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Pulmonary Gas Exchange and Gas Transport

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1 Pulmonary Gas Exchange and Gas Transport
Dr. Meg-angela Christi Amores

2 Physiologic Anatomy One of the most important problems in all the respiratory passages is to keep them open to allow easy passage of air to and from the alveoli Trachea – with cartilage rings 5/6 of the way around Bronchi – walls have less extensive cartilage plates Bronchioles – no plates. Diameter <1.5mm, all smooth muscles Kept expanded by same transpulmonary pressures that expand the alveoli


4 Physiologic Anatomy All areas of the trachea and bronchi not occupied by cartiliage plates, walls are composed of smooth muscles Resistance to flow is greatest NOT in the minute air passages of terminal bronchioles but in some of the larger bronchi near to the trachea. Smaller airways are easily occluded ; smooth muscles = contract easily

5 Pulmonary Circulatory System
Pulmonary Vessels Pulmonary artery (5 cm, thin, 2x VC, 1/3 aorta) Right and Left main pulmonary branches – lungs Large compliance (7 mL/mmHg) Allows pulmonary arteries to accommodate 2/3 of stroke volume output of Right Ventricle Bronchial Vessels – arterial supply to the lungs 1/3 of cardiac output Supplies supporting tissues (CT, septa, bronchi) Drains to pulmonary veins

6 Pulmonary vs. Alveolar Ventilation
Pulmonary Ventilation Inflow and outflow of air between the atmosphere and the lung alveoli Alveolar Ventilation Rate at which new air reaches the areas in the lung where it is in proximity to the pulmonary blood or gas exchange areas (alveolar sacs, ducts, respiratory bronchioles)

7 Diffusion of Gases Diffusion
Random molecular motion of molecules with energy provided by kinetic motion of the molecules All molecules are continually undergoing motion except in absolute zero temperature Net diffusion Product of diffusion from high to low concentration

8 Gas Pressures Partial Pressure AIR = total Pressure 760 mmHg
Pressure is directly proportional to the concentration of gas molecules; caused by impact of moving molecules against a surface In respiration, there’s mixture of gases: O2, N2, CO2 Rate of diffusion of each gas is directly proportional to the pressure caused by each gas alone AIR = total Pressure 760 mmHg 79% N, 21% O2 = PP N = 600mmHg , PP O2 =160mmHg

9 Gas Pressure in Fluid Determined by its concentration and by solubility coefficient If gas is repelled, pressure increases HENRY’s LAW : Pressure = concentration solubility coefficient

10 Solubility of Gases in body temp.
CO2 = x more soluble than O2 CO = 0.018 N2 = 0.012 He = 0.008

11 Factors that affect Rate of Gas Diffusion thru Respiratory Membrane
Respiratory Unit: Respiratory bronchiole Alveolar ducts Atria Alveoli (300 Million in both lungs) (0.2mm) *their membranes make up the respiratory membrane

12 Respiratory Membrane Layers:
Layer of fluid lining alveolus (surfactant) Alveolar epithelium Epithelial basement membrane Interstitial Space Capillary basement membrane Capillary endothelial membrane Overall thickness: 0.2um (ave: 0.6 um) Total surface area: 70 m2

13 Factors that affect Rate of Gas Diffusion thru Respiratory Membrane
Thickness of membrane Inc. in edema and fibrosis Surface area of membrane Dec. in removal of lung and emphysema Diffusion coefficient of Gas in substance of membrane Gas’ solubility Pressure difference Difference between partial pressure of gas in alveolia and pressure of gas in pulmonary capillary blood

14 Ventilation-Perfusion Ratio
A concept developed to help us understand respiratory exchange where there is imbalance between alveolar ventilation and alveolar blood flow Areas in lung with well ventilation but no bloodflow or excellent blood flow but no ventilation Va – alveolar ventilation Q – blood flow

15 Ventilation-Perfusion Ratio
Va/Q = normal If Va is 0 (zero), but with perfusion: Va/Q = 0 If Va is present, but no perfusion Va/Q = infinity In both: there is no gas exchange

16 Ventilation-Perfusion Ratio
Normal person : Upright: Va and Q are less in Upper part but Q is more At top of lung: Va/Q 2.5x > as ideal = physiologic dead space (ventilation but less blood flow) At bottom: Va is less than Q Va/Q is 0.6 < as ideal = physiologic shunt COPD patient: Smoker, emphysema, alveolar walls destroyed Wasted blood flow = severe shunting

17 Transport of O2 and CO2 Pressure differences causes gas to diffuse
Alveolus Capillaries Tissues (fluid) Tissues (cells) pO2 104 mmHg 95 mmHg 40 mmHg 5-40 (ave 23) mmHg pCO2 45 mmHg 46 mmHg

18 Transport of O2 and CO2 CO2 can diffuse about 20 times as rapidly as O2 Transport of O2 in blood: 97% of O2 from lungs to tissues are carried in combination with hemoglobin O2 combines loosely and reversibly with heme pO2 – O2 combines with heme (pulm capi) pO2 – O2 is released (tissue capillaries)

19 For the next meeting, read on Regulation of Respiration
Guyton Textbook of Medical Physiology

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