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1 Part 3 Respiratory Gases Exchange 2 3 I Physical Principles of Gas Exchange.

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Presentation on theme: "1 Part 3 Respiratory Gases Exchange 2 3 I Physical Principles of Gas Exchange."— Presentation transcript:


2 1 Part 3 Respiratory Gases Exchange

3 2

4 3 I Physical Principles of Gas Exchange

5 4 Partial pressure –The pressure exerted by each type of gas in a mixture Concentration of a gas in a liquid –determined by its partial pressure and its solubility coefficient

6 5 Partial Pressures of Gases Basic Composition of Air 79% Nitrogen 21% Oxygen ~ 0% Carbon Dioxide In a mixture of gases, each gas exerts a partial pressure proportional to its mole fraction. Total Pressure = sum of the partial pressures of each gas P gas = P b x F gas P N 2 = 760 x 0.79 = mm Hg P 0 2 = 760 x 0.21 = mm Hg Total Pressure (at sea level) P barometric = 760 mm Hg PbPbPbPb 760 mm Hg PbPbPbPb

7 6 Consider a container of fluid in a vacuum Partial Pressure of Gases in Fluids Each gas has a specific solubility O 2 Solubility coefficient = ml/100 ml Blood C0 2 = 0.06 ml/100 ml Blood (x 20 of 0 2 ) Gases dissolve in fluids by moving down a Partial Pressure gradient rather than a concentration gradient That is opened to the air Molecules of gas begin to enter the fluid

8 7 Partial Pressure of Gases in Fluids After a short time, the number of molecules the number of molecules ENTERING = LEAVING At equilibrium, if the gas phase has a P O 2 = 100 mm Hg, the liquid phase also has a P O 2 = 100 mm Hg An easy way to talk about gases in fluids.

9 8 Alveolus Blood capillary Time for exchange P O2 Time sec Saturated very quickly Reserve diffusive Capacity of the lung 45 mm Hg P CO2 Diffusion: Blood Transit Time in the Alveolus

10 9 II Gas exchange in the lung and in the tissue

11 10 Diffusion Gradients of Respiratory Gases at Sea Level Total H 2 O O CO N Partial pressure (mmHg) % inDryAlveolarVenousDiffusion Gasdry airairairbloodgradient NB. CO 2 is ~20x more soluble than O 2 in blood => large amounts move into & out of the blood down a relatively small diffusion gradient.

12 11 PO 2 and PCO 2 in Blood

13 12 III. A-a gradient, the efficiency of the gas exchange in alveoli

14 13 Oxygen Content in Alveolus Gas ) (measured during exhalation) Oxygen Content in arterial blood (equivalent to that leaving lungs) What is an A - a gradient ? : The DIFFERENCE between: In a healthy person, what would you expect the A - a to be? No difference, greater than 0, or less than 0 Normal: A – a, up to ~ 10 mm Hg, varies with age

15 14 Factors contributing to A - a Gradient 1.Blood Shunts 2.Blood Mixing 3.Matching 1.Blood Shunts 2.Blood Mixing 3.Matching

16 15 Alveolar SPACE arterial vessel SIMPLE CONCEPT OF A SHUNT BLOOD FLOW CO 2 O2O2O2O2 No Gas Exchange = SHUNT AIR FLOW Blood Mixing Lowered O 2 /l00 ml

17 16 Total Perfusion, Q Total Ventilation NEXT NEW CONCEPT Matching What? Blood to Air Flow Exchange Oxygen If the volumes used for exchange are aligned – We might consider the system to be ideally matched

18 17 Arterial Perfusion (Q c ) Slide or Misalign the distribution volumes Alveolar Ventilation (V A ) ExchangeOxygen Dead Air Space (Airways) Shunt (Q s ) (Bronchial Artery) Some Volumes are wasted, Matching Ratio = V A /Q c = 0.8 Normal Case; Small Shunt, low volume Dead Space

19 18 Matching ventilation & perfusion Ventilation and perfusion (blood flow) are both better at the bottom (base) of the lung than that at the top (apex). the change in blood flow is more steep than in ventilation. the ventilation/perfusion ratio rises sharply from the base to the apex.

20 19 Matching ventilation & perfusion (cont) Result: V/Q is greater or less than 0.8 in different regions If V/Q 0.8 little benefit, Increases A - a gradient

21 20 Alveolar Ventilation V A Arterial Perfusion Q Exchange Oxygen Dead Air Space Shunt = Lung Disease with a Large A – a gradient

22 21 IV Factors Affecting the Gas Diffusion in the Lung 1)Area of the respiratory membrane 2)Distance of the diffusion 3)V A /Q

23 22 V Pulmonary Diffusion Capacity Concept: The ability of the respiratory membrane to exchange a gas between the alveoli and the pulmonary blood defined as the volume of a gas that diffuses through the membrane each minute for a pressure of 1 mmHg. D L = V/(P A – P C ) V is a gas that diffuses through the membrane each minute, P A is the average partial pressure of a gas in the air of alveoli, P C is the average partial pressure of a gas in the blood of pulmonary capillary.

24 23 Factors Affecting the D L 1.Body posture 2.Body height and weight 3.Exercise 4.Pulmonary diseases

25 24 VI Internal Respiration All cells require oxygen for metabolism All cells require means to remove carbon dioxide Gas exchange at cellular level

26 25 Concept: Gas exchange between the capillary and the tissues throughout the body Process: Factors affecting the internal respiration: 1.Distance between the cells and the capillary 2.Rate of metabolic rate 3.Speed of the blood flow in capillary


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