7/31/2019 diffusin 1

1/38

7/31/2019 diffusin 1

2/38

Regional Difference inVentilation

7/31/2019 diffusin 1

3/38

Byprof/Hala Salah

7/31/2019 diffusin 1

4/38

Regional difference in

ventilationVentilation is the amount of

gas moved in and out of aregion irrespective to theinitial volume of that region.

7/31/2019 diffusin 1

5/38

Regional difference in ventilation

Slinky springIn the uprightposition, the lungs

under the effect ofgravity are suspendedin the thoracic cavitylike a suspendedslinky spring.

7/31/2019 diffusin 1

6/38

Regional difference in ventilation

When thisspring is freelysuspended, it willbe stretchedmainly at theupper part .

7/31/2019 diffusin 1

7/38

Regional difference in ventilationWhen it is pulled

down, its lengthincreases mainlyin the lower part.

7/31/2019 diffusin 1

8/38

Regional difference in ventilation

In the standing position, the gravity pullsthe lung down. This makes the IPP at theapex of the lung more negative than at the

base.IPP at the apex -10cmH2O.

At the base of the lung, IPP about -2.5

cmH2O .At the mid thoracic position.

7/31/2019 diffusin 1

9/38

Regional difference in ventilation

Apex of the lung Base of the lung

IPP more negative -10 cmH2O IPP less negative -2.5 cm H2Olarge expanding pressure Small expanding pressureBig resting volume and apical

alveoli are more expandedSmall resting volume and

alveoli are less expanded

Small change in volume duringinspiration Large change in volume duringinspiration

less ventilated under resting

conditionmore ventilation under resting

condition

7/31/2019 diffusin 1

10/38

7/31/2019 diffusin 1

11/38

Regional difference in ventilation

In recumbent position ventilation oflower most part (posterior) exceeds that

of the upper most part (anterior of thelung).

In the lateral position (subject on hisside) the dependent lung is bestventilated.

7/31/2019 diffusin 1

12/38

Regional difference in ventilationDuring exercise ventilation

increases at the apex of the lungs and

the regional difference between apexand base decreases.

7/31/2019 diffusin 1

13/38

7/31/2019 diffusin 1

14/38

100150

47mmHg

28mmHg

120mmHg

565mmHg

760mmHg

Expired air

47mmHg

7/31/2019 diffusin 1

15/38

Causes of differences between atmosphericair and alveolar air

Alveolar air is only partially replaced by

atmospheric air with each breath.O2 continuously diffuses from thealveolar air into the pulmonary blood.

7/31/2019 diffusin 1

16/38

Causes of differences betweenatmospheric air and alveolar air

CO2 continuously diffuses from the

pulmonary blood into the alveoli.

Dry atmospheric air that enters the

respiratory passage is humidified evenbefore it reaches the alveoli.

7/31/2019 diffusin 1

17/38

Diffusion

7/31/2019 diffusin 1

18/38

DiffusionIs the exchange betweenalveolar and blood gases

(O2 and CO2) through

the alveolo-capillarymembrane.

7/31/2019 diffusin 1

19/38

Respiratory Membrane

7/31/2019 diffusin 1

20/38

DiffusionDiffusion through tissues is described byFick's law which states that:

Surface area X solubility X Pressure gradient

D Root M.wtThickness x Sq

7/31/2019 diffusin 1

21/38

Factors affecting the rate of diffusionthrough respiratory membrane

7/31/2019 diffusin 1

22/38

1. Thickness of membrane:

The overall thickness=0.5 micron.

Thickness increases in:

1. Pulmonary fibrosis

2. Pulmonary oedema

7/31/2019 diffusin 1

23/38

2. Surface area of the membrane

The surface area of the respiratorymembrane is very large about 50-100 m2 inthe normal adult.

The surface area is decreased in:

1. Removal of one lung.

2. Emphysema .

7/31/2019 diffusin 1

24/38

7/31/2019 diffusin 1

25/38

3. Pressure gradient

100 mmHg

100mmHg

46mmHg

7/31/2019 diffusin 1

26/38

4. The diffusion coefficient

Depends on :

Solubility of gas in the membrane .

Square root of its molecular weight.

7/31/2019 diffusin 1

27/38

A. The Solubility of the gas

It is calculated as the volume of the gas thatdissolves in 1ml of the liquid.

The greater the solubility of the gas in thefluid, the more the amount that is dissolved.

CO2 Solubility is very high than for O2 i.e. the

solubility of CO2 is 24 times greater than O2.

7/31/2019 diffusin 1

28/38

B.Square root of the molecular weight

Molecular weight of O2 32.

Molecular weight of CO2 44.

7/31/2019 diffusin 1

29/38

The net result is that:

Diffusion coefficient for CO2through the tissue sheet is about

20 times faster than O2.

7/31/2019 diffusin 1

30/38

Diffusion capacity of theRespiratory membrane

It is defined as:

The volume of a gas that

diffuses through themembrane each minute with

a pressure difference of 1mmHg.

7/31/2019 diffusin 1

31/38

Diffusion capacity

Diffusion capacity for O2

At rest : 25ml/min/mmHg.

During exercise :65ml/min/mmHg

It decreases in lungdiseases as in fibrosis.

Diffusion capacity for CO2

At rest: 400ml/min/mmHg.

During exercise: 1200ml/min/mmHg

7/31/2019 diffusin 1

32/38

Equilibration

7/31/2019 diffusin 1

33/38

Equilibration

It is the equalization of pressures ofgases across respiratory membrane.

Blood O2

tension and the alveolar O2

tension equalize in about 0.25 second

(Equilibration time).

In normal lungs blood travels alongpulmonary capillaries in about 0.75second (Transit time).

7/31/2019 diffusin 1

34/38

Po2=100mmHgp

co2=40mmHg

0.25

40

46

0.75

capillary

alveolus

100

40

Po2=40 mmHg

Pco2=46 mmHg

Po2=100 mmHg

Pco2=40mmHg

7/31/2019 diffusin 1

35/38

EquilibrationThere is about 0.5 sec with noincrease in O2 content, this time

provides safety margin .It ensures an adequate O2 uptakeduring periods of stress e.g.

during exercise as the circulation isfast .

7/31/2019 diffusin 1

36/38

Transit time during exercise

7/31/2019 diffusin 1

37/38

For CO2

Diffusion of CO2 is more rapidly than O2but the whole equilibration for CO2 is thesame as that for O2 (0.25 sec) because:-

1. The reactions releasing CO2 from blood isrelatively slow.

2. The pressure gradient driving CO2 from

blood to alveolus is only 6 mm Hg ,whilethat driving O2 in the opposite directionis 60 mm Hg.

7/31/2019 diffusin 1

38/38