Physical principles of gas exchange. O2 and CO2 - PowerPoint Presentation

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Physical principles of gas exchange. O2 and CO2

Molecules move randomly & rapidly in relation to each other

Net diffusion is from [high] to [low]

Partial pr. of the gas is proportional to [gas]

 nitrogen  79% 600 mmHg  Oxygen  21% 160 mmHgAccording to Henry's law the partial pr. of a gas in sln. depend on: 1- concentration 2- solubility coefficientP gas = _concentration of dissolved gas______ solubility coefficient

gas

Sol.

Co.

O

2

0.024

CO

2

0.57

CO

0.018

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Solubility coefficient :

Molecules dissolved in water if they are attracted to water more can dissolved without build up excess partial pressure within the solution as CO2

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Physical Principles of Gas Exchange

Diffusion in response to concentration gradient

Pressure proportional to concentration

Gas contributes to total pressure in direct proportion to concentration

CO

2 20 times as soluble as O2Diffusion depends on partial pressure of gas

Air is humidified yielding a vapor pressure of 47 mmHg.

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Determinants of Diffusion

Ficks

Law

Diffusion = (P

1

-P2 ) * Area * Solubility

Distance

* MW

Pressure Gradient

Area

Distance

Solubility and MW are fixed

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Determinants of Diffusion

Diffusion coefficient proportional to

Solubility

Different gases at the same partial pressure

Will diffuse proportional to their diffusion coefficient

MW

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Composition of Alveolar Air

Pn

2

= (760 - 47) * 0.79 = 713 * 0.79 = 563

Questions:

What is the effect of humidification on the partial pressures? Explain the expired air partial pressures? Calculate Po2 in alveoli

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Diffusion between gas phase & dissolved phase

Net diffusion is determined by gradientVapor pr. of H

2

O is the partial pressure that water excretes to escape through the surface

at normal body temperature 47 mmHg the greater the temperature the greater kinetic activity higher PH2O TemeraturePH2O0 ̊C5 mmHg100 ̊C760 mmHg

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V

A

V

T

F

I

F

E

F

A

V

D

Expired air has alveolar and dead space air

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Po

2

IN THE ALVEOLI

PAlvO

2

= PIO2 - (PCO2/R)PO2 = 149 - (40/0.8) = 99

R is respiratory exchange ratio ~0.8

Remember in a normal person alveolar P

O

2

= arterial P

O

2

, and

alveolar P

CO

2

= arterial P

CO

2

.

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Pco

2

IN THE ALVEOLI

PCO

2

=

CO

2

production

* K

Alveolar Ventilation

K is constant

If ventilation is doubled then Pco

2

is ½

If ventilation is halved then Pco

2 is doubled

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Question

A person is breathing from a gas tank containing 45% oxygen. What is the alveolar PO

2

?

A. 149 mmHg

B. 250 mmHg C. 270 mmHg D. 320 mmHg E. 340 mmHg

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Answer

760 – 47 = 713

713 * 0.45 = 321 mmHg = inspired PO

2

Alveolar PO

2 = 321 - (40/0.8) = 321 - 50 =

271 mmHg

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Question

An alveoli that has normal ventilation and no blood flow (V/Q=0) has an alveolar PO

2

of

A. 40 mmHg

B. 100 mmHgC. 149 mmHgD. 159 mmHg

O

2

=?

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O

2

= 40

CO

2

= 45

O

2

= 40

CO

2

= 45

O

2

= 100

CO

2

= 40

O

2

= 40

CO

2

= 45

O

2

= 100

CO

2

= 40

O

2

= 150

CO

2

= 0

O

2

= 40

CO

2

= 45

O

2

= 150

CO

2

= 0

O

2

= 150

CO

2

= 0

V/Q = 0

V/Q = normal

V/Q =



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Ventilation/perfusion

Physiologic shunt

Va

/Q < normal

low ventilation

Physiologic dead spaceVa/Q > normalwasted ventilation

Abnormalities

Upper lung

Va

/Q 3 x normal

Lower lung

Va

/Q .5 x normal

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Diffusion rate

(D) proportional to

P x

AxS

d x MW S : diffusion coefficient of gas. √MW A-cross- sectional areaS-solubility of the gasd-distance

P- pressure gradient

Gas

Diffusion co.

O

2

1

CO

2

20.3

CO

0.81N

0.53

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Most gasses are lipid soluble so the diffusion in tissue is similar to diff. in water because these gases can pass easily through the cell membrane.

Rate at which alveolar air is renewed by atmospheric air

FRC 2.3 L only 350 ml of new air each breath. One seventh of the total, so many breaths are required to exchange most of the alveolar air.

Half of gas will be removed in 17sec.

Why this graduate clearance

: - to prevent sudden change in [gas] in the blood. - to make respiratory control mechanism much more stable.

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Concentration of gasses in alveoli

O

2

is supplied by inspiration and removed by diffusion

PO2 is controlled by: a- rate of diffusion into blood (250ml/min) b- rate of O2 entry by ventilation normally Po2= 104 mmHg in alveoli  if alv. Ventilation 4.2L/min

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Concentration of gasses in alveoli

CO

2

in alveoli depends on:

a- rate of CO

2 excretion b- ventilation rate If vent. Rate= 4.2 l/min, and rate of excretion = 200ml/min PCO2 40 mmHgExpired air= alveolar air+ air in dead space

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Diffusion through respiratory membrane

300 million alveoli, each alveolus with the diameter of 0.2 mm.Respiratory membrane:

1-fluid layer with surfactant

2-epithelium of alveoli

3-basement membrane of epithelium

4-interstitial space 5-capillary basement membrane 6-endothelial cells of the capillary

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Diffusion through respiratory membrane

Respiratory membrane specifications: 1- 0.2 – 0.6

μ

m

2- 70m2 surface area

3- total volume of blood 60-140ml 4- capillary diameter is 5 μm so RBCs squeeze through

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Diffusion through respiratory membrane

Diffusion rate depends on:

1- thickness

2- surface area

3-

Pgas gradient 4-Diffusion coefficientDiffusion capacity: the volume of a gas that will diffuse through the res. membrane each minute for a partial pressure difference of 1mmHg

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Diffusion through respiratory membrane

O2



21 ml/min/mmHg .

11 mmHg Mean O2 part. pr. In all lungs 230 ml/min “at rest” . 65 ml/min/mmHg “exercise”CO2 400 - 450 ml/min/mmHg “at rest”. 1200-1500 ml/min/mmHg “exercise” average of P CO2 gradient is 1mmHg

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