Part 3 Respiratory Gases Exchange

I Physical Principles of Gas Exchange

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

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 = 600.4 mm Hg P02 = 760 x 0.21 = 159.6 mm Hg

Partial Pressure of Gases in Fluids Each gas has a specific solubility O2 Solubility coefficient = 0.003 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

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.

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

II Gas exchange in the lung and in the tissue

Diffusion Gradients of Respiratory Gases at Sea Level Total 100.00 760.0 760 760 0 H2O 0.00 0.0 47 47 0 O2 20.93 159.1 105 40 65 CO2 0.03 0.2 40 46 6 N2 79.04 600.7 569 573 0 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.

PO2 and PCO2 in Blood

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

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

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

Alveolar SPACE Blood Mixing SIMPLE CONCEPT OF A SHUNT AIR FLOW CO2 O2 arterial vessel BLOOD FLOW Blood Mixing Lowered O2/l00 ml No Gas Exchange = SHUNT

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”

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

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.

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

= 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

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

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.

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

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

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

EXTERNAL AND INTERNAL RESPIRATION TISSUE CELL O2 + FOOD ATMOSPHERE SYSTEMIC CIRCULATION HEART PULMONARY CIRULATION LUNGS CO2 + H2O + ATP