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Part 3

Respiratory Gases Exchange

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I Physical Principles of Gas Exchange

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

in a mixture

• Concentration of a gas in a liquid– determined by its partial pressure and its

solubility coefficient

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

Total Pressure = sum of the partial pressures of each gas

Pgas = Pb x Fgas

PN2 = 760 x 0.79 = 600.4 mm HgP02 = 760 x 0.21 = 159.6 mm Hg

Total Pressure (at sea level) Pbarometric = 760 mm Hg

PPbb

760 mm760 mmHg Hg

PPbb

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Consider a container of fluid in a vacuum

Partial Pressure of Gases in Fluids

Each gas has a specific solubilityO2 Solubility coefficient = 0.003 ml/100 ml BloodC02 = 0.06 ml/100 ml Blood (x 20 of 02)

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

That is opened to the air

Molecules of gas begin to enter the fluid

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Partial Pressure of Gases in Fluids

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

ENTERING = LEAVING

At equilibrium, if the gas phase has a PO2 = 100 mm Hg, the liquid phase also has a PO2 = 100 mm Hg

An easy way to talk about gases in fluids.

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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

Diffusion: Blood Transit Time in the Alveolus

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II Gas exchange in the lung and in the tissue

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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.

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PO2 and PCO2 in Blood

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

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Oxygen Content in Alveolus Gas

(measured during exhalation))

Oxygen Content in

arterial blood

(equivalent to that leaving

lungs)

What is an A - a gradient ?

The DIFFERENCE between::

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

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

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

1. Blood Shunts

2. Blood Mixing

3. Matching

1. Blood Shunts

2. Blood Mixing

3. Matching

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Alveolar SPACE

arterial vessel

SIMPLE CONCEPT OF A SHUNT

BLOOD FLOWBLOOD FLOW

COCO22 OO22

No Gas Exchange = SHUNT

AIR FLOW

Blood

MixingLowered O2/l00 ml

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Total Perfusion, Q

Total VentilationTotal Ventilation

NEXT NEW CONCEPT

Matching What? BloodBlood to to Air FlowAir Flow

ExchangeOxygen

If the volumes used for exchange are aligned – We might consider the system to be

“ideally matched”

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Arterial Perfusion (Qc)

Slide or Misalign the distribution volumes

Alveolar Ventilation (VAlveolar Ventilation (VAA))

ExchangeOxygenOxygen

Dead Air Space (Airways)

Shunt (Qs)(Bronchial Artery)

Some Volumes are wasted, Matching Ratio = VA/Qc = 0.8

Normal Case; Small Shunt, low volume Dead Space

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Matching ventilation & perfusion

Ventilation and perfusion (blood flow) are both better at the bottom (base) of the lung than that at the top (apex).

the change in blood flow is more steep than in ventilation.

the ventilation/perfusion ratio rises sharply from the base to the apex.

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Matching ventilation & perfusion (cont)

Result: V/Q is greater or less than 0.8 in different regions

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

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Alveolar VentilationVA

Arterial Perfusion Q

ExchangeOxygen

Dead Air Space

Shunt

= Lung Disease with a Large A – a gradient

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IV Factors Affecting the Gas Diffusion in the Lung

1) Area of the respiratory membrane

2) Distance of the diffusion

3) VA/Q

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V Pulmonary Diffusion CapacityConcept:

The ability of the respiratory membrane to exchange a gas between the alveoli and the pulmonary blood

defined as the volume of a gas that diffuses through the membrane each minute for a pressure of 1 mmHg.

DL = V/(PA – PC)

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.

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Factors Affecting the DL

1. Body posture

2. Body height and weight

3. Exercise

4. Pulmonary diseases

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VI Internal Respiration

• All cells require oxygen for metabolism

• All cells require means to remove carbon

dioxide

• Gas exchange at cellular level

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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

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EXTERNAL AND INTERNAL RESPIRATION

HEART

TISSUECELL

O2 + FOOD

CO2 + H2O+ ATP

LUNGS

ATMOSPHERE

PULMONARYCIRULATION

SYSTEMICCIRCULATION