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M1 – Cardiovascular/Respiratory Sequence Louis D’Alecy, Ph.D. Alveolar Ventilation M1 – Cardiovascular/Respiratory Sequence Louis D’Alecy, Ph.D. Fall 2008 3

Monday 11/17/08, 9:00 Ventilation Static Volumes Tidal volume 20 slides, 50 minutes Static Volumes Tidal volume Dead space volume Alveolar volume Minute volumes Alveolar Ventilation Composition of Respiratory Gases

Functional Volumes (mL) VT VD VA Tidal volume is the volume of each breath. Dead space volume has no gas exchange. - conducting airways (anatomic) - non-perfused alveoli (alveolar) Alveolar volume has gas exchange.

DEAD SPACE VD Physiological Dead space (volume) sum of anatomical and alveolar dead space Anatomical Dead Space (volume) volume of air in airways that can not exchange gases with blood - typical value about 150 ml or 1 ml per pound body weight. Alveolar Dead Space (volume) volume of alveoli that are ventilated but do not receive a blood flow and thus no gas exchange. Small in normal lung but can be very large in some pulmonary diseases.

VT = VD + VA VA = VT - VD Tidal Volume (VT) The tidal volume is the sum of the dead space volume and the alveolar volume. The alveolar volume is the difference between the tidal volume and the dead space volume.

Breaths per Minute & Alveolar Ventilation Normal respiratory rate is about 12 to 15 breaths /minute. Alveolar ventilation (VA) is calculated by multiplying the respiratory rate times the volume. Indicates a rate or “per min” as in mL/min.

Minute Volume (Rate X VT) or Total Ventilation or Minute Ventilation VT = VE VT = Rate X VT 6000 mL/min = 12 b/min X 500 mL/b or from previous example 7500 mL/min = 15 b/min X 500 mL/b BUT Same rate applies to VD + VA

Breaths per Minute (book example) Source Undetermined VT = VE

Alveolar Ventilation VT = VE VA = VT - VD or VA = VE - VD But VA = VT - VD or VA = VE - VD which is the same as VA = Rate (VE - VD) or = Rate (VT - VD)

Alveolar Ventilation end expiration end inspiration tidal volume 450 ml anatomical dead space 150 ml 300 ml 450 ml 3000 ml FRC end expiration end inspiration D’Alecy

Alveolar Ventilation Is the exchange of gas between the alveoli and the external environment. Bulk Flow Diffusion Source Undetermined

Alveolar ventilation = Total ventilation - Dead space ventilation b/min X mL/b = b/min X mL/b - b/min X mL/b X 350 = 15 X 500 - 15 X 150 5250 ml/min = 7500 ml/min - 2250 ml/min = 15 X 500 = b/min X mL/b Dead space ventilation = b/min X mL/b = 15 X 150 = 2250 mL/min Fig2-1.gif Source Undetermined Alveolar ventilation is approximately equal to pulmonary blood flow (cardiac output).

VA = R (VT - VD) 4200 mL/min = 12 ( 500 - 150) Fixed VE & VD Effect of rate & tidal volume on ALVEOLAR VENTILATION VA = R (VT - VD) 4200 mL/min = 12 ( 500 - 150) Fixed VE & VD VT R VE VA ml/min ml/min 150 40 6000 0 no alveolar ventilation 500 12 6000 4200 normal ventilation 1000 6 6000 5100 excess ventilation

Composition of Respiratory Gas What do we breath? Air What do we breath in? Air & moisture What do we breath out? Air, CO2 & H2O What is air made of? N2 and O2 mostly How measure? Partial Pressure Partial pressure of a gas is equal to its fractional concentration times the total pressures of all gases in mixture. (Dalton’s Gas Law)

STEPS IN RESPIRATION Bulk Flow Diffusion Source Undetermined

Dalton’s Law of Partial Pressures The total pressure exerted by a mixture of gases is equal to the sum of the partial pressures that each gas would exert if it alone occupied the entire volume. Total pressure of air (barometric) Pair = PO2 + PCO2 + PN2 + Px 100% = 21% + 0.03% + 78% + 0.9% 760 mm Hg = 160 + 0.2 + 593 + 7 PO2 = 21% x 760 = 160 mm Hg Where does the 760 mmHg come from ??

Origin of 760 mmHg - effect of ALTITUDE Altitude PB % O2 PO2 ft mm Hg mm Hg Sea level 0 760 21% 160 Ann Arbor 800 737 21% 155 Denver 5,200 640 21% 134 Mt Everest 29,028 253 21% 53

T = 273 ° absolute (0 ° Celsius) Standard Conditions for Measuring Gas Volumes The volume of a pure gas (V) at STPD is directly proportional to the number of moles (n) of that gas (1 mole gas = 22.4 liters STPD), R (gas constant) and T PV = nRT n RT P V = constant Standard Temperature, Pressure, Dry = STPD T = 273 ° absolute (0 ° Celsius) Pb = 760 mm Hg at sea level 21% of dry air pressure is due to oxygen thus 0.21 X 760 = 160 mmHg PO2 Dry (no water vapor)

Pb = PH2O + Pdry WATER PARTIAL PRESSURE The gas partial pressure of water in equilibrium with liquid water depends only on the temperature. The higher the temperature the higher partial pressure due to water. At body temperature (37 C °) the PH2O = 47 mmHg Pb = PH2O + Pdry The water added by the body dilutes the other gases such that all their partial pressures go down !! 760 = 47 + 713 P = 0.21 ( - ) = 150 mm Hg Not the 160 mmHg of dry air P P H O 2 O b 2

P = inspired oxygen PI = % O2 (Pb - PH20) That is why the partial pressure of oxygen in inspired air is lower than dry room air. P = inspired oxygen I O 2 The partial pressure of oxygen in the air that enters the body is reduced by the addition of water vapor. Inspired air is diluted with water vapor until saturated. PI = % O2 (Pb - PH20) O 2

So why alveolar PO2 so low?? Dry air P = % O2 Pb = 160 mm Hg b O 2 Humidified air PI = % O2 (Pb - PH20) = 0.21 x 713 = 150 mm Hg O 2 P = 104 mm Hg A O 2 ??Alveolar air partial pressure of oxygen ?? D’Alecy

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