DR NILESH KATE

MBBS,MD ASSOCIATE PROF

DEPT. OF PHYSIOLOGY

PULMONARY DIFFUSION

OBJECTIVES Physics of gas diffusion & gas partial

pressure. Alveolar ventilation. Alveolar ventilation : perfusion ratio Alveolar air. Diffusion of gases through respiratory

membrane.

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INTRODUCTION. External Respiration done by

Pulmonary ventilation Pulmonary diffusion Transport of gases.

Main content Physics of gas diffusion & gas partial pressure Alveolar ventilation Ventilation-Perfusion ratio Diffusion of gases through respiratory membrane.

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PHYSICS OF GAS DIFFUSION & GAS PARTIAL PRESSURE.

Gas Pressure – depend upon following factors Concentration of molecules.

P α n Volume P α 1/v By Boyle’s

law of gases. Absolute Temperature

By Charle’s law at constant pressure

V α T

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PHYSICS OF GAS DIFFUSION & GAS PARTIAL PRESSURE.

Partial PressureAs per Dalton’s Law – the total pressure exerted by mixture

of gases is equal to the sum of partial pressure of all gases. Partial pressure of gases in water and tissue

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PHYSICS OF GAS DIFFUSION & GAS PARTIAL PRESSURE.

As per Henry’s Law –when temperature is constant content of gases dissolved in any solution is directly proportional to partial pressure of gas.

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PHYSICS OF GAS DIFFUSION & GAS PARTIAL PRESSURE.

Partial Pressure – At equilibrium partial pressure in liquid

phase is equal to partial pressure in gas

In absence of equilibrium partial pressure of gas in liquid

phase is less than in gas phase.

Solubility Coefficient – solubility coefficient of CO2 is 24

times more & that of N2 is half of that of oxygen.

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WATER VAPOUR PRESSURE Vapour pressure of

water depend on its temperature.

At body temp vapour pressure of water in alveolar air is 47 mm Hg.

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ALVEOLAR VENTILATION. Volume of fresh air which

reaches the gas exchange area of the lung each min.

Alveolar Ventilation = respiratory rate × (tidal volume – dead space volume)

= 12 × (500-150) 4200 ml/min

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PHYSIOLOGICAL SIGNIFICANCE OF ALVEOLAR VENTILATION

Subject 1 – TV = 500 ml, RR = 12/min Pulmonary ventilation = 12 × 500 = 6L/min Alveolar ventilation = 12 × 500-150 = 4.2 L/min

Subject 2 – TV = 200 ml, RR = 30/min Pulmonary ventilation = 30 × 200 = 6L/min Alveolar ventilation = 30 × 200-150 = 1.5 L/min

Though in both pulmonary ventilation is same other subject alveolar ventilation is much less & will suffer from hypoxia & Hypercapnia.

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DEAD SPACE

DEAD SPACE AIR Anatomical dead space air. Alveolar dead space air. Physiological dead space. Measurement of anatomical dead space. Measurement of physiological dead space.

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Introduction Total 23 generations of

airways b/w trachea & alveolar sac.

First 16 generations: Conducting zone No gaseous exchange Up to terminal bronchiole

Last 7 generations Transitional & respiratory zone Gaseous exchange Include respiratory bronchiole,

alveolar ducts & alveoli

DEAD SPACE Part of the tidal volume that does not take

part in gaseous exchange with pulmonary capillary blood.

This can be: Anatomical dead space Alveolar dead space Total (Physiological) dead space

ANATOMICAL DEAD SPACE Gas in the conducting areas of the

respiratory passage, where no gaseous exchange occurs.

Volume of air from nose to terminal bronchiole.

Approximately equal to the body weight in pounds.

So, in a 68 kg (150 lb) manSo, in a 68 kg (150 lb) manAnatomical dead space = 150 mlAnatomical dead space = 150 ml

i.e. out of 500 ml inspired air, only 350 ml i.e. out of 500 ml inspired air, only 350 ml reaches the alveoli for gaseous exchange.reaches the alveoli for gaseous exchange.

rest 150 ml just fills the anatomical dead spacerest 150 ml just fills the anatomical dead space

During expiration,During expiration,First 150 ml – dead space airFirst 150 ml – dead space airLast 350 ml – alveolar air Last 350 ml – alveolar air

ANATOMICAL DEAD SPACE

Alveolar ventilation (amount of air reaching the alveoli per min) is less than the respiratory minute volume.

If, tidal volume = 500 ml & RR = 12/minIf, tidal volume = 500 ml & RR = 12/minDead space volume = 150 mlDead space volume = 150 mlThen, air reaching the alveoli = 500-150 mlThen, air reaching the alveoli = 500-150 ml = 350 ml= 350 ml

Minute volume = 500 x 12 = 6 l/minMinute volume = 500 x 12 = 6 l/min

Alveolar ventilation = (500-150) x 12Alveolar ventilation = (500-150) x 12 = 350 x 12= 350 x 12 = 4200 ml= 4200 ml

ANATOMICAL DEAD SPACE

Rapid shallow breathing produces much less alveolar ventilation than slow deep breathing at the same respiratory minute volume.

Respiratory rate Respiratory rate 30/min 30/min 10/min 10/minTidal volume Tidal volume 200 mL 200 mL 600 mL 600 mLMinute volume Minute volume 6 L 6 L 6 L 6 LAlveolar ventilation (200 – 150) x 30 (600 – 150) x 10Alveolar ventilation (200 – 150) x 30 (600 – 150) x 10

= 1500 mL = 4500 mL= 1500 mL = 4500 mL

ALVEOLAR DEAD SPACE

Gas present in under-perfused or non-perfused alveoli and excess gas present in over-ventilated alveoli.

Alveolar air that is not equilibrating with the pulmonar capillary blood.

If, Tidal volume = 500 mlIf, Tidal volume = 500 mlAnatomical dead space = 150 mlAnatomical dead space = 150 mlAlveolar dead space = 100 mlAlveolar dead space = 100 ml

Effective alveolar ventilation = 500 – 150 – 100Effective alveolar ventilation = 500 – 150 – 100 = 250 ml= 250 ml

TOTAL (PHYSIOLOGICAL) DEAD SPACE

Total volume of inspired air that does not equilibrate with the pulmonary capillary blood.

Total DS = Anatomical DS + Alveolar DS

In a healthy individual, Total DS and Anatomical DS are equal.

MEASUREMENT OF DEAD SPACE Anatomic dead space – Single breath N2 curve

Total dead space – Bohr’s equation PECO2 x VT = PaCO2 x (VT – VD) + PICO2 x VD

PCO 2 of the expired gas (PECO 2)Arterial PCO 2 (PaCO 2)PCO 2 of inspired air (PICO 2)Tidal volume (VT)Dead space volume (VD)

SINGLE BREATH N2 CURVE Subject is asked to take a deep

breath of Oxygen. This fills the entire dead space

with pure Oxygen. Some Oxygen also mixes with

the alveolar air but does not completely replace their air.

Then the person expires through a rapidly recording Nitrogen meter end exp

VT

VD

VA

RESULTS OBTAINED First portion- from the dead

space regions-Nitrogen concentration is zero.

After some time- Nitrogen concentration rises rapidly because alveolar air containing Nitrogen + dead space air.

At end- only air from alveoli- high steady concentration of nitrogen.

CALCULATION :

VE = total volume of expired air.

VD = dead space air

Suppose gray area = 30 cm ² Suppose gray area = 30 cm ² Pink area Pink area = 70 cm ² = 70 cm ² Total volume expired is 500 mlTotal volume expired is 500 ml

Then dead space would be : Then dead space would be : 30 x 50030 x 500 30+7030+70 = 150 ml= 150 ml

EFFECT OF GRAVITY ON ALVEOLAR VENTILATION

In Supine Position – alveolar ventilation evenly distributed

In Upright Position – Alveolar pressure is zero throughout lung Intrapleural pressure – at apex -10 mmHg & at base -2 mm Hg. So transpulmonary pressure -10 & -2 at apex & base

respectively. So linear reduction in regional alveolar ventilation from base

to apex.

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CLINICAL SIGNIFICANCE So arterial

oxygenation in unilateral lung diseases is improved by keeping good lung in Dependent Position.

Opposite is done in INFANT.

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ALVEOLAR VENTILATION : PERFUSION RATIO

Ratio of alveolar ventilation per minute to quantity of blood flow to alveoli per min.

VA/Q = 4.2/5 = 0.84-0.9

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EFFECT OF GRAVITY Linear Reduction of blood flow and

alveolar ventilation from base to apex. But gravity affects perfusion more

than ventilation. So as we go up from middle VA/Q goes

on increasing , about 3 at apex. At the base it is over perfused than

over ventilated so at the base is 0.6

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CAUSES OF ALTERATION. Causes of altered

alveolar ventilation Bronchial asthma Emphysema Pulmonary fibrosis Pneumothorax Congestive heart failure

Causes of altered pulmonary perfusion. Anatomical shunts Pulmonary embolism Decrease in pulmonary

vascular bed in emphysema

Increase pulmonary resistance in pulmonary fibrosis, Pneumothorax, CHF

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EFFECTS OF ALTERATION IN VA/Q RATIO.

Normal VA/Q ratio –both normal alveolar pO2 = 104 mmHg, pCO2 =40 mmHg.

Increased VA/Q ratio. – alveolar dead space air, VA/Q = infinity, pO2 = 149 mmHg, pCO2 = 0 mmHg.

Decreased VA/Q ratio, pO2 = 40 mmHg, pCO2 = 45 mmHg.

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ALVEOLAR AIR. Volume of air available for exchange of gases

in alveoli per breath Composition of alveolar air.

Water vapors dilute the other gases in the inspired air. Alveolar air is renewed very slowly by atmospheric

air. Oxygen is constantly being absorbed from the alveolar

air. Carbon dioxide is constantly diffusing from the

pulmonary blood to alveoli.

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COMPOSITION OF EXPIRED AIR

First Portion – Dead space air , composition is similar to typical humidified air.

Middle Portion – mixture of dead space air & alveolar air.

Last Portion – alveolar air.

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ALVEOLAR GAS EQUATION Relationship between alveolar pO2 & pCO2

Pao2 = pIO2 – pACO2 × {FIO2 + 1-FIO2/RQ} pAO2 = alveolar air PO2 pIO2 = Inspired air. pACO2 = alveolar air pCO2 FIO2= fraction of O2 in dry air. RQ = Respiratory quotient (0.8)

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DIFFUSION OF GASES THROUGH RESPIRATORY MEMBRANE.

Respiratory unit & respiratory membrane. Factors affecting diffusion across respiratory

membrane. Diffusion & equilibrium of gases through respiratory

membrane. Perfusion limited versus diffusion limited gas

exchange. Effect of VA/Q ratio on pulmonary gas exchange. Diffusion capacity of lungs.

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RESPIRATORY UNIT & RESPIRATORY MEMBRANE.

Respiratory Unit – composed of respiratory bronchiole, alveolar ducts, atria & alveoli.

Respiratory Membrane – separate capillary blood from alveolar air.

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STRUCTURE OF RESPIRATORY MEMBRANE

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FACTORS AFFECTING DIFFUSION ACROSS RESPIRATORY MEMBRANE.

Thickness of respiratory membrane – rate of diffusion inversely proportional to thickness. Thickness increases in pulmonary oedema & fibrosis.

Surface area of respiratory membrane – R@A Diffusion coefficient V@ D

DC of CO2 20 times that of O2 Pressure gradient across respiratory

membrane – V@(Pc-PA)

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DIFFUSION & EQUILIBRIUM OF GASES THROUGH RESPIRATORY

MEMBRANE. Diffusion of O2 Alveolar PO2 = 104 mmHg,

pulmonary capillary PO2 – 40 mm Hg.

Pressure Gradient = 64 mmhg.

By the time blood passes 1/3rd of distance in capillary the PO2 of blood equals that of alveoli.

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EQUILIBRATION TIME. Blood remains in capillary

for about 0.75 sec – Transit time

Blood PO2 & alveolar PO2 equalize in 0.25 sec

Provide safety margin to ensure O2 uptake during stress.(exercise, high altitude)

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DIFFUSION OF CO2 PCO2 in capillary

blood – 46 mmHg, in alveoli – 40 mmHg.

Pressure gradient – 6 mmHg.

EQUILIBRATION TIME – for PCO2 is also 0.25 sec.

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EFFECT OF VA/Q RATIO ON PULMONARY GAS EXCHANGE.

Optimum gas exchange across respiratory membrane occurs when VA/Q ratio is normal – 0.8-1

Decrease as well as increase in VA/Q ratio reduces gas exchange.

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DIFFUSION CAPACITY OF LUNGS.

Quantitative expression of the ability of the respiratory membrane to exchange a gas between alveoli & blood.

Def – Volume of gas that diffuses through respiratory membrane of lung each min for a pressure gradient of 1 mmHg.

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FACTORS AFFECTING DIFFUSION CAPACITY

Diffusion Distance – Inversely proportional to thickness of membrane.

Surface Area – Directly Proportional Diffusion Coefficient - Directly Proportional Pressure Gradient - Directly Proportional

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DIFFUSION CAPACITY OF LUNGS FOR O2

O2 Pressure Gradient = 11 mmhg,

So DLCO - At Rest – 20-25 ml/min/mm Hg.

During Exercise – 65 ml/min/mmHg Due to increase in

surface area Increase in VA/Q ratio.

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DIFFUSION CAPACITY OF LUNGS FOR CO2

At Rest – about 20 times that of O2

400-500 ml/min/mmHg

During exercise – 1200-1300 ml/min/mmHg.

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MEASUREMENT OF DIFFUSION CAPACITY OF LUNGS

By Fick’s law v DL =-------- (pA-pC)DL – diffusion capacityV = volume of gas uptake in

1 minpA-pC – presure gradient

between alveoli & blood.

So DLO2 = O2 consumption/min

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pAO2-pO2

CO is preferred for measuring DLCO.

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Thank You