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An Overview of Gas Exchange

Arthur B. Otis

10.1002/cphy.cp030401

Source: Supplement 13: Handbook of Physiology, The Respiratory System, Gas Exchange

Originally published: 1987

Published online: January 2011

Full Article on Wiley Online Library



Abstract

The sections in this article are:

1 Gas‐Exchange System: A Series of Conductances
2 Physiological Components of Conductance
2.1 Inspired Gas to Alveolar Gas
2.2 Alveolar Gas to Pulmonary Capillary Blood
2.3 Pulmonary Capillary Blood to Tissue Capillary Blood
2.4 Tissue Capillary Blood to Tissue
Figure 1. Figure 1.

Gas‐exchange system for O2 considered as 5 compartments and 4 conductances. Each conductance is characterized by a partial pressure, P. Oxygen flows along the system at rate M. Each conductance is defined as G = M/ΔP.
Figure 2. Figure 2.

Relationships between O2 flow, M; conductance, G; resistance, R; and pressure drop, ΔP. A: O2 flux as a function of conductance at different pressure drops. B: O2 flux as a function of resistance at different pressure drops. C: pressure loss as a function of conductance at different O2 fluxes. D: pressure loss as a function of resistance at different O2 fluxes.
Figure 3. Figure 3.

Hydraulic analogue of O2‐transport system. Height of water in each rectangular compartment represents the partial pressure of O2 (Po2) in that compartment. Last, circular compartment represents participation of O2 in metabolic energy generation, a process in which PO2 is reduced to zero. In any given situation atmospheric Po2 is considered constant; the height in the 4 succeeding compartments is determined by O2 flow and 4 conductances. Volume of compartment is immaterial in steady state but is of great importance in transient events.
Figure 4. Figure 4.

Electrical analogue of O2‐transport system. Each compartment is represented as a capacitor and each conductance (G) as a resistor. At any given metabolic rate the mitochondria constitute a constant current device in which the potential is reduced to zero ground level.
Figure 5. Figure 5.

Relationship showing physical energy loss per unit pressure drop as a function of pressure. Loss is greater at low than at high pressures. Thus raising the partial pressure of a mole of gas by a given increment is more costly when the initial pressure is low than when it is high.
Figure 6. Figure 6.

Oxygen cascade as 2 diffusive and 2 convective conductances. Difference, ΔP, between 2 successive regions is determined by the metabolic O2 consumption and the conductance between the 2 regions. Convective conductance may be regarded as the mixing of the 2 extreme regions of a conductive element by stirring (alveolar ventilation or blood flow). The faster the stirring the smaller the PI ‐ PA or c,L ‐ c,ti. PI, partial pressure of O2 (PO2) in inspired gas; PA, PO2 in alveolar gas; c,L, mean PO2 in pulmonary capillary blood; c,t (c,ti), mean PO2 in tissue capillary blood; Pt (Pti), PO2 in tissue.
Figure 7. Figure 7.

Electrical model of O2 transport system to demonstrate factors determining the rate of O2 transfer into pulmonary blood and out of tissue capillary blood. Blood is represented as an assembly of capacitors mounted on an endless belt rotating at a rate . Rate of charging of capacitors in the lung is determined by DL (diffusing capacity of the lung) and PA ‐ c,L (PA, alveolar PO2; c,L, mean PO2 in pulmonary capillary blood). Rate of discharge in the tissue is determined by (use rate). βg, Molar concentration of a gas per unit partial pressure; A, alveolar ventilation; PI, inspired PO2; RI, pulmonary resistance; Pc, capillary PO2; Vc, capillary volume; βb, slope of the O2 dissociation curve; P, mixed venous PO2; Pa, arterial PO2; Pt (Pti), tissue PO2; PO, PO2 = 0.
Figure 8. Figure 8.

Change in alveolar‐pulmonary capillary PO2 with time (t) (or distance) along a pulmonary capillary at different values of D/βb. D(DL), diffusing capacity of the lung; βb, slope of the O2 dissociation curve; , blood flow; PA, alveolar PO2; Pc,L, PO2 in pulmonary capillary blood; P, mixed venous PO2; t′, total time. •, Mean ΔPO2 for indicated D/βb.
Figure 9. Figure 9.

Increase in pulmonary capillary Po2 with time (or distance) along a pulmonary capillary at different values of D/βb.
Figure 10. Figure 10.

Mean alveolar‐pulmonary capillary Po2 difference as a function of D/βb.
Figure 11. Figure 11.

Difference between mean pulmonary capillary Po2 and mixed venous Po2 as a function of D/βb.
Figure 12. Figure 12.

Difference between mean pulmonary capillary Po2 and mean tissue capillary Po2 as a function of D/βb. Upper limit for this difference is half the alveolar‐mixed venous difference.
Figure 13. Figure 13.

Arterial, mean pulmonary capillary, and mean tissue capillary Po2 as a function of D/βb.
Figure 14. Figure 14.

Changes in Po2 along lung capillaries and along tissue capillaries at 2 different values of D/βb. •, Mean Po2 values in each region.
Figure 15. Figure 15.

Changes of Po2 in a tissue capillary and in tissue as a function of distance (x) along the capillary.


Figure 1.

Gas‐exchange system for O2 considered as 5 compartments and 4 conductances. Each conductance is characterized by a partial pressure, P. Oxygen flows along the system at rate M. Each conductance is defined as G = M/ΔP.



Figure 2.

Relationships between O2 flow, M; conductance, G; resistance, R; and pressure drop, ΔP. A: O2 flux as a function of conductance at different pressure drops. B: O2 flux as a function of resistance at different pressure drops. C: pressure loss as a function of conductance at different O2 fluxes. D: pressure loss as a function of resistance at different O2 fluxes.



Figure 3.

Hydraulic analogue of O2‐transport system. Height of water in each rectangular compartment represents the partial pressure of O2 (Po2) in that compartment. Last, circular compartment represents participation of O2 in metabolic energy generation, a process in which PO2 is reduced to zero. In any given situation atmospheric Po2 is considered constant; the height in the 4 succeeding compartments is determined by O2 flow and 4 conductances. Volume of compartment is immaterial in steady state but is of great importance in transient events.



Figure 4.

Electrical analogue of O2‐transport system. Each compartment is represented as a capacitor and each conductance (G) as a resistor. At any given metabolic rate the mitochondria constitute a constant current device in which the potential is reduced to zero ground level.



Figure 5.

Relationship showing physical energy loss per unit pressure drop as a function of pressure. Loss is greater at low than at high pressures. Thus raising the partial pressure of a mole of gas by a given increment is more costly when the initial pressure is low than when it is high.



Figure 6.

Oxygen cascade as 2 diffusive and 2 convective conductances. Difference, ΔP, between 2 successive regions is determined by the metabolic O2 consumption and the conductance between the 2 regions. Convective conductance may be regarded as the mixing of the 2 extreme regions of a conductive element by stirring (alveolar ventilation or blood flow). The faster the stirring the smaller the PI ‐ PA or c,L ‐ c,ti. PI, partial pressure of O2 (PO2) in inspired gas; PA, PO2 in alveolar gas; c,L, mean PO2 in pulmonary capillary blood; c,t (c,ti), mean PO2 in tissue capillary blood; Pt (Pti), PO2 in tissue.



Figure 7.

Electrical model of O2 transport system to demonstrate factors determining the rate of O2 transfer into pulmonary blood and out of tissue capillary blood. Blood is represented as an assembly of capacitors mounted on an endless belt rotating at a rate . Rate of charging of capacitors in the lung is determined by DL (diffusing capacity of the lung) and PA ‐ c,L (PA, alveolar PO2; c,L, mean PO2 in pulmonary capillary blood). Rate of discharge in the tissue is determined by (use rate). βg, Molar concentration of a gas per unit partial pressure; A, alveolar ventilation; PI, inspired PO2; RI, pulmonary resistance; Pc, capillary PO2; Vc, capillary volume; βb, slope of the O2 dissociation curve; P, mixed venous PO2; Pa, arterial PO2; Pt (Pti), tissue PO2; PO, PO2 = 0.



Figure 8.

Change in alveolar‐pulmonary capillary PO2 with time (t) (or distance) along a pulmonary capillary at different values of D/βb. D(DL), diffusing capacity of the lung; βb, slope of the O2 dissociation curve; , blood flow; PA, alveolar PO2; Pc,L, PO2 in pulmonary capillary blood; P, mixed venous PO2; t′, total time. •, Mean ΔPO2 for indicated D/βb.



Figure 9.

Increase in pulmonary capillary Po2 with time (or distance) along a pulmonary capillary at different values of D/βb.



Figure 10.

Mean alveolar‐pulmonary capillary Po2 difference as a function of D/βb.



Figure 11.

Difference between mean pulmonary capillary Po2 and mixed venous Po2 as a function of D/βb.



Figure 12.

Difference between mean pulmonary capillary Po2 and mean tissue capillary Po2 as a function of D/βb. Upper limit for this difference is half the alveolar‐mixed venous difference.



Figure 13.

Arterial, mean pulmonary capillary, and mean tissue capillary Po2 as a function of D/βb.



Figure 14.

Changes in Po2 along lung capillaries and along tissue capillaries at 2 different values of D/βb. •, Mean Po2 values in each region.



Figure 15.

Changes of Po2 in a tissue capillary and in tissue as a function of distance (x) along the capillary.

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Article Title: An Overview of Gas Exchange
 

How to Cite

Arthur B. Otis. An Overview of Gas Exchange. Compr Physiol 2011, Supplement 13: Handbook of Physiology, The Respiratory System, Gas Exchange: 1-11. First published in print 1987. doi: 10.1002/cphy.cp030401