5 L V/Q = 0.84 (optimal ratio) Ventilation-perfusion ratio VA/QC Effect of gravity on V/Q Gas transport in the blood Forms of gas transported Physical dissolve Chemical combination AlveoliBloodTis

8、sue O2 dissolvecombinedissolve O2 CO2 dissolvecombinedissolve CO2 Transport of oxygen Forms of oxygen transported Physical dissolve: 1.5% Chemical combination: 98.5% Hemoglobin (Hb) is essential for the transport of O2 by blood Adding hemoglobin to compartment B substantially increases the total amo

9、unt of oxygen in that compartment, since the bound oxygen is no longer part of the diffusional equilibrium. Hb + O2 HbO2 High PO2 Low PO2 Oxygen capacity The maximal amount of O2 that can combine with Hb at high PO2 Oxygen content The amount of O2 that combines with Hb Oxygen saturation (O2 content

10、/ O2 capacity) x 100% Cyanosis Hb50g/L Carbon monoxide poisoning CO competes for the O2 sides in Hb CO has extremely high affinity for Hb Oxygen-hemoglobin dissociation curve The relationship between O2 saturation of Hb and PO2 Factors that shift oxygen dissociation curve PCO2 and H+ Temperature 2,3

11、-diphosphoglycerate (DPG) Bohr Effect Increased delivery of oxygen to the tissue when carbon dioxide and hydrogen ions shift the oxygen dissociation curve Chemical and thermal factors that alter hemoglobins affinity to bind oxygen alter the ease of “loading” and “unloading” this gas in the lungs and

12、 near the active cells. Transport of carbon dioxide Forms of carbon dioxide transported Physical dissolve: 7% Chemical combination: 93% Bicarbonate ion: 70% Carbaminohemoglobin: 23% tissue capillaries tissues CO2 transport in tissue capillaries CO2 + HbHbCO2 CO2 tissues capillaries CO2 + H2OH2CO3 H+

13、 +HCO3- carbonic anhydrase CO2 pulmonary capillaries CO2 + HbHbCO2 H+ +HCO3- H2CO3 carbonic anhydrase CO2 + H2O alveoli Cl- pulmonary capillaries CO2 transport in pulmonary capillaries CO2 Carbon Dioxide Dissociation Curve Haldane Effect When oxygen binds with hemoglobin, carbon dioxide is released

14、PO2=40 mmHg PO2=100 mmHg Bohr effect and Haldane effect H2CO3H+ +HCO3- HbO2Hb + O2 CO2 HbCO2 HbH Bohr effect Haldane effect HbO2Hb + O2 tissue capillaries Regulation of respiration Breathing is autonomically controlled by the central neuronal network to meet the metabolic demands of the body Breathi

15、ng can be voluntarily changed, within certain limits, independently of body metabolism Respiratory center A collection of functionally similar neurons that help to regulate the respiratory movement Respiratory center Medulla Pons Higher respiratory center: cerebral cortex, hypothalamus these sensory

16、 afferent neurons alter CNS regulation of the rate of ventilation. Carotid body Effect of carbon dioxide on pulmonary ventilation CO2 respiratory activity Central and peripheral chemosensory neurons that respond to increased carbon dioxide levels in the blood are also stimulated by the acidity from

17、carbonic acid, so they “inform” the ventilation control center in the medulla oblongata to increase the rate of ventilation. Effect of hydrogen ion on pulmonary ventilation H+ respiratory activity Regardless of the source, increases in the acidity of the blood cause hyperventilation, even if carbon dioxide levels are driven to abnormally low levels. Effect of low arterial PO2 on pulmonary ventilation PO2 respiratory activity Chemosensory neurons that respond to decreased oxygen levels in the blood “inform” the ventilation control center in the medulla