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Competency

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No. PY6.2 (Respiratory Physiology)

CompetencyDescribe the V/P ratio & diffusion capacity of lungs.

D/L/Core K/KH/Y

AM Written/Viva voce

Integration No

Imp. Concept Clinical significance of V/P

Phy. TriviaCO is diffusion-limited & N2O is perfusionlimited

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OUTLINE

• Physics law

• Respiratory membrane

• Diffusion capacity of lung

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Physics law

• Gas pressure:

P=nRT/V

• Dalton law:

Partial pressure

• Henry’s law:

n=pc

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According to the Henry’s law, whentemperature is kept constant, the contentof gas (n) dissolved in any solution isdirectly proportional to the partialpressure of a gas,

The total pressure exerted by amixture of gases is equal to the sumof the partial pressure of all gasespresent in the mixture

Partial Pressures of Respiratory Gases as They Enter and Leave the Lungs (at Sea Level)

Atmospheric Air* Alveolar Air Expired Air(mm Hg) (mm Hg) (mm Hg)

N2 596

O2 158

CO2 0.3

H2O 5.7

TOTAL 760

On an average cool, clear day.

Partial Pressures of Respiratory Gases as They Enter and Leave the Lungs (at Sea Level)

Atmospheric Air* Alveolar Air (mm Hg) (mm Hg)

N2 596 573

O2 158 100

CO2 0.3 40

H2O 5.7 47

TOTAL 760 760

On an average cool, clear day.

Partial Pressures of Respiratory Gases as They Enter and Leave the Lungs (at Sea Level)

Atmospheric Air* Alveolar Air Expired Air(mm Hg) (mm Hg) (mm Hg)

N2 596 573 565

O2 158 100 116

CO2 0.3 40 32

H2O 5.7 47 47

TOTAL 760 760 760

On an average cool, clear day.

Alveolar air

i) Composition-

ii) Characteristics-

iii) Collection-

Haldane-

Priestly tube

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Diffusion across the R membrane…..

Site

Respiratory zone/unit;

• Respiratory bronchioles,

• Alveolar ducts

• Alveoli

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Diffusion across the R membrane…..

Respiratory membrane

(alveolar-capillary Membrane)

1. Structure-

2. Surface area-

3. Thickness-

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Structure of respiratory mem.

→ Fluid with surfactant

→ Alveolar squamous epithelium

→ Basement membrane

→ Interstitial fluid

→ Capillary basement membrane

→ Capillary endothelium.

Thickness only .5 micron

Surface area 70 m2

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Process-

O2 & CO2 being lipid soluble diffuses across respiratory

membrane according to fick’s law

Factors affecting the diffusion-

1) Inversely proportional to thickness (d) of R. membrane.

2) Directly ∝ to surface area (A) of R. mem.

3) Directly ∝ to partial pressure gradient of gasses

4) Directly ∝ to diffusion coefficient of gasses, which in

turn Is directly ∝ to gas solubility (S) & inversely ∝ to

square root of molecular weight (MW)

Diffusion capacity (DL) of lungs

1) Definition-,

2) Normal Value-

3) Measurement-

Factors affecting (3 Ps)

5) Physical (determinants)-

6) Physiological- (exercise)

7) Pathological (Clinical)-

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N2O is flow-limited

CO is diffusion-limited

https://youtu.be/h-IfQBVcnqM

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Diffusion capacity (DL) of lungs

i) Definition

is the amount of the gas that diffuses thro.

respiratory membrane / minute /unit or mm Hg difference in

partial pressure of the gas on the two sides of the

membrane.

ii) Value

DL of O2 is 20-25 ml/min/mmHg at rest while of CO2 is 400

ml/min/mmHg, (20 times more than O2)

iii) Measurement

indirectly by measuring DL of CO

iv) Physical factors (determinants) affecting DL-

∝ to surface area (A ) of R membrane

Inversely ∝ thickness (d) of R membrane

∝ to gas solubility (S) of the gas

Inversely ∝ to square root molecular weight (MW) of gas

v) Physiological factors affecting DL-

exercise (as new capillaries open & SA can ↑ up to 3 times)

vi) Pathological (clinical) factors ↓ing DL-

in lobectomy & emphysema due to ↓ SA while in

pulmonary edema, pulmonary fibrosis due to ↑ thickness

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Ventilation perfusion ratio- VA/Q ratio

1) Definition-,

2) Normal Value-

Factors affecting

5) Physical (relation with PO2 & PCO2

6) Physiological- (gravity)

7) Pathological (Clinical)-

Ventilation perfusion ratio- VA/Q ratio

i) Definition (VA/Q ratio)-

is the ratio of alveolar ventilation and pulmonary blood

flow

ii) Value-

VA/Q ratio = alveolar ventilation 4200 ml/ pulmonary blood

flow 5000 ml = 0.8 (normal gas exchange)

iii) Physical factors affecting VA/Q ratio-

Relation with PAO2 / PACO2-

↑ value of VA/Q ratio ↑es PAO2 & ↓es PACO2

while ↓ value of VA/Q ratio ↓es PAO2 & ↑es PACO2

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Ventilation perfusion ratio-VA/Q ratio

Ventilation perfusion ratio-VA/Q ratio

At apex-Physiological Dead space, T.B.

At base-Physiological shunt (shunted

blood)

(iv) Physiological factors affecting VA/Q ratio-

Effect of gravity-

at the apex of lungs (wasted ventilation)

- VA/Q ratio ↑es & is 3 x normal (as Q ↓es due to gravity)

- So ↑ PAO2 & ↓ PACO2

- This create physiological dead space at apex and

- make apex vulnerable to T.B. (tuberculosis) due to ↑ PAO2

while at the base of lungs (wasted perfusion)

- VA/Q ratio ↓es & is .6 x normal (as Q ↑ es due to gravity)

- so ↓ PAO2 & ↑ PACO2

- & this create physiological shunt (shunted blood) at base.

in extreme case (no gas exchange)

complete obstruction of

pulmonary artery

complete bronchial obstruction

complete obstruction of

pulmonary artery

i) PO2/PCO2 equals

humidified inspired air

ii) No gaseous exchange

complete bronchial obstruction

i) PO2/PCO2 equals

Venous blood

ii) No gaseous exchange

vi) Pathological (clinical) factors VA/Q ratio-

in bronchial obstruction VA/Q ratio ↓es while

obstruction of pulmonary artery ↑es VA/Q ratio.

in extreme case (no gas exchange)

- ratio is infinite if there is complete obstruction of

pulmonary artery then PAO2 will be 149 & PACO2 will be

0.3 mmHg equal to humidified inspired air.

- ratio is zero if there is complete bronchial obstruction

then PAo2 will be 40 & PAco2 45 mmHg that is equal to

mixed venous blood.

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