11.17.08(a): Alveolar Ventilation

Author(s): Louis D’Alecy, 2009

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3

Alveolar Ventilation

M1 – Cardiovascular/Respiratory Sequence

Louis D Alecy, Ph.D.

Fall 2008

4

Monday 11/17/08, 9:00

Ventilation 20 slides, 50 minutes

1. Static Volumes a)Tidal volume b)Dead space volume c) Alveolar volume

2. Minute volumes 3. Alveolar Ventilation 4. Composition of Respiratory Gases

5

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.

6

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 exchangegases with blood - typical value about 150 mlor 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.

7

Tidal Volume (VT)VT = VD + VA

VA = VT - VD

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.

8

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.

9

Minute Volume (Rate X VT) orTotal Ventilation or Minute Ventilation

VT = Rate X VT

6000 mL/min = 12 b/min X 500 mL/bor from previous example

7500 mL/min = 15 b/min X 500 mL/b

BUT

Same rate applies to VD + VA

VT = VE

10

Breaths per Minute (book example)

VT = VE

Source Undetermined

11


VA = VE - VD

which is the same as

VA = Rate (VE - VD) or = Rate (VT - VD)

VT = VE

VA = VT - VD or

But

12

end expiration

end inspiration

anatomical

dead space 150 ml

tidal volume

450 ml

450 ml

300 ml

3000 ml

FRC


D’Alecy

13

Alveolar VentilationIs the exchange of gas between the

alveoli and the external environment.

Bulk Flow

Diffusion

Source Undetermined

14

= 15 X 500= b/min X mL/b

Dead space ventilation

= b/min X mL/b = 15 X 150= 2250 mL/min

Alveolar ventilation = Total ventilation - Dead space ventilation

b/min X mL/b = b/min X mL/b - b/min X mL/b17 X 350 = 15 X 500 - 15 X 1505250 ml/min = 7500 ml/min - 2250 ml/min

Alveolar ventilation is approximately

equal to pulmonary blood flow (cardiac output).

Source Undetermined

15

Effect of rate & tidal volume on ALVEOLAR VENTILATION

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

VA = R (VT - VD)

4200 mL/min = 12 ( 500 - 150)

Fixed VE & VD

16

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)

17

STEPS IN RESPIRATION

Bulk Flow

Diffusion

Source Undetermined

18

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

760 mm Hg = 160 + 0.2 + 593 + 7

100% = 21% + 0.03% + 78% + 0.9%

PO2 = 21% x 760 = 160 mm Hg

Where does the 760 mmHg come from ??

19

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

20

Standard Conditions for Measuring Gas Volumes

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)

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

n RT

P V =

PV = nRT

constant

21

WATER PARTIAL PRESSURE

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

Pb PH O

2O2

Pb = PH2O + Pdry

PH2O = 47 mmHg

The gas partial pressure of water in equilibrium with liquidwater depends only on the temperature. The higher the temperature the higher partial pressure due to water.

At body temperature (37 C °) the

22

P = inspired oxygenI O

2

PI = % O2 (Pb - PH20)O2

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.

That is why the partial pressure of oxygen

in inspired air is lower than dry room air.

23

PI = % O2 (Pb - PH20) = 0.21 x 713 = 150 mm Hg O

2

P = 104 mm Hg A

O 2

P = % O2 Pb = 160 mm Hg bO

2

Dry air

Humidified air

??Alveolar air partial pressure of oxygen ??

So why alveolar PO2 so low??

D’Alecy

Slide 10: Source Undetermined

Slide 12: D’Alecy




Slide 23: D’Alecy

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11.17.08(a): Alveolar Ventilation