Transcript CO 2
Respiration 2 Xia Qiang, PhD Department of Physiology Zhejiang University School of Medicine Email: [email protected] Gas exchange Pulmonary gas exchange O2 CO2 Tissue gas exchange CO 2 O2 CO2 O2 Pulmonary capillary Tissue cells CO2 O2 CO2 O2 Tissue capillaries Physical principles of gas exchange Laws governing gas diffusion • Henry’s law The amount of dissolved gas is directly proportional to the partial pressure of the gas Boyle’s 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 • Graham's Law When gases are dissolved in liquids, the relative rate of diffusion of a given gas is proportional to its solubility in the liquid and inversely proportional to the square root of its molecular mass Laws governing gas diffusion • Fick’s 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 proportional to the thickness of the membrane Factors affecting gas exchange P S T A D d MW • • • • D: T: A: S: Rate of gas diffusion Absolute temperature Area of diffusion Solubility of the gas • • • P: d: MW: Difference of partial pressure Distance of diffusion Molecular weight Changes in the concentration of dissolved gases 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 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 alveolus capillary endothelial cell surfactant CO2 epithelial cell O2 red blood cell interstitial space Ventilation-perfusion ratio • Alveolar ventilation (V) = 4.2 L • Pulmonary blood flow (Q) = 5 L • V/Q = 0.84 (optimal ratio) Ventilation-perfusion ratio Effect of gravity on V/Q VA/QC Gas transport in the blood • Forms of gas transported • Physical dissolve • Chemical combination Alveoli O2 Blood Tissue →dissolve→combine→dissolve→ O2 CO2 ←dissolve←combine←dissolve← 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 amount of oxygen in that compartment, since the bound oxygen is no longer part of the diffusional equilibrium. High PO2 Hb + O2 HbO2 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 / O2 capacity) x 100% Cyanosis • Hb>50g/L Carbon monoxide poisoning • CO competes for the O2 sides in Hb • CO has extremely high affinity for Hb O2 O CO 2 COCO O2 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-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 hemoglobin’s affinity to bind oxygen alter the ease of “loading” and “unloading” this gas in the lungs and near the active cells. Transport of carbon dioxide • Forms of carbon dioxide transported • Physical dissolve: 7% • Chemical combination: 93% • Bicarbonate ion: 70% • Carbaminohemoglobin: 23% CO2 transport in tissue capillaries tissues CO2 CO2 CO2+H2O tissue capillaries H2CO3 H+ CO2 + + HCO 3 R-NH2 R-NHCOO+ H+ CO2 + Hb HbCO2 CO2 + H2O carbonic anhydrase H2CO3 HCO3- H+ +HCO3Cl - plasma tissues capillaries CO2 transport in pulmonary capillaries alveoli CO2 pulmonary capillaries CO2 CO2 + Hb HbCO2 CO2 + H2O carbonic anhydrase H2CO3 HCO3H+ +HCO3plasma Clpulmonary capillaries Cl- Carbon Dioxide Dissociation Curve Haldane Effect • When oxygen binds with hemoglobin, carbon dioxide is released PO2=40 mmHg PO2=100 mmHg Bohr effect and Haldane effect tissue capillaries H2CO3 Bohr effect HbO2 HbO2 H+ +HCO3- HbH Hb + O2 Hb + O2 Haldane effect CO2 HbCO2 Regulation of respiration • Breathing is autonomically controlled by the central neuronal network to meet the metabolic demands of the body • Breathing 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 Basic respiratory center • Pons • Higher respiratory center: cerebral cortex, hypothalamus & limbic system Respiratory center • Dorsal respiratory group (medulla) – mainly causes inspiration • Ventral respiratory group (medulla) – causes either expiration or inspiration • Pneumotaxic center (pons) – helps control the rate and pattern of breathing Pulmonary mechanoreceptors A:Slowly Adapting Receptor (SAR) B: Rapidly Adapting Receptor (RAR) C: J-receptors (C-fibers) Location Fibers Stimulus Effect SAR trachea-terminal large bronchioles myelinated (smooth muscle) Stretch (lung volume) termination of inspiration RAR trachearespiratory bronchioles (epithelium) lung volume, noxious gases, cigarette smoke, histamine, lung deflation bronchocontriction, (rapid & shallow breathing) Cfibers alveolar capillary membrane volume of interstitial fluid Apnea followed by a rapid & shallow breathing HR&BP small myelinated nonmyelinated Hering-Breuer inflation reflex (Pulmonary stretch reflex) • The reflex reactions originating in the lungs and mediated by the fibers of the vagus nerve: inflation of the lungs, eliciting expiration, and deflation, stimulating inspiration Hering-Breuer reflex End of inspiration FRC FRC Chemical control of respiration • Chemoreceptors • Central chemoreceptors • Peripheral chemoreceptors • Carotid body • Aortic body Central chemoreceptors Chemosensory neurons that respond to changes in blood pH and gas content are located in the aorta and in the carotid sinuses; these sensory 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 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 to increase the rate of ventilation. The levels of oxygen, carbon dioxide, and hydrogen ions in blood and CSF provide information that alters the rate of ventilation. An integrated perspective recognizes the variety and diversity of factors that alter the rate of ventilation. End.