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Respiratory Physiology Manuel Otero Lopez Department of Anaesthetics and Intensive Care Hôpital Européen Georges Pompidou, Paris, France Conflict of interest declaration I do not have conflict of interest www.esahq.org Programme • Functional respiratory anatomy • Ventilation • Mechanics of breathing (compliance & airway resistance) • The inefficiency of respiratory gas exchange (Respiratory Dead space and Shunt) • Ventilation-perfusion relationship • Gas transport • Control of ventilation www.esahq.org Functional respiratory anatomy • Upper respiratory tract (from nostrils to vocal cords) • Lower respiratory tract (from vocal cords to alveoli) www.esahq.org Functional respiratory anatomy Muscles of respiration Innervation Diaphragm => Phrenic nerves (C3-C5 nerve roots) Intercostal muscles => by their respective thoracic nerves roots Vagus => provide sensory innervation to the tracheobronchial tree (bronchoconstriction, bronchial secretions). Sympathetic activity (T1-T4) causes bronchodilatation and secretions via β2 –receptors. α1 receptors cause bronchoconstriction α1 and β2 receptors are also present in the pulmonary vessels. www.esahq.org Weibel classification of airways Ewald Weibel Weibel ER. Morphometry of the human lung. Heidelberg: Springer-Verlag, New York: Academic Press; 1963 www.esahq.org Functional respiratory anatomy Each alveolar sac contains, on average, 17 alveoli. An estimated 300 million alveoli provide a membrane of 50 to 100 m2 for gas exchange. Pulmonary epithelium: - Type 1 pneumocytes - Type 2 pneumocytes (surfactant) www.esahq.org Increase in total cross-sectional area of the airways in the respiratory zone www.esahq.org Ventilation www.esahq.org . Total ventilation = tidal volume (VT) x respiratory frequency • Minute volume of ventilation . Anatomic dead space = the volume of the conducting airways, which does not take part in gas exchange (VD) . Alveolar ventilation = (VT - VD) x respiratory frequency • the amount of fresh inspired air available for gas exchange . Physiologic dead space = the volume of gas that does not eliminate CO2 • VD / VT = 0.3 in spontaneous ventilation • VD / VT = 0.5 in mechanical ventilation www.esahq.org www.esahq.org Lung volumes and capacities Volume vs. Capacity Volume is the amount of 3D space taken up by an object, e.g. a solid, a liquid or a gas. Capacity is the measure of an object’s ability to hold a substance, e.g. a solid, a liquid or a gas. Morgan and Mikhail’s Clinical Anesthesiology, 5th ed. www.esahq.org Expiratory spirogram Morgan and Mikhail’s Clinical Anesthesiology, 5th ed. www.esahq.org Peak flowmeter, PEFR Measurement of the peak expiratory flow rate which is sustained for 10 ms www.esahq.org Respiratory failure An obstructive pattern (COAD) will show : Low FEV1.0 / FVC ratio, low PEFR, low VC and high RV. A restrictive pattern will show : Normal FEV1.0 / FVC ratio, low VC, low PEFR and low RV. (after resection of the lung, kyphoscoliosis,…) www.esahq.org Closing capacity • The lung volume present after a maximum expiratory effort is called RV (residual volume). At this minimum volume some of the dependent alveoli are close off. • The closing capacity is the lung volume at which this closure is first recorded using a marker gas expirogram such as helium. • Closing capacity is independent of body position but increases with age. If it exceeds FRC, there is some degree of airway closure during respiration (intrapulmonary shunt). Responsible for the normal age-related decline in arterial O2 tension. www.esahq.org Closing capacity (Morgan and Mikhail’s Clinical Anesthesiology, 5th ed.) www.esahq.org Mechanics of breathing www.esahq.org Pressure volume relationship and compliance Compliance is an index of distensibility of elastic organs and defined as the change in volume per unit change in pressure (ΔV/ΔP). Compliance Elastance www.esahq.org Factors which modifie compliance • Body size • Posture • Volume history of the lungs • Pulmonary blood volume • Fibrosis Normal lung compliance ~ 0.2-0.3 L/cm H2O (2-3 L/kPa) www.esahq.org Surface Tension www.esahq.org Surface Tension and Surfactant www.esahq.org Surface Tension www.esahq.org Type II pneumocyte Start to develop at about 24 weeks of gestation secreting small amounts of surfactant Adequate amounts are not secreted until about 35 weeks of gestation Electron micrograph of type II epithelial cell (x 10 000) www.esahq.org Pulmonary Surfactant • Reduces the surface tension of the alveolar lining layer • Increases lung compliance • Increases the stability of alveoli • Prevents pulmonary edema • Has a short half-life Absence • Low lung compliance, alveolar atelectasis, tendency to pulmonary edema www.esahq.org Airway Resistance Derived from Hagen–Poiseuille equation Jean Louis Marie Poiseuille www.esahq.org Main site of airway resistance www.esahq.org Laminar versus turbulent flow Laminar flow Turbulent flow Hagen-Poiseuille equation In tubulent flow the resistance to flow is greater and increases more rapidly when the flow increases. Reynolds’ number: V x diameter x gas density gas viscosity www.esahq.org The inefficiency of respiratory gas exchange (Respiratory Dead space and Shunt) Ventilation-perfusion relationship www.esahq.org John B. West Video Lectures in Respiratory Physiology http://meded.ucsd.edu/ifp/jwest/resp_phys/index.html www.esahq.org www.esahq.org Gravitational Distribution of Blood Flow in the Lung The uneven distribution of blood flow can be explained by the hydrostatic pressure differences within the blood vessels (« 30 cm H2O pressure from top to bottom ») On exercise these regional differences become less. John B. West www.esahq.org VENTILATION-PERFUSION RELATIONSHIP Distribution of V, Q and V/Q ratio in the normal, upright lung www.esahq.org Ventilation /perfusion ratios in an erect subject. (Textbook of Anaesthesia, AR Aitkenhead & G Smith, 2nd ed. 1990) www.esahq.org Dead space The amount of ventilation not taking part in the gas exchange www.esahq.org Ventilation-Perfusion Relationship www.esahq.org Shunt Cc’ O2 can be calculated from the alveolar gas equation : Clinically, the alveolar – arterial oxygen partial pressure difference is often used as an approximation for « shunt » www.esahq.org Hypoxic Pulmonary Vasoconstriction • alveolar hypoxia constricts small pulmonary arteries • a compensatory mechanism aimed at reducing blood flow in hypoxic lung regions • the precise mechanism is not known • occurs in excised isolated lung • probably a direct effect of the low PO2 on vascular smooth muscle www.esahq.org DIFFUSION www.esahq.org DIFFUSION Fick's Law of Diffusion Adolf Fick, 1855 www.esahq.org DIFFUSION www.esahq.org DIFFUSION Diffusion of Oxygen Across the Blood-Gas Barrier • At rest PaO2 virtually reaches PAO2 after about 1/3 of its time in capillary • The diffusion process is challenged by exercise, alveolar hypoxia, and thickening of the blood-gas barrier • True diffusion defects that create arterial hypoxemia are rare www.esahq.org GAS TRANSPORT O2 • dissolved • combined with Hb www.esahq.org GAS TRANSPORT Dissolved O2 -1 . For each mmHg of PO2 0.003 ml O2 · 100 ml of blood or 0.003 vol. % -1 . In normal arterial blood PO2 of 100 mmHg 0.3 ml O2 · 100 ml www.esahq.org GAS TRANSPORT O2 capacity: 1 g Hb - 1.39 ml O2 O2 combined with Hb O2 saturation of Hb = X 100 O2 capacity CO2 = 1.39 x Hb x SO2 (%)/100 + 0.003 PO2 Oxygen carrying capacity of Hb = Hüfner’s constant (1,39 ml/gr) www.esahq.org O2 dissociation curve www.esahq.org Shifts of the O2 dissociation curve www.esahq.org GAS TRANSPORT CO2 • dissolved • as bicarbonate • in combination with proteins as carbamino compounds www.esahq.org CO2 carriage in the blood www.esahq.org www.esahq.org CONTROL OF RESPIRATION www.esahq.org Basic elements of the respiratory control system West, John B. Respiratory Physiology: The Essentials, 9th Edition Copyright © 2012 Lippincott Williams & Wilkins, a Wolters Kluwer business www.esahq.org www.esahq.org www.esahq.org Carotid bodies • respond to PO2, PCO2, pH • little response to normoxia • very high blood flow • respond to arterial, not venous PO2 • response to PCO2, pH is < important • fast response www.esahq.org Lung receptors • pulmonary stretch receptors (slowly adapting pulmonary stretch receptors) • Hering-Breuer inflation reflex • irritant receptors (rapidly adapting pulmonary stretch receptors) • J receptors (juxtacapillary) • bronchial C fibers www.esahq.org Mechanisms of hypoxemia Morgan and Mikhail’s Clinical Anesthesiology, 5th ed. www.esahq.org www.esahq.org NON-RESPIRATORY FUNCTIONS OF THE RESPIRATORY SYSTEM www.esahq.org NON-RESPIRATORY FUNCTIONS OF THE RESPIRATORY SYSTEM • Protective functions of respiratory tract • Non-respiratory functions of pulmonary circulation • Metabolic functions of the lung www.esahq.org • Protective functions of respiratory tract • Warming Raises incoming air to 37 Celsius • Humidification Raises incoming air to 100% humidity • Filtration ------------------------------> • Removal of filtered particles • (cough, cilia) • Defense mechanisms of terminal respiratory units (macrophages and other relevant cells) • Olfaction www.esahq.org NON-RESPIRATORY FUNCTIONS OF THE RESPIRATORY SYSTEM • Non-respiratory functions of pulmonary circulation • Reservoir for left ventricle (contains about 500 ml blood) • Fluid and electrolyte exchange • Filter to protect the systemic circulation including: small fibrin or blood clots, fat cells, bone marrow, detached cancer cells, gas bubbles, agglutinated RBC's, masses of platelets or WBC's, debris in stored blood, particles in i.v. solutions www.esahq.org NON-RESPIRATORY FUNCTIONS OF THE RESPIRATORY SYSTEM • Metabolic functions of the lung • Uptake or conversion of chemical substances by lungs (conversion of angiotensin I to angiotensin II) • Formation of chemical substances • Pulmonary surfactant • Release into blood of substances stored in pulmonary tissues • Bradykinin • Histamine • Serotonin • PGE2, PGF2 • Heparin www.esahq.org Thank you for attention Manuel Otero Lopez [email protected] Many tanks to : Dr. Armen Varosyan Department of Anaesthesiology and Intensive Care Yerevan State Medical University, Yerevan, Armenia www.esahq.org .
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