GAF FPP, Wk12, Tutorial 1, Gas Exchange and the Alveolus 23.pptx

Respiratory system – gas exchange Week 12 |Tutorial 1 Foundations for Physiotherapy Practice University Hertfordshire | FPP | 2023-24 Learning outcomes Following this session and appropriate independent study you should be able to: ‐ Explain how gaseous exchange occurs in the lungs. ‐ Describe ho...

Respiratory system – gas exchange Week 12 |Tutorial 1 Foundations for Physiotherapy Practice University Hertfordshire | FPP | 2023-24 Learning outcomes Following this session and appropriate independent study you should be able to: ‐ Explain how gaseous exchange occurs in the lungs. ‐ Describe how the partial pressures of oxygen and carbon dioxide vary between the atmosphere, the alveoli and the blood. University Hertfordshire | FPP | 2023-24 Movement of air in the lungs • Mechanics of breathing produce inspiration and air flow • During inspiration air initially moves into lungs via convection • Air passes through the conducting zone to respiratory zone of the bronchial tree • Air in smallest airways move into the alveoli sacs by diffusion • The actual exchange of gases occurs across the capillaries in the alveoli sacs University Hertfordshire | FPP | 2023-24 Movement of air in the lungs • Convection in the most general terms refers to the movement of currents within fluids (i.e. liquids, gases) • Diffusion refers to the process by which molecules intermingle as a result of their kinetic energy of random motion • Diffusion is an extremely rapid process • Can only occur over very small distances University Hertfordshire | FPP | 2023-24 Diffusion & gas exchange Gas exchange takes place in alveoli sac across the alveolar membrane Diffusion Provides a boundary and Gas between the external environment and interior Exchange of the body Gases cross the respiratory membrane by diffusion In accordance with Fick’s Law University Hertfordshire | FPP | 2023-24 Fick’s Law The rate of transfer of a gas through a sheet of tissue is proportional to: 1. Tissue area 2. Difference in gas partial pressure between the 2 sides 3. Diffusion coefficient (determined by solubility of the gas and its molecular weight) Inversely proportional to tissue thickness University Hertfordshire | FPP | 2023-24 Diffusion Across the Alveolar Membran e Diffusion Across Alveolar Membran e In accordance with Fick’s law diffusion is dependent on 1. Concentration / pressure gradient 2. Gas solubility 3. Surface area of alveolar membrane 4. Ventilation / perfusion coupling 5. Thickness of alveolar membrane University Hertfordshire | FPP | 2023-24 Respiratory Zone • Respiratory bronchioles • alveolar ducts, alveolar sacs (clusters of alveoli) • ~300 million alveoli account for most of the lungs’ volume and are the main site for gas exchange • The total alveolar surface area is approximately 30-50 square metres – equivalent to a tennis court University Hertfordshire | FPP | 2023-24 The micro structure of an alveolus Note: a whole alveoli is only 0.2-0.3mm and the membrane is 0.2 thousandths of a millimetre Fig. 16.5 Stanfield University Hertfordshire | FPP | 2023-24 Fig. 16.5 Germann The respiratory membrane • Three layers but very thin to facilitate gaseous exchange: – Alveolar epithelium • mainly made up of a single layer of cells called Type I cells – Fused basement membrane – Capillary endothelium • Thickness: – 0.2micrometres (10-6 metres or 10-3mm) • Large overall surface area • 300 million alveoli in each lung University Hertfordshire | FPP | 2023-24 Anatomy of the respiratory membrane Red blood cell Nucleus of type I (squamous epithelial) cell Alveolar pores Capillary O2 Capillary CO2 Alveolus Alveolus Type I cell of alveolar wall Macrophage Endothelial cell nucleus Respiratory membrane Red blood cell Alveoli (gas-filled in capillary Type II (surfactantair spaces) secreting) cell Alveolar epithelium Fused basement membranes of the alveolar epithelium and the capillary endothelium Capillary endothelium University Hertfordshire | FPP | 2023-24 Martini Figure 22.9c And… remember the very important Type II cells • As each alveolus is surrounded by elastic fibrous tissue that tends to make the alveoli collapse inwards and there is surface tension from the fluid on the internal surface of the alveoli that also encourages it to collapse in on itself • So the surfactant produced by Type II (surfactant secreting) cells is crucial to the alveoli remaining inflated. • Surfactant – is an oily secretion – contains phospholipids and proteins – coats internal alveolar surfaces and reduces surface tension • Neonates – born without surfactant – can go into respiratory distress syndrome • Adults can develop Adult Respiratory Distress Syndrome University Hertfordshire | FPP | 2023-24 Alveolar structure: Pulmonary arterioles/capillaries and pulmonary venules University Hertfordshire | FPP | 2023-24 Blood supply to the lung tissue itself • Bronchial arteries provide oxygenated blood to lung tissue itself (down to the bronchioles) –Arise from aorta –Supply all lung tissue except the alveoli • Bronchial veins anastomose with pulmonary veins • Pulmonary veins carry most of the bronchial venous blood back to the heart University Hertfordshire | FPP | 2023-24 Any Questions so far? University Hertfordshire | FPP | 2023-24 Concept of partial pressure of a gas in a gaseous mixture • The total pressure (P) exerted by a gas mixture is the sum of it's constituent parts (Dalton’s Law) • Atmospheric pressure (Patm) at sea level = 760mmHg = 101.325kPa • Gas percentages stay the same as altitude rises but the overall pressure P (atm) exerted by each gas reduces in relation to the overall pressure of the air – the reduced effect of gravity reduces the number of gas molecules in a certain volume of air…. University Hertfordshire | FPP | 2023-24 Composition of air/the atmosphere is always…. Oxygen (~21%) Nitrogen (~79%) Carbon dioxide (~0.04%) (Water – variable) P(atm) = PO2+PCO2+PN2 = 760mmHg (101kPa) • Nitrogen plays a vital role in keeping the alveoli inflated as it does not participate in gaseous exchange and it is very insoluble in water therefore 100% O2 for extended periods is not a good idea University Hertfordshire | FPP | 2023-24 The concept that the greater the number of gas molecules in an area the greater the pressure they exert Relates to Boyles Law University Hertfordshire | FPP | 2023-24 Very diagrammatically… The percentage of oxygen molecules in the air is 21% per unit volume of the atmosphere Earth whether you are at sea level or at the top pf Everest- but on Everest it is 21% of a much lower number of molecules per unit University Hertfordshire | FPP | 2023-24 volume So……… going up a mountain/flying Gas Percentage TOTALs Air atmosphe re at sea level At 8,000 feet On top of EVEREST Partial Partial pressure pressure ~760mm 564mmHg/ Hg/ 75.2kPa 101.3kPa Partial pressure 260mmH g/ 34.7kPa Nitrogen 79 597mmH g / 79.6 kPa 446mmHg/ 59 kPa 205mmHg / 27.3kPa Oxygen 21 160mmH g/ 21.3kPa 118mmHg/ 15.7kPa About 75% of normal O2 54mmHg/ 7.2kPa About 35% of normal O2 0.04 0.3mmHg / Carbon dioxide University Hertfordshire | FPP | 2023-24 Factors affecting partial pressures of gases in the alveoli and availability for gas exchange • The levels of O2 in inspired air (normally constant but varies at altitude, when receiving oxygen therapy, if someone is artificially ventilated and on an aircraft) • The rate and depth of ventilation i.e. inspiration and expiration. Rapid shallow breathing may move the same volume of air in a minute as slow deep breaths. • If tissue metabolism rises (and hence the rate of O2 consumption and CO2 production rises) then the body should increase the rate and depth of ventilation (hyperpnoea) to compensate to maintain the amounts of O2 in the alveoli available for gaseous exchange and to remove CO2 • The fact that alveolar air is fully saturated with water vapour (vs. atmospheric air that is not) University Hertfordshire | FPP | 2023-24 Another important concept: Gases equilibrate between a gaseous mixture and a solution • If a gas is next to a fluid then gas particles will move into the liquid until the partial pressures are the same in the liquid and the gas mixture • However this is also dependent on: • The solubility of the gas in the liquid • The temperature of the liquid • In the alveoli the oxygen and carbon dioxide meet across the respiratory membrane which is permeable so gas exchange occurs by diffusion University Hertfordshire | FPP | 2023-24 Gas exchange in the lungs Diffusion across the respiratory membrane occurs: –From an area of higher partial pressure of gas to an area of lower partial pressure of the same gas across the respiratory membrane (down its partial pressure gradient) –This gradient is greater for oxygen (~100mmHg in alveolar air versus ~40mmHg in the deoxygenated blood in the alveolar capillary) than carbon dioxide (~40mmHg in the alveolar air versus ~45mmHg in the capillary blood) BUT –Carbon Dioxide (CO2) is around 20 times more soluble than oxygen (O2) University Hertfordshire | FPP | 2023-24 Tidal Volume – A normal breath is termed a person’s tidal volume and is ~ 500mls, this comprises –Air in the conducting zone (~150mls) • = 1/3 rd of a breath –Air in the respiratory zone (~ 350mls) • What happens to the amount of fresh air reaching the alveoli if you take: –Deeper breaths –Very shallow breaths? • Clinical significance: –in relation to what happens to the respiratory rate and depth in a patient with respiratory disease? University of Hertfordshire FPP 2015-2016 University Hertfordshire | FPP | 2023-24 Assistance of alveolar ventilation University Hertfordshire | FPP | 2023-24 25 Collateral ventilation assists equal alveolar expansion: integral to physio techniques M L K University Hertfordshire | FPP | 2023-24 Summary Efficient gas exchange is dependant on: 1. Concentration / pressure gradient 2. Gas solubility 3. Surface area of alveolar membrane 4. Ventilation / perfusion coupling 5. Thickness of alveolar membrane University Hertfordshire | FPP | 2023-24 Overview Inspired air: P O2 160 mm Hg P CO 0.3 mm Hg Alveoli of lungs: P O2 104 mm Hg P CO 40 mm Hg 2 2 External respiration Clinically termed Venous Blood Gases Pulmonary arteries Pulmonary veins (PO2 100 mm Hg) Blood leaving tissues and entering lungs: PO2 40 mm Hg PCO2 45 mm Hg Blood leaving lungs and entering tissue capillaries: P O2 100 mm Hg P CO2 40 mm Hg Clinically termed Arterial Blood Gases (ABGs) Heart Systemic veins Internal respiration Systemic arteries Tissues: P O2 less than 40 mm Hg P CO greater than 45 mm Hg 2 University Hertfordshire | FPP | 2023-24 Marieb and Hoehn 2010 Figure 22.17 Any questions? University Hertfordshire | FPP | 2023-24 29 Thank you for your attention. University Hertfordshire | FPP | 2023-24 30 Bibliography Fox, S. I. (2009). Human physiology. (11th ed.). Boston: McGraw Hill Higher Education. Marieb, E. N, and Hoehn, K.(2013). Human anatomy and physiology. (9th ed.). Pearson Benjamin Cummings. San Francisco: London. Martini, F. H. (2006). Fundamentals of anatomy and physiology. (7th ed.). San Francisco: Pearson Stanfield, C. L. & Germann, W. J. (2008). Principles of human physiology. (3rd ed.). San Francisco: Pearson Thibodeau, G. A. & Patton, K. T. (2007). Anatomy and physiology. (6th ed.). St Louis, Missouri: Mosby Elsevier. Widmaier, E. P., Raff, H. & Strang. K.Y. (2006). Vander’s human physiology: The mechanisms of body function. (10th ed.). Boston: McGraw Hill. University Hertfordshire | FPP | 2023-24