Chapter 11 Gas Exchange and Transport. Mosby items and derived items © 2009 by Mosby, Inc., an...

Chapter 11 Chapter 11

Gas Exchange and TransportGas Exchange and Transport

Mosby items and derived items © 2009 by Mosby, Inc., an affiliate of Elsevier Inc. 2

ObjectivesObjectives Describe how oxygen and carbon dioxide move Describe how oxygen and carbon dioxide move

between the atmosphere and tissues.between the atmosphere and tissues.

Identify what determines alveolar oxygen and Identify what determines alveolar oxygen and carbon dioxide pressures.carbon dioxide pressures.

Calculate the alveolar partial pressure of oxygen Calculate the alveolar partial pressure of oxygen (P(PAOAO22) at any give barometric pressure and F) at any give barometric pressure and FIOIO22..

State the effect that normal regional variations in State the effect that normal regional variations in ventilation and perfusion have on gas exchange.ventilation and perfusion have on gas exchange.


Objectives (cont.)Objectives (cont.)

Describe how to compute total oxygen Describe how to compute total oxygen contents for arterial blood.contents for arterial blood.

State the factors that cause the arteriovenous State the factors that cause the arteriovenous oxygen content difference to change.oxygen content difference to change.

Identify the factors that affect oxygen loading Identify the factors that affect oxygen loading and unloading from hemoglobin.and unloading from hemoglobin.

Describe how carbon dioxide is carried in the Describe how carbon dioxide is carried in the blood.blood.


Objectives (cont.)Objectives (cont.)

Describe how oxygen and carbon dioxide Describe how oxygen and carbon dioxide transport are interrelated.transport are interrelated.

Describe the factors that impair oxygen Describe the factors that impair oxygen delivery to the tissues and how to distinguish delivery to the tissues and how to distinguish among them.among them.

State the factors that impair carbon dioxide State the factors that impair carbon dioxide removal.removal.


IntroductionIntroduction

Respiration is the process of moving oxygen to Respiration is the process of moving oxygen to tissues for aerobic metabolism and removal tissues for aerobic metabolism and removal of carbon dioxide of carbon dioxide Involves gas exchange at the lungs and tissuesInvolves gas exchange at the lungs and tissues

• OO22 from atmosphere to tissues for aerobic metabolism from atmosphere to tissues for aerobic metabolism

• Removal for CORemoval for CO22 from tissues to atmosphere from tissues to atmosphere


DiffusionDiffusion

Whole-body diffusion gradientsWhole-body diffusion gradients Gas moves across system by simple diffusionGas moves across system by simple diffusion Oxygen cascade moves from POxygen cascade moves from POO22 of 159 mm Hg in of 159 mm Hg in

atmosphere to the intracellular Patmosphere to the intracellular POO22 of ~5 mm Hg of ~5 mm Hg COCO22 gradient is the reverse from an intracellular gradient is the reverse from an intracellular

COCO22 ~60 mm Hg to the atmosphere where it is <1 ~60 mm Hg to the atmosphere where it is <1 mm Hgmm Hg

Determinants of alveolar CODeterminants of alveolar CO22

PPACOACO22 = (V = (VCOCO22 0.863)/V 0.863)/VAA

PPACOACO22 will increase with will increase with ↑↑VVCOCO22 or or ↓↓VVAA

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Diffusion (cont.)Diffusion (cont.) Alveolar oxygen tensions (PAlveolar oxygen tensions (PAOAO22))

PPIOIO22 is the primary determinant. is the primary determinant.

In lungs it is diluted by water vapor and COIn lungs it is diluted by water vapor and CO22..

Alveolar air equation accounts for all these factors:Alveolar air equation accounts for all these factors:

PPAOAO22 = F = FIOIO22 (P (PBB – 47) – (P – 47) – (PACOACO22/0.8)/0.8)

Dalton’s law of partial pressures accounts for first part Dalton’s law of partial pressures accounts for first part of the formula; the second relates to the rate at which of the formula; the second relates to the rate at which COCO22 enters the lung compared to oxygen exiting. enters the lung compared to oxygen exiting.

• This ratio is normally 0.8.This ratio is normally 0.8.


Diffusion (cont.)Diffusion (cont.)

Changes in alveolar gas partial tensionsChanges in alveolar gas partial tensions OO22, CO, CO22, H, H22O, and NO, and N22 normally comprise alveolar normally comprise alveolar

gas.gas.• NN22 is inert but occupies space and exerts pressure. is inert but occupies space and exerts pressure.

• PPANAN22 is determined by Dalton’s law . is determined by Dalton’s law .

PPANAN22 = P = PBB – (P – (PAOAO22 + P + PACOACO22 + P + PHH22OO))

The only changes seen will be in OThe only changes seen will be in O22 and CO and CO22..

• Constant FConstant FIOIO22, P, PAOAO22 varies inversely with P varies inversely with PACOACO22..

• Prime determinant of PPrime determinant of PACOACO22 is V is VAA..


Diffusion: MechanismsDiffusion: Mechanisms

Diffusion occurs along pressure gradientsDiffusion occurs along pressure gradients

Barriers to diffusionBarriers to diffusion A/C membrane has three main barriersA/C membrane has three main barriers..

• Alveolar epitheliumAlveolar epithelium• Interstitial space and its structuresInterstitial space and its structures• Capillary endotheliumCapillary endothelium

RBC membraneRBC membrane

Fick’s law: The greater the sFick’s law: The greater the surface area, urface area, diffusion constant, and pressure gradient, the diffusion constant, and pressure gradient, the more diffusion will occur.more diffusion will occur.



Pulmonary diffusion gradientsPulmonary diffusion gradients Diffusion occurs along pressure gradients.Diffusion occurs along pressure gradients. Time limits to diffusion:Time limits to diffusion:

• Pulmonary blood is normally exposed to alveolar gas for 0.75 Pulmonary blood is normally exposed to alveolar gas for 0.75 second, during exercise may fall 0.25 second second, during exercise may fall 0.25 second

• Normally equilibration occurs in 0.25 secondNormally equilibration occurs in 0.25 second

• With a diffusion limitation or blood exposure time of less then With a diffusion limitation or blood exposure time of less then 0.25 second there may be inadequate time for equilibratiion0.25 second there may be inadequate time for equilibratiion


Normal Variations From Normal Variations From Ideal Gas ExchangeIdeal Gas Exchange

PaPaOO22 normally 5 – 10 mm Hg less than P normally 5 – 10 mm Hg less than PAOAO22 due due to the presence of anatomic shuntsto the presence of anatomic shunts

Anatomic shuntsAnatomic shunts Two right-to-left anatomic shunts exist.Two right-to-left anatomic shunts exist.

• Bronchial venous drainageBronchial venous drainage• Thebesian venous drainageThebesian venous drainage• These drain poorly oxygenated blood into arterial circulation These drain poorly oxygenated blood into arterial circulation

which lowers the Cawhich lowers the CaOO22

Regional inequalities in V/QRegional inequalities in V/Q Changes in either V or Q affect gas tensions.Changes in either V or Q affect gas tensions.

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Normal Variations From Normal Variations From Ideal Gas Exchange (cont.)Ideal Gas Exchange (cont.)

V/Q ratio and regional differences V/Q ratio and regional differences An ideal ratio is 1, where V/Q is in perfect balanceAn ideal ratio is 1, where V/Q is in perfect balance In reality the lungs don’t function at the ideal level In reality the lungs don’t function at the ideal level

• High V/Q ratio at apices >1 V/Q (~3.3)High V/Q ratio at apices >1 V/Q (~3.3) ↑↑PPAOAO22 (132 mm Hg), (132 mm Hg), ↓↓PPACOACO22 (32 mm Hg) (32 mm Hg)

• Low V/Q ratio at bases <1.0 (~0.66)Low V/Q ratio at bases <1.0 (~0.66) Blood flow is ~20 times higher at basesBlood flow is ~20 times higher at bases Ventilation is greater at bases but not 20Ventilation is greater at bases but not 20 ↓↓PPAOAO22 (89 mm Hg), (89 mm Hg), ↑↑PPACOACO22 (42 mm Hg) (42 mm Hg)

• See Table 11-1.See Table 11-1.

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Oxygen TransportOxygen Transport Transported in two forms: dissolved and boundTransported in two forms: dissolved and bound Physically dissolved in plasmaPhysically dissolved in plasma

Gaseous oxygen enters blood and dissolves. Gaseous oxygen enters blood and dissolves. Henry’s law allows calculation of amount dissolved.Henry’s law allows calculation of amount dissolved.

• Dissolved ODissolved O22 (ml/dl) = P (ml/dl) = POO22 0.003 0.003

Chemically bound to hemoglobin (Hb)Chemically bound to hemoglobin (Hb) Each gram of Hb can bind 1.34 ml of oxygen.Each gram of Hb can bind 1.34 ml of oxygen. [Hb g] [Hb g] 1.34 ml O 1.34 ml O22 provides capacity. provides capacity. 70 times more O70 times more O22 transported bound than dissolved transported bound than dissolved


Oxygen Transport (cont.)Oxygen Transport (cont.)

Hemoglobin saturationHemoglobin saturation Saturation is percentage of Hb that is carrying oxygen Saturation is percentage of Hb that is carrying oxygen

compared to total Hbcompared to total Hb• SaOSaO22 = [HbO = [HbO22/total Hb] /total Hb] 100 100• Normal SaONormal SaO22 is 95% to 100% is 95% to 100%

HbOHbO22 dissociation curve dissociation curve The relationship between PaOThe relationship between PaO22 and SaO and SaO2 2 isis S-shaped.S-shaped. Flat portion occurs with SaOFlat portion occurs with SaO2 2 >90% >90%

• Facilitates OFacilitates O22 loading at lungs even with low PaO loading at lungs even with low PaO22

Steep portion (SaOSteep portion (SaO2 2 <90%) occurs in capillaries<90%) occurs in capillaries• facilitates Ofacilitates O22 unloading at tissues unloading at tissues



Total oxygen content of bloodTotal oxygen content of blood Combination of dissolved and bound to HbCombination of dissolved and bound to Hb CaCaOO22 = (0.003 = (0.003 Pa PaOO22) + (Hb ) + (Hb 1.34 1.34 Sa SaOO22)) Normal is 16Normal is 1620 ml/dl20 ml/dl

Normal arteriovenous difference (~5 ml/dl)Normal arteriovenous difference (~5 ml/dl)



Fick equationFick equation C(a C(a v)O v)O22 indicates tissue oxygen extraction in indicates tissue oxygen extraction in

proportion to blood flow (per 100 ml of blood)proportion to blood flow (per 100 ml of blood) Combined with total oxygen consumption (VCombined with total oxygen consumption (VOO22) )

allows the calculation of cardiac output (Qallows the calculation of cardiac output (Qtt))

QQtt = V = VOO22/[C(a /[C(a v)O v)O22 10] 10]

Normal adult QNormal adult Qtt is 4 is 48 L/min. 8 L/min.

VVOO22 constant, changes in C(a constant, changes in C(a v)O v)O22 are due to are due to

changes in Qchanges in Qtt; i.e. ; i.e. ↑C(a↑C(a v)Ov)O22 signifies ↓Q signifies ↓Qtt

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Factors affecting oxygen loading and unloadingFactors affecting oxygen loading and unloading Besides the shape of the HbOBesides the shape of the HbO22 curve, many factors curve, many factors

affect Oaffect O22 loading and unloading. loading and unloading.

pH (Bohr effect) pH (Bohr effect) Describes the affect pH has on Hb affinity for ODescribes the affect pH has on Hb affinity for O22

pH alters the position of HbOpH alters the position of HbO22 curve. curve.• Low pH shifts curve to right, high pH shifts to left.Low pH shifts curve to right, high pH shifts to left.

Enhances oxygen transportEnhances oxygen transport• At tissue pH is ~7.37 shift right, more OAt tissue pH is ~7.37 shift right, more O22 unloaded unloaded

• Lungs pH ~7.4 shifts back left, enhancing OLungs pH ~7.4 shifts back left, enhancing O22 loading loading



Body temperature (T) and HbOBody temperature (T) and HbO22 curve curve Changes in T alter the position of the HbOChanges in T alter the position of the HbO22 curve. curve.

• Decreased T shifts the curve left.Decreased T shifts the curve left.• Increased T shifts the curve right.Increased T shifts the curve right.• T is directly related to metabolic rate.T is directly related to metabolic rate.

When T is higher, the right shift facilitates more oxygen When T is higher, the right shift facilitates more oxygen unloading to meet metabolic demands.unloading to meet metabolic demands.

With lower metabolic demands, curve shifts left as not as With lower metabolic demands, curve shifts left as not as much oxygen is required.much oxygen is required.

See Figure 11-10.See Figure 11-10.



2,3-Diphosphoglycerate (DPG) and the HbO2,3-Diphosphoglycerate (DPG) and the HbO22

curvecurve Found in quantity in RBCs, it stabilizes deoxygenated Found in quantity in RBCs, it stabilizes deoxygenated

Hb, decreasing oxygen’s affinity for HbHb, decreasing oxygen’s affinity for Hb• Without 2,3-DPG Hb affinity is so great that OWithout 2,3-DPG Hb affinity is so great that O22 cannot cannot

unloadunload

↑↑2,3-DPG shifts curve to right, promoting O2,3-DPG shifts curve to right, promoting O22

unloadingunloading ↓↓2,3-DPG shifts curve to left, promoting loading2,3-DPG shifts curve to left, promoting loading

• Stored blood loses 2,3-DPG, large transfusions can Stored blood loses 2,3-DPG, large transfusions can significantly impair tissue oxygenationsignificantly impair tissue oxygenation


Oxygen Transport (cont.)Oxygen Transport (cont.)Abnormal hemoglobinsAbnormal hemoglobins HbS (sickle cell): fragile leads to hemolysis and HbS (sickle cell): fragile leads to hemolysis and

thrombi.thrombi. Acute chest syndrome most common cause of deathAcute chest syndrome most common cause of death

Methemoglobin (metHb): abnormal iron (FeMethemoglobin (metHb): abnormal iron (Fe3+3+) ) cannot bind with oxygen and alters HbOcannot bind with oxygen and alters HbO22 affinity affinity (left shift)(left shift) Commonly caused by NO, nitroglycerin, lidocaineCommonly caused by NO, nitroglycerin, lidocaine

Carboxyhemoglobin (HbCO): Hb binds CO, which Carboxyhemoglobin (HbCO): Hb binds CO, which has 200 times greater Hb affinity than Ohas 200 times greater Hb affinity than O22.. Displaces ODisplaces O22 and shifts curve left and shifts curve left

• That OThat O22 which is bound cannot unload (left shift) which is bound cannot unload (left shift) Treat with hyperbaric therapyTreat with hyperbaric therapy


Oxygen Transport (cont.)Oxygen Transport (cont.) Measurement of Hb affinity for oxygenMeasurement of Hb affinity for oxygen


Carbon Dioxide TransportCarbon Dioxide TransportTransport mechanismsTransport mechanisms Dissolved in blood: ~8% as high solubility Dissolved in blood: ~8% as high solubility

coefficientcoefficient Combined with protein: ~12% binds with amino Combined with protein: ~12% binds with amino

groups on plasma proteins and Hb groups on plasma proteins and Hb Ionized as bicarbonate: ~80% is transported as Ionized as bicarbonate: ~80% is transported as

HCOHCO33 due to hydrolysis reaction due to hydrolysis reaction

Majority of hydrolysis occurs in RBCs as they contain Majority of hydrolysis occurs in RBCs as they contain carbonic anhydrase which serves as a catalyst carbonic anhydrase which serves as a catalyst

HCOHCO33 diffuses out of RBCs in exchange for Cl diffuses out of RBCs in exchange for Cl, called , called

the the chloride shiftchloride shift or or hamburger phenomenonhamburger phenomenon..


Carbon Dioxide Transport (cont.)Carbon Dioxide Transport (cont.)

COCO22 dissociation curve dissociation curve Relationship between PaRelationship between PaCOCO22 and Ca and CaCOCO22

HbOHbO22 affects this relationship affects this relationship

• The Haldane effect describes this relationshipThe Haldane effect describes this relationship

• As HbOAs HbO2 2 increases, Caincreases, CaCOCO22 decreases decreases Facilitates COFacilitates CO22 unloading at lungs unloading at lungs

• At tissues, HbOAt tissues, HbO2 2 decreases and facilitates a higher decreases and facilitates a higher

CaCaCOCO22 for transport to the lungs for transport to the lungs

See Figure 11-14.See Figure 11-14.


Abnormalities of Gas Exchange Abnormalities of Gas Exchange and Transportand Transport

Impaired oxygen delivery (DImpaired oxygen delivery (DOO22)) DDOO22 = Ca = CaOO22 Q Qtt

When DWhen DOO22 is inadequate, tissue hypoxia ensues. is inadequate, tissue hypoxia ensues.

Hypoxemia: Defined as an abnormally low PaHypoxemia: Defined as an abnormally low PaOO22

Most common cause is V/Q mismatchMost common cause is V/Q mismatch• Because of shape HbOBecause of shape HbO22 curve, areas of high V/Q cannot curve, areas of high V/Q cannot

compensate for areas of low V/Q, so compensate for areas of low V/Q, so ↓Pa↓PaOO22

See Figure 11-16. See Figure 11-16.

Other causes: hypoventilation, diffusion defect, Other causes: hypoventilation, diffusion defect, shunting, and a low Pshunting, and a low PIOIO22 (altitude) (altitude)

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Abnormalities of Gas Exchange Abnormalities of Gas Exchange and Transport (cont.)and Transport (cont.)

Impaired DImpaired DOO22 due to Hb deficiencies due to Hb deficiencies Majority of OMajority of O22 is carried bound to Hb, for Ca is carried bound to Hb, for CaOO22 to to

be adequate there must be enough normal Hbbe adequate there must be enough normal Hb If Hb is low, although the PaIf Hb is low, although the PaOO22 and Sa and SaOO22 are normal, are normal,

the Cathe CaOO2 2 will be low.will be low. Absolute low Hb is caused by anemiaAbsolute low Hb is caused by anemia Relative deficiency may be due to COHb or metHb.Relative deficiency may be due to COHb or metHb.



Reduction in blood flow (shock or ischemia)Reduction in blood flow (shock or ischemia) Hypoxia can occur with a normal CaHypoxia can occur with a normal CaOO22 if Q if Qtt is is

lowlow May be due toMay be due to

• ShockShock Results in widespread hypoxiaResults in widespread hypoxia Limited ability to compensate Limited ability to compensate

– Prolonged shock becomes irreversibleProlonged shock becomes irreversible

• IschemiaIschemia Local reductions in blood flow that may result in hypoxia Local reductions in blood flow that may result in hypoxia

and tissue death, i.e., myocardial infarction and strokeand tissue death, i.e., myocardial infarction and stroke



DysoxiaDysoxia DDOO22 is normal but cells undergo hypoxia. is normal but cells undergo hypoxia.

Cells are unable to adequately utilize oxygenCells are unable to adequately utilize oxygen• Cyanide poisoning prevents cellular use of OCyanide poisoning prevents cellular use of O22..

• In very sick individuals (sepsis, ARDS), oxygen debt may In very sick individuals (sepsis, ARDS), oxygen debt may occur at normal levels of Doccur at normal levels of DOO22. .

If oxygen uptake increases with increased DIf oxygen uptake increases with increased DOO22, then D, then DOO22

was inadequate.was inadequate. Demonstrates a low VDemonstrates a low VOO22/D/DOO22 (extraction ratio), thus (extraction ratio), thus

dysoxiadysoxia



Impaired COImpaired CO22 removal removal Disorders that decreases VDisorders that decreases VAA relative to metabolic relative to metabolic

needneed Inadequate VInadequate VEE

• Usually result of Usually result of ↓↓VVTT, , ↓f rare (drug overdose)↓f rare (drug overdose)

Increased VIncreased VDD/V/VTT

• Caused by rapid shallow breathing or high V/QCaused by rapid shallow breathing or high V/Q V/Q imbalanceV/Q imbalance

• Typically, COTypically, CO22 does not rise; instead, increase V does not rise; instead, increase VEE..

• If patient is unable to If patient is unable to ↑↑VVEE, then hypercarbia with acidosis , then hypercarbia with acidosis occurs.occurs.

Seen in severe chronic disorders, i.e., COPDSeen in severe chronic disorders, i.e., COPD

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Chapter 11 Gas Exchange and Transport. Mosby items and derived items © 2009 by Mosby, Inc., an...

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