Pulmonary Gas Exchange and Gas Transport

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Pulmonary Gas Exchange and Gas Transport

• Dr. Meg-angela Christi Amores

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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 lt1.5mm, all
  smooth muscles
• Kept expanded by same transpulmonary pressures
  that expand the alveoli

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Physiologic Anatomy

• All areas of the trachea and bronchi not occupied
  by cartilage 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

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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

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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)

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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

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Gas Pressures

• Partial Pressure
• Pressure is directly proportional to the
  concentration of gas molecules caused by impact
  of moving molecules against a surface
• In respiration, theres 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

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Gas Pressure in Fluid

• Determined by its concentration and by solubility
  coefficient
• If gas is repelled, pressure increases
• HENRYs LAW Pressure concentration
• solubility coefficient

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Solubility of Gases in body temp.

• O2 0.024
• CO2 0.57 - 20x more soluble than O2
• CO 0.018
• N2 0.012
• He 0.008

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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

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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

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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
  alveoli and pressure of gas in pulmonary
  capillary blood

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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

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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

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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 gt as ideal
  physiologic dead space (ventilation but less
  blood flow)
• At bottom Va is less than Q
• Va/Q is 0.6 lt as ideal physiologic shunt
• COPD patient
• Smoker, emphysema, alveolar walls destroyed
• Wasted blood flow severe shunting

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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 40 mmHg 45 mmHg 45 mmHg 46 mmHg
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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)

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• For the next meeting, read on Regulation of
  Respiration
• Guyton Textbook of Medical Physiology, 10th
  edition Chapter 41