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Part 3 Respiratory Gases Exchange.

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Presentation on theme: "Part 3 Respiratory Gases Exchange."— Presentation transcript:

1 Part 3 Respiratory Gases Exchange


3 I Physical Principles of Gas Exchange

4 Concentration of a gas in a liquid
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

5 Partial Pressures of Gases
Total Pressure (at sea level) Pbarometric = 760 mm Hg Pb 760 mm Hg 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 Pgas = Pb x Fgas PN2 = 760 x 0.79 = mm Hg P02 = 760 x 0.21 = mm Hg

6 Partial Pressure of Gases in Fluids
Each gas has a specific solubility O2 Solubility coefficient = ml/100 ml Blood C02 = 0.06 ml/100 ml Blood (x 20 of 02) Gases dissolve in fluids by moving down a Partial Pressure gradient rather than a concentration gradient Consider a container of fluid in a vacuum That is opened to the air Molecules of gas begin to enter the fluid

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 PO2 = 100 mm Hg, the liquid phase also has a PO2 = 100 mm Hg An easy way to talk about gases in fluids.

8 Diffusion: Blood Transit Time in the Alveolus
Blood capillary Time for exchange PO2 Time 0.75 sec 40 100 Saturated very quickly Reserve diffusive Capacity of the lung 45 mm Hg PCO2

9 II Gas exchange in the lung and in the tissue

10 Diffusion Gradients of Respiratory Gases at Sea Level
Total H2O O CO N Partial pressure (mmHg) % in Dry Alveolar Venous Diffusion Gas dry air air air blood gradient NB. CO2 is ~20x more soluble than O2 in blood => large amounts move into & out of the blood down a relatively small diffusion gradient.

11 PO2 and PCO2 in Blood

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

13 The DIFFERENCE between:
What is an A - a gradient ? The DIFFERENCE between: Oxygen Content in arterial blood (equivalent to that leaving lungs) Oxygen Content in Alveolus Gas (measured during exhalation) 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

14 Factors contributing to A - a Gradient
Blood Shunts Blood Mixing Matching

arterial vessel BLOOD FLOW Blood Mixing Lowered O2/l00 ml No Gas Exchange = SHUNT

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

17 Matching Dead Air Space (Airways) Alveolar Ventilation (VA) Oxygen
Exchange Oxygen Arterial Perfusion (Qc) Slide or Misalign the distribution volumes Shunt (Qs) (Bronchial Artery) Some Volumes are wasted, Matching Ratio = VA/Qc = 0.8 Normal Case; Small Shunt, low volume Dead Space

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.

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

20 = Lung Disease with a Large A – a gradient
Severe Mismatch Dead Air Space Alveolar Ventilation VA Exchange Oxygen Arterial Perfusion Q blood mixing Shunt = Lung Disease with a Large A – a gradient

21 IV Factors Affecting the Gas Diffusion in the Lung
Area of the respiratory membrane Distance of the diffusion VA/Q

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. DL = V/(PA – PC) V is a gas that diffuses through the membrane each minute, PA is the average partial pressure of a gas in the air of alveoli, PC is the average partial pressure of a gas in the blood of pulmonary capillary.

23 Factors Affecting the DL
Body posture Body height and weight Exercise Pulmonary diseases

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

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


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