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 mmHg • According to Henry's law the partial pr. of a gas in sln. depend

on: 1- concentration 2- solubility coefficient P gas = _concentration of dissolved gas______ solubility coefficient

gas Sol. Co.

O2 0.024

CO2 0.57

CO 0.018

• 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

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

• CO2 20 times as soluble as O2

• Diffusion depends on partial pressure of gas

• Air is humidified yielding a vapor pressure of 47

mmHg.

Determinants of Diffusion

Ficks Law Diffusion = (P1-P2 ) * Area * Solubility

Distance * MW

• Pressure Gradient

• Area

• Distance

• Solubility and MW are fixed

Determinants of Diffusion

Diffusion coefficient proportional to Solubility

Different gases at the same partial pressure Will diffuse proportional to their diffusion coefficient

MW

Composition of Alveolar Air

Pn2 = (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

Diffusion between gas phase & dissolved phase

• Net diffusion is determined by gradient

• Vapor pr. of H2O 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

Temerature PH2O

0 ̊C 5 mmHg

100 ̊C 760 mmHg

VA

VT

F I F E

F A

VD

Expired air has alveolar and dead space air

Po2 IN THE ALVEOLI

PAlvO2= PIO

2 - (PCO

2/R)

PO2 = 149 - (40/0.8) = 99

R is respiratory exchange ratio ~0.8

Remember in a normal person alveolar PO2 = arterial PO2, and

alveolar PCO2 = arterial PCO2 .

Pco2 IN THE ALVEOLI

PCO2 = CO2 production * K

Alveolar Ventilation

K is constant

If ventilation is doubled then Pco2 is ½

If ventilation is halved then Pco2 is doubled

Question

A person is breathing from a gas tank containing 45% oxygen. What is the alveolar PO2?

A. 149 mmHg

B. 250 mmHg

C. 270 mmHg

D. 320 mmHg

E. 340 mmHg

Answer

760 – 47 = 713

713 * 0.45 = 321 mmHg = inspired PO2

Alveolar PO2 = 321 - (40/0.8) = 321 - 50 =

271 mmHg

Question

An alveoli that has normal ventilation and no

blood flow (V/Q=0) has an alveolar PO2 of

A. 40 mmHg

B. 100 mmHg

C. 149 mmHg

D. 159 mmHg O2=?

O2 = 40

CO2 = 45

O2 = 40

CO2 = 45

O2 = 100

CO2 = 40

O2 = 40

CO2 = 45 O2 = 100

CO2 = 40

O2 = 150

CO2 = 0

O2 = 40

CO2 = 45

O2 = 150

CO2 = 0 O2 = 150

CO2 = 0

V/Q = 0 V/Q = normal V/Q =

Ventilation/perfusion

• Physiologic shunt

– Va/Q < normal

– low ventilation

• Physiologic dead space

– Va/Q > normal

– wasted ventilation

• Abnormalities

– Upper lung Va/Q 3 x normal

– Lower lung Va/Q .5 x normal

Diffusion rate (D) proportional to P x AxS d x MW • S : diffusion coefficient of gas. • √MW

A-cross- sectional area S-solubility of the gas

d-distance P- pressure gradient

Gas Diffusion co.

O2 1

CO2 20.3

CO 0.81

N 0.53

• 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.

Concentration of gasses in alveoli

• O2 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

Concentration of gasses in alveoli

• CO2 in alveoli depends on:

a- rate of CO2 excretion

b- ventilation rate

If vent. Rate= 4.2 l/min, and rate of excretion = 200ml/min PCO2 40 mmHg

• Expired air= alveolar air+ air in dead space

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

Diffusion through respiratory membrane

• Respiratory membrane specifications:

1- 0.2 – 0.6 μm

2- 70m2 surface area

3- total volume of blood 60-140ml

4- capillary diameter is 5 μm so RBCs

squeeze through

Diffusion through respiratory membrane

• Diffusion rate depends on:

1- thickness

2- surface area

3- Pgas gradient

4-Diffusion coefficient

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

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