Exchange of Gases in the Exchange of Gases in the LungsLungs

Week 3Week 3

Dr. Walid DaoudDr. Walid DaoudA. ProfessorA. Professor

Diffusion of Gases through respiratory Diffusion of Gases through respiratory membranemembrane

Layers of the respiratory membraneLayers of the respiratory membrane::

11 - -Fluid lining alveoli containing surfactantFluid lining alveoli containing surfactant..

22 - -One layer of alveolar epitheliumOne layer of alveolar epithelium..

33 - -Basement membrane of alveolar epitheliumBasement membrane of alveolar epithelium..

44 - -Interstitial spaceInterstitial space..

55 - -Basement membrane of capillaryBasement membrane of capillary..

66 - -Endothelial cells of capillaryEndothelial cells of capillary..

Gas ExchangeGas Exchange

Normal adult lungs contain 300 million alveoliNormal adult lungs contain 300 million alveoli..

Respiratory membraneRespiratory membrane::

- -Thickness 0.2 micronsThickness 0.2 microns..

- -Surface area 100 square meterSurface area 100 square meter..

Blood traverse pulmonary capillary in 0.75 secBlood traverse pulmonary capillary in 0.75 sec..

At rest only 0.25 sec for equilibrium between At rest only 0.25 sec for equilibrium between Alveolar and capillary PO2Alveolar and capillary PO2..

Factors affecting rate of diffusion Factors affecting rate of diffusion through respiratory membranethrough respiratory membrane

Rate of diffusion is directly proportional Rate of diffusion is directly proportional toto::

11 - -Pressure gradient across the membranePressure gradient across the membrane..

22 - -Surface area of respiratory membraneSurface area of respiratory membrane..

33 - -TemperatureTemperature..

44 - -Solubility of the gas in the mediumSolubility of the gas in the medium..

Factors affecting rate of diffusion Factors affecting rate of diffusion through respiratory membranethrough respiratory membrane

Rate of diffusion is inversely proportional Rate of diffusion is inversely proportional toto::

11 - -Square root of molecular weightSquare root of molecular weight..

22 - -Thickness of the membraneThickness of the membrane..

It increases with edema and lung fibrosisIt increases with edema and lung fibrosis..

Oxygen Transport by BloodOxygen Transport by Blood

11 - -O2 in physical solutionO2 in physical solution:: 0.30.3 ml O2 in 100 ml arterial bloodml O2 in 100 ml arterial blood

0.130.13 ml O2 in 100 ml venous bloodml O2 in 100 ml venous blood 0.170.17 ml transported to tissues by 100 mlml transported to tissues by 100 ml

plasmaplasma O2 in physical solution determines theO2 in physical solution determines the

PO2 in blood and the direction of diffusion PO2 in blood and the direction of diffusion of O2 of O2..

Oxygen Transport by BloodOxygen Transport by Blood

22 - -O2 carried by HbO2 carried by Hb::

19.519.5 ml O2 in 100 ml arterial bloodml O2 in 100 ml arterial blood

98%98% of O2 is transported in loose chemicalof O2 is transported in loose chemical

combination with Hbcombination with Hb

Hemoglobin moleculeHemoglobin molecule contains 4 ferrouscontains 4 ferrous

atoms, each reversibly combines with oneatoms, each reversibly combines with one

O2 moleculeO2 molecule . .

O2 ContentsO2 Contents

It is the volume of O2 carried by blood It is the volume of O2 carried by blood combined with Hb / 100 ml bloodcombined with Hb / 100 ml blood..

O2 contents depends onO2 contents depends on::

11--Amount of Hb presentAmount of Hb present..

22 - -O2 tensionO2 tension..

33 - -O2 affinity of HbO2 affinity of Hb..

44 - -Metabolic state of the organMetabolic state of the organ..

O2 Capacity of BloodO2 Capacity of Blood

It is the maximum volume of O2 that can It is the maximum volume of O2 that can be carried by Hb when Hb is fully be carried by Hb when Hb is fully saturated with O2saturated with O2..

11 gram Hb combines with 1.34 ml O2gram Hb combines with 1.34 ml O2..

O2 capacity = 1.34 ml O2 x 15 gmO2 capacity = 1.34 ml O2 x 15 gm

= = 20.120.1 ml O2 / 100 ml bloodml O2 / 100 ml blood

O2 capacity varies with Hb contentO2 capacity varies with Hb content..

Hemoglobin Saturation with O2Hemoglobin Saturation with O2

% %Saturation of Hb with O2Saturation of Hb with O2::

% %HbO2 = HbO2 = O2 contentO2 content x 100 x 100

O2 capacityO2 capacity

% %Hb saturation does not vary with Hb Hb saturation does not vary with Hb content because both O2 content and content because both O2 content and capacity are affected to the same extentcapacity are affected to the same extent..

Hb O2 Dissociation CurveHb O2 Dissociation Curve

It is the relationship between PaO2 and % It is the relationship between PaO2 and % HbO2. It is not linear (S-shaped)HbO2. It is not linear (S-shaped)..

--O2 bound to Hb is relatively constant inO2 bound to Hb is relatively constant in PaO2 above 70 mmHg (flat part of thePaO2 above 70 mmHg (flat part of the

curvecurve.(.(--At lower Pao2 levels large change inAt lower Pao2 levels large change in

volume of O2 bound to Hb (steep part of thevolume of O2 bound to Hb (steep part of the curvecurve.(.(

Physiological significance of Physiological significance of S-shaped oxy-Hb dissociation curveS-shaped oxy-Hb dissociation curve

- -At the flat portion of the curve, Alveolar and At the flat portion of the curve, Alveolar and arterial PO2 can decrease to 60 mmHg arterial PO2 can decrease to 60 mmHg

with little change in % saturation (90%) e.gwith little change in % saturation (90%) e.g , , at high altitudeat high altitude . .

- -Below PO2 60 mmHg (steep part of theBelow PO2 60 mmHg (steep part of the curve) desaturation of Hb is rapid and O2curve) desaturation of Hb is rapid and O2

is readily given off to tissues with littleis readily given off to tissues with little further fall in PO2further fall in PO2..

Physiological significance of Physiological significance of S-shaped oxy-Hb dissociation curveS-shaped oxy-Hb dissociation curve

- -At 40 mmHg O2 tension (tissues during rest At 40 mmHg O2 tension (tissues during rest andand

venous blood), blood is 70% saturated, saovenous blood), blood is 70% saturated, sao tissues take 27% of O2 of arterial bloodtissues take 27% of O2 of arterial blood..

- -Below 40 mmHg (tissues during muscularBelow 40 mmHg (tissues during muscular exercise). The curve becomes more or lessexercise). The curve becomes more or less

vertical (steep) enable tissue to extract vertical (steep) enable tissue to extract relativelyrelatively

large amount of O2fro blood with relatively large amount of O2fro blood with relatively smallsmall

change in PaO2change in PaO2..

Shift of O2 Hb dissociation curve to the rightShift of O2 Hb dissociation curve to the right

- -Increase temperatureIncrease temperature.. - -Increase PCO2Increase PCO2..

- -Decrease pHDecrease pH.. - -Increase 2,3-diphosphoglycerate (2,3-DPG)Increase 2,3-diphosphoglycerate (2,3-DPG)

This occurs during muscular exerciseThis occurs during muscular exercise Decrease affinity of Hb to O2Decrease affinity of Hb to O2 . .

O2 is given easily to tissuesO2 is given easily to tissues

Shift of O2 Hb dissociation curve to the leftShift of O2 Hb dissociation curve to the left

- -Decrease temperatureDecrease temperature..

- -Decrease PCO2Decrease PCO2..

- -Increase pHIncrease pH..

- -Carbon monoxide (CO) poisoningCarbon monoxide (CO) poisoning

Increase affinity of Hb to O2Increase affinity of Hb to O2

Hb gives its O2 with difficulty to tissuesHb gives its O2 with difficulty to tissues

CO2 Transport by BloodCO2 Transport by Blood100100 ml of arterial blood contain 48 ml CO2ml of arterial blood contain 48 ml CO2

100100 ml of venous blood contain 52 ml CO2ml of venous blood contain 52 ml CO2

11 - -CO2 in blood in physical solution (3 ml)CO2 in blood in physical solution (3 ml)

Responsible for PO2 in blood (PaCO2 40Responsible for PO2 in blood (PaCO2 40

mmHgmmHg.(.(

22 - -CO2 ion blood in chemical; combinationCO2 ion blood in chemical; combination

As bicarbonaters (42 ml)As bicarbonaters (42 ml)

As Carbamino compounds (3 ml)As Carbamino compounds (3 ml)

Tidal CO2Tidal CO2

It is CO2 given by tissues to 100 ml bloodIt is CO2 given by tissues to 100 ml blood..

At rest tidal CO2 = 52 – 48At rest tidal CO2 = 52 – 48

= = 44 ml/100 mlml/100 ml..

It is mainly carried in chemical combination It is mainly carried in chemical combination with Hb which buffer tidal CO2, so pH of with Hb which buffer tidal CO2, so pH of blood does not markedly changeblood does not markedly change..

Transport of Tidal CO2Transport of Tidal CO2

11 - -In physical solution (0.4 ml)In physical solution (0.4 ml)

22 - -As bicarbonate (2.6 ml)As bicarbonate (2.6 ml)

33 - -As carbamino compounds (1 ml)As carbamino compounds (1 ml)

Transport of Tidal CO2Transport of Tidal CO2

Chloride Shift or Hamburger PhenomenonChloride Shift or Hamburger Phenomenon