Section 2 Respiratory Gases Exchange

I Physical Principles of Gas Exchange

• Partial pressure– The pressure exerted by each type of gas in a

mixture

• Diffusion of gases through liquids– Concentration of a gas in a liquid is determined

by its partial pressure and its solubility coefficient

Partial Pressures of GasesPartial Pressures of GasesBasic Composition of AirBasic Composition of Air• 79% Nitrogen79% Nitrogen• 21 % Oxygen21 % Oxygen• ~ 0% Carbon Dioxide~ 0% Carbon Dioxide In a mixture of gases, each gas exerts a partial In a mixture of gases, each gas exerts a partial pressure proportional to its mole fractionpressure proportional to its mole fraction. .

Total Pressure = sum of the partial pressures of each gasTotal Pressure = sum of the partial pressures of each gas

PPgasgas = P = Pbb x F x Fgasgas

PPNN = 760 x 0.79 = 600.4 mm Hg = 760 x 0.79 = 600.4 mm HgPP0022 = 760 x 0.21 = 159.6 mm Hg = 760 x 0.21 = 159.6 mm Hg

Total Pressure (at sea level) Total Pressure (at sea level)

PPbarometric barometric = 760 mm Hg= 760 mm Hg

PPbb

760 mm760 mmHg Hg

PPbb

Consider a container of fluid in a vacuumConsider a container of fluid in a vacuum

Partial Pressure of Gases in FluidsPartial Pressure of Gases in Fluids

Each gas has a specific solubilityEach gas has a specific solubilityOO22 Solubility coefficient = 0.003 ml/100 ml Blood Solubility coefficient = 0.003 ml/100 ml BloodC0C02 = 0.06 ml/100 ml Blood (x 20 of 0 = 0.06 ml/100 ml Blood (x 20 of 022))

Gases dissolve in fluids by moving down aGases dissolve in fluids by moving down aPartial Pressure gradient rather than a concentration gradientPartial Pressure gradient rather than a concentration gradient

That is opened to the airThat is opened to the air

Molecules of gas begin to Molecules of gas begin to enter the fluidenter the fluid

Partial Pressure of Gases in FluidsPartial Pressure of Gases in Fluids

After a short time, After a short time, the number of molecules the number of moleculesthe number of molecules the number of molecules

ENTERING ENTERING = = LEAVINGLEAVING

At equilibrium, if the gas phase has a PAt equilibrium, if the gas phase has a POO22 = 100 mm Hg, = 100 mm Hg, the liquid phase also has a Pthe liquid phase also has a POO22 = 100 mm Hg = 100 mm Hg

An easy way to talk An easy way to talk about gases in fluidsabout gases in fluids. .

• Transport of gases between the alveoli and (pulmonary) capillaries and eventually from the capillaries to the tissues

• diffusion dependent on perfusion and the partial pressure (pp) exerted by each gas

• gases diffuse from area of conc. (pp) to conc. (pp)

concentration pp of gas diffusion

CCO2 more soluble than O2, therefore it diffuses faster

Diffusion

Diffusion: Blood Transit time in the AlveolusDiffusion: Blood Transit time in the Alveolus

AlveolusAlveolus

Blood capillaryBlood capillary

Time for exchangeTime for exchangePO2PO2

Time0 0.75 sec

40

100

Saturated very quickly

Reserve diffusive Capacity of the lung

45

mm Hg

PCO2PCO2

II Gas exchange in the lung and in the tissue

Oxygen and Carbon Dioxide Diffusion Gradients

• Oxygen– Moves from alveoli into

blood. Blood is almost completely saturated with oxygen when it leaves the capillary

– P02 in blood decreases because of mixing with deoxygenated blood

– Oxygen moves from tissue capillaries into the tissues

• Carbon dioxide– Moves from tissues into

tissue capillaries

– Moves from pulmonary capillaries into the alveoli

Diffusion Gradients of Respiratory Gases at Sea Level

Total 100.00 760.0 760 760 0

H2O 0.00 0.0 47 47 0

O2 20.93 159.1 105 40 65

CO2 0.03 0.2 40 46 6

N2 79.04 600.7 569 573 0

Partial pressure (mmHg)

% in Dry Alveolar Venous DiffusionGas dry air air air blood gradient

NB. CO2 is ~20x more soluble than O2 in blood => large amounts move into & out of the blood down a relatively small diffusion gradient.

PO2 and PCO2 in Blood

III. A-a gradient, the efficiency of the gas exchange in alveoli

Oxygen Content in Oxygen Content in Alveolus GasAlveolus Gas

(measured during exhalation)(measured during exhalation)

Oxygen Oxygen Content in Content in

arterial bloodarterial blood

(equivalent to (equivalent to that leaving that leaving

lungs)lungs)

What is an A - a gradient ?What is an A - a gradient ?

The The DIFFERENCEDIFFERENCE between between::

In a healthy person, what would you expect the A - a to be?In a healthy person, what would you expect the A - a to be?No difference, greater than 0, or less than 0No difference, greater than 0, or less than 0

Normal: A – a, up to ~ 10 mm Hg, varies with ageNormal: A – a, up to ~ 10 mm Hg, varies with age

Factors contributing to A - a GradientFactors contributing to A - a GradientFactors contributing to A - a GradientFactors contributing to A - a Gradient

1.1. Blood ShuntsBlood Shunts

2.2. Blood MixingBlood Mixing

3.3. MatchingMatching

1.1. Blood ShuntsBlood Shunts

2.2. Blood MixingBlood Mixing

3.3. MatchingMatching

Alveolar Alveolar SPACESPACE

arterial vesselarterial vessel

SIMPLE CONCEPT OF A SHUNTSIMPLE CONCEPT OF A SHUNT

BLOOD FLOWBLOOD FLOW

COCO22 OO22

No Gas Exchange = No Gas Exchange = SHUNTSHUNT

AIR FLOWAIR FLOW

BloodBlood

MixingMixingLowered OLowered O22/l00 ml/l00 ml

Total Perfusion, QTotal Perfusion, Q

Total VentilationTotal Ventilation

NEXT NEW NEXT NEW CONCEPT CONCEPT

Matching What?Matching What? BloodBlood to to Air FlowAir Flow

ExchangeExchangeOxygenOxygen

If the volumes used for exchange are aligned If the volumes used for exchange are aligned

– – We might consider the system to beWe might consider the system to be “ideally matched” “ideally matched”

Arterial Perfusion (QArterial Perfusion (Qcc))

Slide or Misalign the distribution volumesSlide or Misalign the distribution volumes

Alveolar Ventilation (VAlveolar Ventilation (VAA))

ExchangeExchangeOxygenOxygen

Dead Air Space (Airways)Dead Air Space (Airways)

Shunt (QShunt (Qss))

(Bronchial (Bronchial Artery)Artery)

Some Volumes are wasted, Some Volumes are wasted, Matching Ratio = VMatching Ratio = VAA/Q/Qcc = 0.8 = 0.8

Normal Case; Small Shunt, low volume Dead SpaceNormal Case; Small Shunt, low volume Dead Space

Matching ventilation & perfusionVentilation and perfusion (blood flow) are both better at the bottom (base) of the lung than that at the top (apex). But the change in blood flow is more steep than in ventilation. Therefore the ventilation/perfusion ratio rises sharply from the base to the apex.Result:Result: V/Q is greater or less V/Q is greater or less than 0.8 in different than 0.8 in different regionsregions

If V/Q <0.8 = shunt like, If V/Q > 0.8 little benefit, Increases A - a gradient

Alveolar VentilationAlveolar VentilationVVAA

Arterial Perfusion QArterial Perfusion Q

ExchangeExchangeOxygenOxygen

Dead Air Space Dead Air Space

ShuntShunt

= Lung Disease with a Large A – a gradient= Lung Disease with a Large A – a gradient

IV Factors Affecting the Gas Diffusion in the Lung

1. The Properties of the Gas1) Molecular weight. Diffusion rate is inversely proportional

to the square root of the molecular weight2) Temperature3) Solubility in waterEach gas has a specific solubilityEach gas has a specific solubility

OO22 Solubility coefficient = 0.003 ml 0 Solubility coefficient = 0.003 ml 022/100 ml Blood/100 ml Blood

C0C02 = 0.06 ml/100 ml Blood (x 20 of 0 = 0.06 ml/100 ml Blood (x 20 of 02))

PO2PO2

Time0 0.75 sec

40

100

Saturated very quickly

Reserve diffusive Capacity of the lung

45

mm HG

PCO2PCO2

2. Partial Pressure of the Gases

1) Alveoli ventilation

2) Blood perfusion in the lung capillary

3) Speed of the chemical reaction

The slow speed of the chemical reaction HCO3- + H+

----- H2CO3 ---H2O + CO2 reduces the CO2 exchange in the lung. So, during the gas exchange in the external respiration, the exchange of CO2 is a little lower than that of O2.

3. Properties of the Lung

1) Area of the respiratory membrane

2) Distance of the diffusion

3) Viscosity of the medium during diffusion

V Pulmonary Diffusion CapacityThe ability of the respiratory membrane to exchange a gas between the alveoli and the pulmonary blood can be expressed in quantitative terms by its diffusing capacity, which is defined as the volume of a gas that diffuses through the membrane each minute for a pressure of 1 mmHg.

DL = V/(PA – PC)

Where V is a gas that diffuses through the membrane each minute, PA is the average partial pressure of a gas in the air of alveoli, PC is the average partial pressure of a gas in the blood of pulmonary capillary.

Factors Affecting the DL

1. Body posture

2. Body height and weight

3. Exercise

4. Pulmonary diseases

Internal Respiration

VI Internal Respiration

• All cells require oxygen for metabolism

• All cells require means to remove carbon dioxide

• Gas exchange at cellular level

Concept: Gas exchange between the capillary and the tissues throughout the body

Process:

Factors affecting the internal respiration:

1. Distance between the cells and the capillary

2. Rate of metabolic rate

3. Speed of the blood flow in capillary

EXTERNAL AND INTERNAL RESPIRATION

HEART

TISSUECELL

O2 + FOOD

CO2 + H2O+ ATP

LUNGS

ATMOSPHERE

PULMONARYCIRULATION

SYSTEMICCIRCULATION