Review of Breathing Mechanics & Gas Exchange (PDF)

Review of mechanics of breathing / gas exchange Prof. Donna Sellers [email protected] 5_3_19 Learning objectives At the end of this session, you will be able to: 1.Describe the pressure changes in the thoracic cavity 2.Outline the mechanics of breathing 3.Describe the...

Review of mechanics of breathing / gas exchange Prof. Donna Sellers [email protected] 5_3_19 Learning objectives At the end of this session, you will be able to: 1.Describe the pressure changes in the thoracic cavity 2.Outline the mechanics of breathing 3.Describe the factors affecting pulmonary ventilation 4.Explain the principles of gas exchange – transport of oxygen and carbon dioxide Reading: Human Anatomy and Physiology, Marieb and Hoehn, Chapter 22 Textbook of Medical Physiology, Guyton and Hall, Chapters 38, 40, 41 ** look out for the colour code to link to the LOs Reminder: Respiratory System Components Nose and nasal cavity Pharynx Larynx Trachea Bronchi and their branches Lungs Alveoli Conducting zones Respiratory zone LO1. Describe the pressure changes in the thoracic cavity Reminder: Pleural cavity and pleural membranes Double layered membrane surrounding the lungs Pleura are serous membranes lining the pleural cavity  Parietal – attaches to the walls of the thoracic cavity & diaphragm  Visceral – adheres to the surface of the lungs Pleural fluid fills and lubricates the space between the pleura - pleural cavity/sac Respiratory Pressures Described relative to atmospheric pressure (P atm) ~ 760 mmHg  Negative respiratory pressure (760 mmHg) Intrapulmonary pressure (Ppul)  ↑ and ↓ with the phases of breathing  Eventually equalizes with atmospheric pressure  Determines direction of air flow Intrapleural pressure (Pip)  Pressure in the plural cavity  Always ~ 4 mmHg less than Ppul  Maintains pull on lungs Transpulmonary pressure (PTpul)  Difference between Ppul and Pip  Prevents lungs from collapsing LO1. Describe the pressure changes in the thoracic cavity Volume and Pressure Changes  Boyle’s Law  At constant temperature the pressure exerted by a gas varies inversely with the volume of the gas   volume will result in  pressure   volume will result in  pressure  Volume changes in the thoracic cavity lead to pressure changes which leads to flow of gas to equalise the pressure LO1. Describe the pressure changes in the thoracic cavity LO1. Describe the pressure changes in the thoracic cavity  Mechanics of breathing - Inspiration Diaphragm and external intercostal muscles (inspiratory muscles) contract and the rib cage rises The lungs are stretched and intrapulmonary volume increases Intrapulmonary pressure drops below atmospheric pressure Air flows into the lungs, down its pressure gradient, until intrapulmonary pressure = atmospheric pressure LO2.Outline the mechanics of breathing Inspiration continued  Normal quiet breathing (Eupnea): relative contribution of diaphragm and intercostal muscles varies  Rest: Diaphragm dominates  Pregnancy: Changes to costal breathing  Forced breathing (Hypernea)  Extra muscle groups involved LO2.Outline the mechanics of breathing Mechanics of breathing - Expiration Inspiratory muscles relax, rib cage descends due to gravity Thoracic cavity volume decreases Elastic lungs recoil passively, intrapulmonary volume decreases Intrapulmonary pressure rises above atmospheric pressure Gases flow out of the lungs down the pressure gradient until intrapulmonary pressure equals atmospheric pressure LO2.Outline the mechanics of breathing Factors Affecting Pulmonary Ventilation Airway resistance  Resistance inversely proportional to airway diameter  Normally insignificant  Disease increases resistance  Mucus narrows airways  Irritants, inflammatory chemicals activate parasympathetic reflexes causing bronchoconstriction  Bronchodilators decrease resistance and increase air flow LO3.Describe the factors affecting pulmonary ventilation  Airway resistance Resistance to airflow  Greatest in bronchi near trachea and in large bronchioles  Smooth muscle in bronchiolar wall is very sensitive to neural control and chemicals  Resistance in smaller bronchioles important in some disease states:  smaller in size  muscle contraction  oedema in walls  mucus collection in lumen Factors Affecting Pulmonary Ventilation Alveolar surface tension  At the water/air interface, the water molecules on the surface have a strong attractive force for one another  Inside alveoli: water surfaces attempting to contract and force air out of lungs  Alveoli collapse  Net effect: Elastic contractile force Surfactant  Secreted by Type II alveolar epithelia  Detergent like substance containing phospholipids, proteins and ions  Reduces the surface tension in water  ↑ lung compliance  Infant respiratory distress syndrome  Premature babies little or no surfactant LO3.Describe the factors affecting pulmonary ventilation Factors Affecting Pulmonary Ventilation Lung compliance  Extent to which the lung volume will expand for a given increase in transpulmonary pressure, due to:  Elastic forces of the lung tissue  Alveolar surface tension  Healthy lungs have high compliance   compliance results in  force required to fill and empty the lungs   elasticity of lungs (fibrosis) leads to  compliance   surfactant production leads to  compliance   thoracic mobility (arthritis, paralysis) leads to  compliance  Alveolar damage (emphysema) leads to  compliance, air moves in and out of the lung more easily but loss of respiratory surfaces restricts gas exchange LO3.Describe the factors affecting pulmonary ventilation Pneumothorax  Normally air does not enter the pleural cavity  If the chest wall is punctured (stab wound or a broken rib)  Air flows down pressure gradient from atmosphere in to pleural space  Lung collapses  Hole in lung wall (disease state) LO3.Describe the factors affecting pulmonary ventilation GENERAL PRINCIPLES OF GAS EXCHANGE Gas exchange: diffusion of gases and gas laws Dalton’s Law of Partial Pressures  Total pressure exerted by a mixture of gases is equal to the sum of the pressures exerted by each gas  Atmospheric air: PN2 (78.6%)+ PO2 (20.9%) + PCO2 (0.04%) + PH2O (0.46%) = 760 mm Hg (100%) Henry’s Law  When a mixture of gases is in contact with a liquid each gas will dissolve in proportion to its partial pressure  How much gas dissolves is dependent on its solubility  CO2 is twenty times more soluble in water than O2 LO4.Explain the principles of gas exchange – transport of oxygen and carbon dioxide Composition of Alveolar Air  Atmospheric air is mostly O2 and N2  Alveolar air is mostly CO2 and water vapour  Differences are due to:  Gas exchange in the lungs  Humidification  Mixing of alveolar gas LO4. Explain the principles of gas exchange – transport of oxygen and carbon dioxide Pulmonary Gas Exchange Factors that influence movement of O2 and CO2 across the respiratory membrane: 1.Partial pressure gradients and gas solubilities 2.Matching of alveolar ventilation and pulmonary blood perfusion 3.Structural features of the respiratory membranes LO4.Explain the principles of gas exchange – transport of oxygen and carbon dioxide Partial Pressure Gradients & Gas Solubilities Alveolar partial pressure vs Venous partial pressure  PO2 of venous blood is 40mmHg, PO2 in the alveoli is 104mmHg  O2 diffuses rapidly down its pressure gradient from the alveoli into the capillary blood  Reaches equilibrium in 0.25s  RBCs remain in capillaries 0.75s  WHAT DOES THIS MEAN?  PCO2 45mmHg in capillaries  PCO2 40mmHg in alveoli  Although steeper gradient for O2  Equal amounts of these gases are exchanged  WHY? LO4.Explain the principles of gas exchange – transport of oxygen and carbon dioxide LO4.Explain the principles of gas exchange – transport of oxygen and carbon dioxide Ventilation-Perfusion Coupling For gas exchange to be efficient there needs to be close coupling between alveolar ventilation and blood flow in pulmonary capillaries (perfusion): LO4.Explain the principles of gas exchange – transport of oxygen and carbon dioxide Features of Respiratory Membranes Thickness of Respiratory Membrane  Healthy lungs 0.5 - 1.0 μm thick  Gas exchange is very efficient  Accumulation of fluid in lungs  Exchange membranes thicken  Inadequate gas exchange Surface Area of Respiratory Membrane  Large SA (~90m2 in adult male) in healthy lungs  > SA > gas exchange  Functional surface area reduced  Emphysema, tumours, inflammatory material LO4.Explain the principles of gas exchange – transport of oxygen and carbon dioxide Oxygen Transport  98.5% carried bound to hemoglobin within RBCs  1.5% dissolved in plasma  Rate at which Hb binds or releases O2 is regulated by:  PO2  Temperature  Blood pH  PCO2  2,3-DPG  These factors ensure adequate delivery of O2 to tissue cells LO4.Explain the principles of gas exchange – transport of oxygen and carbon dioxide Oxygen Transport Oxygen Hemoglobin Saturation Curve LO4.Explain the principles of gas exchange – transport of oxygen and carbon dioxide Carbon Dioxide Transport Transported from tissue cells to lungs: Dissolved in plasma (7-10%) Chemically bound to hemoglobin (~20%)  Carboxyhemoglobin  Globin As bicarbonate ions in plasma (~70%)  Converted to bicarbonate mainly in RBCs then released into the plasma  Some conversion in plasma LO4.Explain the principles of gas exchange – transport of oxygen and carbon dioxide  Tissue cell Interstitial fluid CO2 CO2 (dissolved in plasma) Binds to Slow plasma CO2 CO2 + H2O H2CO3 HCO3– + H+ proteins CO2 HCO3– Chloride Fast Cl– shift CO2 CO2 + H2O H2CO3 HCO3– + H+ Carbonic Cl– (in) via CO2 anhydrase transport HHb protein CO2 CO2 + Hb HbCO2 (Carbamino- hemoglobin) Red blood cell HbO2 O2 + Hb CO2 O2 O2 O2 (dissolved in plasma) Blood plasma (a) Oxygen release and carbon dioxide pickup at the tissues LO5.Outline how oxygen and carbon dioxide are transported in the blood  Alveolus Fused basement membranes CO2 CO2 (dissolved in plasma) Slow CO2 CO2 + H2O H2CO3 HCO3– + H+ HCO3– Chloride Fast Cl– CO2 CO2 + H2O H2CO3 HCO + H 3 – + shift Carbonic Cl– (out) via anhydrase transport CO2 CO2 + Hb HbCO2 (Carbamino- protein hemoglobin) Red blood cell O2 + HHb HbO2 + H+ O2 O2 O2 (dissolved in plasma) Blood plasma (b) Oxygen pickup and carbon dioxide release in the lungs LO5.Outline how oxygen and carbon dioxide are transported in the blood Learning objectives At the end of this session, you will be able to: 1.Describe the pressure changes in the thoracic cavity 2.Outline the mechanics of breathing 3.Describe the factors affecting pulmonary ventilation 4.Explain the principles of gas exchange – transport of oxygen and carbon dioxide Reading: Human Anatomy and Physiology, Marieb and Hoehn, Chapter 22 Textbook of Medical Physiology, Guyton and Hall, Chapters 38, 40, 41 ** look out for the colour code to link to the LOs