Respiratory System Lecture Notes PDF

L29- RESPIRATORY SYSTEM Dr.Pugazhandhi Bakthavatchalam Assistant Professor of Anatomy and Physiology, AUACAS, American University of Antigua  LEARNING OUTCOMES At the end of this session the student would be able to: Describe the factors affecting resistance to airflow Describe the...

L29- RESPIRATORY SYSTEM Dr.Pugazhandhi Bakthavatchalam Assistant Professor of Anatomy and Physiology, AUACAS, American University of Antigua  LEARNING OUTCOMES At the end of this session the student would be able to: Describe the factors affecting resistance to airflow Describe the factors affecting work of breathing with special reference to lower airway resistance Outline gas laws as applicable to pulmonary diffusion Describe the factors affecting net diffusion of gas across membrane Describe the factors affecting pulmonary diffusion of gases and their clinical significance Describe the pulmonary diffusion capacity of oxygen at rest and during exercise 9-Oct-24 2 Work of Breathing 9-Oct-24 3 Work of Breathing  Work of inspiration Performed by the respiratory muscles in stretching the elastic tissues of the chest wall and lungs  Compliance work or elastic work (65%) Work that required to expand the lungs against the lung and chest elastic forces Presence of elastin & collagen fibres Surface tension  Tissue resistance work (7%) Work that required to overcome the viscosity of the lung and chest wall structures  Airway resistance work (28%) Work that required to overcome airway resistance Increases during bronchial constriction 9-Oct-24 4 Factors Affecting Airflow Resistance  Parasympathetic constriction of the bronchioles Cause mild to moderate constriction Vagus nerves secrete acetylcholine Ach acts on the target cells through M1, M2 & M3 receptors G protein coupled Stimulation causes increased intracellular Ca2+ & Smooth muscle contraction  Sympathetic dilation of the bronchioles Direct control of the bronchioles is relatively weak Norepinephrine and epinephrine Release by sympathetic stimulation of the adrenal gland medullae Epinephrine stimulate beta-adrenergic receptors Cause bronchodilatation 9-Oct-24 5 Cough Reflex Receptors located in the airways Stimulated by Irritants Activate vago-vagal reflex Causing bronchoconstriction Rapid inspiration Epiglottis closes Vocal cords shut tightly Abdominal muscles contract forcefully Expiratory muscles contract forcefully Pulmonary pressure rises rapidly Vocal cords and epiglottis open widely Lungs explodes outward 9-Oct-24 6 Airway Resistance: Effect of Airway Radius Resistance offered by the respiratory airways is determined by Poiseuille’s formula: R = 8 v l / π r4 R = peripheral resistance, v = viscosity r = radius of tube, l = length of tube Viscosity and length does not change under normal circumstances Diameter of the airways is the major factor Slight change in radius causes a marked change in airway resistance 9-Oct-24 7 Airway Resistance  Intrapleural pressure Negative intrapleural pressure keeps the airway open during inspiration Decreases resistance Positive intrapleural pressure collapse some of the smaller airways and during forced expiration Increases resistance Excessive mucus secretion - Increases resistance In exercise - Increases resistance 9-Oct-24 8 Gas Laws Applicable to Pulmonary Diffusion  Charles law: Volume of a given mass of gas is directly proportional to the temperature (when the pressure is constant) V α T  Boyle’s law: The pressure exerted by a given mass of gas is inversely proportional to its volume (when the temperature is constant) P α 1/V 9-Oct-24 9 Partial Pressures of Gases  Partial pressure Total pressure is the summated force of all component gases in a gas mixture Dry air has 21% O2 , 78% N2 , 0.04% CO2 and 0.096% other inert constituents Barometric pressure at sea level is 760 mm Hg (1 Atmosphere) The presence of water vapor in air reduces partial pressures Inside the body air is saturated with water vapor and PH2O is 47 mm Hg o Partial pressures of gases in alveoli: PO2 is 149 mm Hg , PN2 is 564 mm Hg and PCO2 is 0.3 mm Hg 9-Oct-24 10 Respiratory Membrane Total surface area of the respiratory membrane is 70 square meters Made up of the walls of Alveolar epithelium Capillary endothelium Alveoli are in close contact with capillary basket 1. Thin layer of fluid & surfactant 2. Alveolar wall 3. Basement membrane of alveoli 4. Interstitial space 5. Basement membrane of capillary Average thickness of 6. Wall of capillary endothelium respiratory membrane is 0.6 μ 9-Oct-24 11 Respiratory Membrane 9-Oct-24 12 Factors Affecting Diffusion: Diffusion Coefficient  Diffusion depends on (Fick’s law) D α (P1 – P2) x A x S / d x Mol wt Pressure gradient of partial pressure of a gas between alveoli & pulmonary capillary D α (P1 – P2) Total Surface area of the alveolar membranes DαA Thickness of the respiratory membrane D α 1/d Solubility of the gas in the respiratory membrane DαS Molecular weight of the gas D α 1/Mol wt 9-Oct-24 13 Factors Affecting Diffusion: Diffusion Coefficient Greater the pressure gradient of the gases greater the diffusion Greater the solubility of the gases greater the diffusion Lesser the molecular weight of the gases greater the diffusion Greater the surface area of respiratory membrane greater the diffusion The thickness of respiratory membrane increases the diffusion resistance  Diffusing Capacity Volume of gas that diffuses across the respiratory membrane in 1 minute for a pressure difference of 1 mm Hg Diffusing capacity for O2 = 23 ml / min/ mm Hg CO2 diffuses 20 times more rapidly than O2 9-Oct-24 14 Changes in O2 Diffusion During Exercise During exercise Blood flow through the pulmonary capillaries increases More time is taken for PO2 to reach equilibrium Increase in the total surface area of the respiratory membrane Increased ventilation stretches the alveolar wall Increased blood flow stretches the capillary wall Further enhancement by opening of capillaries which are dormant during rest 9-Oct-24 15  REFERENCES Drake R.L., Gray’s Anatomy for Students, 2nd Edition, 2009, Churchill Livingstone Moore, Clinically Oriented Anatomy, 6th Edition, 2009, Lippincott Williams & Wilkins Textbook of Medical Physiology – Guyton & Hall Medical Physiology – R.K Marya 9-Oct-24 16