Comprehensive Physiology Wiley Online Library

Respiratory Gas Exchange in the Placenta

Full Article on Wiley Online Library



Abstract

The sections in this article are:

1 The Placenta
1.1 Placental Exchange and Fetal Oxygenation
1.2 Questions to Consider
2 Maternal Blood O2 Affinity and Capacity
2.1 Hemoglobin
2.2 Blood O2 Affinity and HbO2 Saturation Curve
2.3 Blood O2 Capacity
3 Fetal Blood O2 Affinity and Capacity
3.1 Fetal Hb
3.2 Blood O2 Affinity
3.3 Blood O2 Capacity
4 Interrelations of Maternal and Fetal Blood O2 Affinity and Capacity
4.1 General Considerations
4.2 Species Differences
5 Transplacental Diffusion
5.1 Mean Maternal‐to‐Fetal O2 Tension Differences
5.2 Placental Diffusing Capacity
5.3 Role of Hb Reaction Rates
5.4 Possible Facilitated O2 Transport
5.5 Alterations in Placental Diffusing Capacity
5.6 Placental O2 Consumption
5.7 Summary
6 Variations in Maternal and Fetal O2 Tensions
6.1 Theoretical Considerations
6.2 Experimental Studies
6.3 Clinical Implications
7 Variations in Maternal and Fetal O2 Affinity
7.1 Theoretical Considerations
7.2 Decrease in Fetal Blood O2 Affinity by Intrauterine Transfusion
7.3 Changes in O2 Affinity Due to Hemoglobinopathies
7.4 Maternal Hyperventilation
7.5 Carbon Monoxide Effects
8 Placental CO2 Exchange
8.1 Forms of CO2
8.2 Carbon Dioxide Dissociation Curves
8.3 Transplacental CO2 Exchange
8.4 Haldane and Bohr Effects
8.5 Importance of Various Factors on CO2 Exchange
8.6 Umbilical‐to‐ Uterine Venous CO2 Tension Differences
8.7 Interrelations of Placental CO2 and H2O Exchange
8.8 Conclusions
9 Variations in Rate of Maternal and Fetal Placental HB and O2 Flows
9.1 General Considerations
9.2 Theoretical Considerations
9.3 Experimental Studies
9.4 Clinical Implications of Altered Placental Flows
10 Interrelations of Maternal and Fetal Placental Flows and O2 Exchange
10.1 Distribution of Maternal and Fetal Placental Flows
10.2 “Sluice” or “Waterfall” Flow in Placental Vessels
10.3 Uterine Contractions and O2 Exchange
10.4 Importance of Various Parameters in O2 Transfer
10.5 Summary Equations
10.6 Graphical Analysis of Placental O2 and CO2 Exchange
11 Some Comparisons of Blood Flow and Gas Exchange in the Placenta and Lung
12 Conclusions
Figure 1. Figure 1.

A. Near‐term human placental villus showing well‐differentiated syncytiotrophoblast with its border folded with tufts of microvilli facing maternal intervillous space. Large cytotrophoblast cell is on left and fetal capillary contains several erythrocytes. × 7,350. B. Scanning electron micrograph of 28‐day rhesus monkey villus with fetal capillary containing erythrocytes. × 2,250.

Courtesy of B. F. King
Figure 2. Figure 2.

Schematic illustration of functional anatomy and morphology of placental barrier of several species. Numerical values indicate typical O2 tensions ( , Torr) for inflowing and outflowing maternal and fetal blood.

Figure 3. Figure 3.

Cascade of from air to alveolar gas, arterial and uterine venous blood of maternal organism, fetal umbilical venous and arterial blood, and cristae of fetal mitochondria.

Figure 4. Figure 4.

Changes in human maternal (line 2) and fetal (line 1) hemoglobin (Hb) concentrations and fetal mean corpuscular volume (line 3) during course of gestation.

Figure 5. Figure 5.

HbO2 saturation curves for human maternal and near‐term fetal blood. Maternal and fetal Hb‐O2 affinities (P50) are 26.5 and 20 Torr, respectively. A and V, maternal arterial and venous values, respectively, under standard conditions; a and v, umbilical arterial and venous values, respectively; V′, a′, and v′, probable in vivo maternal venous, umbilical arterial, and umbilical venous values, respectively.

Figure 6. Figure 6.

Blood O2 content as function of for maternal ([Hb]m = 12 g · dl−1 and Pm50 = 26.5 Torr) and near‐term fetal ([Hb]f = 16.5 g · dl−1 and Pf50 = 20 Torr) blood. Asterisks, maternal and fetal ; A and V, maternal arterial and venous values; a and v, umbilical arterial and venous values.

Figure 7. Figure 7.

Relation of blood O2 capacity of fetus to that of mother in various species. □, Camel; ☆, cat; , chicken; ▪, cow; ♦, elephant; +, goat; , guinea pig; , hamster; ▴, human; , llama; , pig; ⋄, rabbit; ⊖, rat; Δ and , rhesus monkey; ○, seal; , sheep. Line of identity is shown.

Figure 8. Figure 8.

Relation of blood O2 affinity of fetus to that of mother in various species. Symbols are as in Fig. , with the addition of × for baboon and for dog.

Figure 9. Figure 9.

Diagrammatic representation of time course of change in in maternal and fetal blood during single transit in placental exchange vessels. Dashed lines, values assuming that values in uterine and umbilical veins are the same as those of maternal and fetal end capillaries, which is probably incorrect; long arrow, average maternal‐to‐fetal difference of ∼30 Torr calculated on this basis. Solid lines, time courses of changes in values in maternal and fetal placental exchange vessels calculated from measurements of placental CO diffusing capacity; short arrow, probable true maternal‐to‐fetal difference of 5–7 Torr based on calculations of placental diffusing capacity for CO ( ); under these conditions, values in maternal and fetal blood probably reach near equilibrium in end‐capillary vessels.

Figure 10. Figure 10.

Schematic representation of maternal‐to‐fetal partial pressure differences and resistance to diffusion. Total pressure difference (Pm — Pf) consists of 1) difference between interior of maternal erythrocyte and plasma (Pm), 2) difference across membrane between maternal and fetal plasma ( ), and 3) difference between fetal plasma and interior of fetal erythrocyte ( , ‐ Pt). Overall resistance to diffusion (1/DP) is sum of individual resistances: 1) effective resistance of maternal blood to diffusion (1/θmVm), 2) resistance of placental membrane per se (1/DM), and 3) resistance of fetal blood (1/θfVf).

Figure 11. Figure 11.

Calculated changes in transient O2 exchange rate and maternal and fetal placental end‐capillary values (dashed curves) and steady‐state maternal and fetal end‐capillary (solid curves) as functions of Dp. Vertical dashed lines, assumed normal value of Dp. During steady‐state conditions, a normal ( ) may be maintained within limits by decreases of fetal arterial (bottom panel).

Figure 12. Figure 12.

Theoretical effects of maternal exercise on placental O2 transfer. Top, contribution of individual factors to net ( ). Bottom, net effect of these changes on transient and steady‐statetransfer rates. Tf, fetal temperature; Tm, maternal temperature; Hbm, maternal Hb concentration; , maternal arterial ; pHf, fetal arterial pH; pHm, maternal arterial pH; , uteroplacental blood flow.

Figure 13. Figure 13.

Calculated effects of changes in maternal arterial ( ) on transient maternal and fetal end‐capillary values and O2 exchange rate (dashed curves) and steady‐state end‐capillary values and fetal arterial ( ) (solid curves). Moderate increases in above normal values (95 Torr, vertical dashed lines) increase end‐capillary and transient ( ) only slightly, whereas decreases in produce substantial decreases in ( ) and end‐capillary values. Decreases in alone cannot maintain normal ( ) when is below ∼40 Torr.

Figure 14. Figure 14.

Calculated effects of changes in on end‐capillary and O2 exchange rate. Changes in from normal values (20 Torr, vertical dashed lines) result in increases in either end‐capillary or ( ) but not both and may serve to restore to a normal value.

Figure 15. Figure 15.

Effects of experimentally varying on O2 transfer rate and . Solid lines, theoretical curves.

Adapted from Power and Jenkins
Figure 16. Figure 16.

Effects of varying on ( ) and outflowing umbilical . Solid lines, theoretical curves.

Adapted from Power and Jenkins
Figure 17. Figure 17.

Relation of to that of mother in chronically catheterized sheep (□) and monkeys (Δ) and acutely anesthetized sheep (○).

Figure 18. Figure 18.

values in uterine arterial (•) and venous (○) blood and in umbilical arterial (Δ) and venous (▴) blood as function of . When Pmvo2 was >200 Torr, uterine venous HbO2 was completely saturated and O2 crossing the placenta was almost entirely from that physically dissolved in maternal plasma. In all cases, was < .

Figure 19. Figure 19.

Cascade of from ambient air to maternal and fetal blood at ∼5,000 m (dotted line) as compared with that at sea level (solid line).

Figure 20. Figure 20.

Calculated maternal and fetal blood O2 contents as function of when relative positions of maternal and fetal HbO2 saturation curves are identical; i.e., both Pm50 and Pf50 are 26.5 Torr. Asterisks, and ; A and V, maternal arterial and venous values; a and v, umbilical arterial and venous values.

Figure 21. Figure 21.

Maternal and fetal blood O2 contents as function of when relative positions of maternal and fetal HbO2 saturation curves are reversed. In this instance, affinity of maternal blood is normal (Pm50 = 26.5 Torr), whereas that of fetus is increased (Pf50 = 35 Torr) due to Hb Kansas replacing Hb A. Symbols are as in Fig. .

Figure 22. Figure 22.

Maternal and fetal blood O2 contents as function of when maternal and fetal HbO2 saturation curves are reversed. In this instance, Pm50 = 12 Torr and [Hb]m = 15 g · dl−1, whereas Pf50 = 20 Torr. Symbols are as in Fig. .

Figure 23. Figure 23.

Human maternal and fetal HbO2 saturation curves showing CO effects. The HbO2 saturation ([HbO2]) is that percentage of Hb not bound as HbCO.

Figure 24. Figure 24.

Calculated O2 content vs. of human blood with [HbCO]f = 0% and 10% and [HbCO]m = 0% and 9.4%. [Hb]m and [Hb]f are assumed to be 12 and 16.5 g · dl−1, respectively. Normal arteriovenous O2 difference of 5 ml · dl−1 is assumed for both the uterus and its contents and the fetus. Figure depicts mechanism accounting for reduction of umbilical artery and vein O2 tensions and contents resulting from elevated [HbCO].

Figure 25. Figure 25.

Dissociation curves for CO2 in human maternal and fetal blood showing effects of O2 saturation and base deficit, a, Adult deoxygenated blood; b, adult oxygenated blood; c, pregnant women and fetuses; d, fetal blood. Heavy lines, positions expected for maternal and fetal blood based on degree of oxygenation.

Adapted from Eastman et al.
Figure 26. Figure 26.

Effect of changes in various factors on transient placental CO2 transfer rate. To facilitate comparison, values are plotted as percent changes from assumed normal values. Slope of each curve at 100% may be used as index of relative sensitivity of CO2 transfer to that factor.

Adapted from Hill et al.
Figure 27. Figure 27.

Time course of during single transit through exchange vessels. Solid lines, values within erythrocytes; broken lines, values in plasma. Changes occurring after blood leaves exchange area are plotted on right with scale compressed 40 times.

Adapted from Hill et al.
Figure 28. Figure 28.

Calculated effects of changes in maternal placental blood flow, [Hb], Hb flow, and O2 flow on transient maternal and fetal placental end‐capillary values and ( ) (dashed lines) and on steady‐state end‐capillary values and (solid lines).

Figure 29. Figure 29.

Calculated effects of changes in fetal placental blood flow, [Hb], Hb flow, and O2 flow on transient maternal and fetal placental end‐capillary values and ( ) (dashed lines) and on steady‐state end‐capillary values and (solid lines).

Figure 30. Figure 30.

Relations between venous outflow , placental O2 transfer, and fetal cotyledonary flow in perfused isolated sheep and rabbit placentas. Solid lines, predicted relations using mathematical model.

Adapted from Power and Jenkins
Figure 31. Figure 31.

Calculated blood O2 content as function of in presence of maternal anemia ([Hb]m = 5.75 g · dl−1) with associated slight increase in O2 affinity (Pm50 = 28 Torr). A and V, maternal arterial and venous values; a and v, umbilical arterial and venous values.

Figure 32. Figure 32.

Calculated blood O2 content as function of in presence of fetal anemia ([Hb]f = 8.25 g · dl−1). Symbols are as in Fig. .

Figure 33. Figure 33.

Total uterine vein‐to‐umbilical vein difference at various values of (solid line). Contribution of distribution effect is shown over entire range of values (dashed line) assuming that distribution of maternal to fetal flow ( ) at higher values is similar to that during normal oxygenation. Diffusion effect (shaded area) is shown to be insignificant when exceeds 100 Torr. Dotted line, maternal‐to‐fetal venous difference produced by 15% maternal shunt. Sum of various factors must represent total of this difference.

Adapted from Longo and Power
Figure 34. Figure 34.

Volume of maternal ( ) and fetal ( ) placental flows required to exchange 1 ml of O2. As ratio of to increases above 1.0, progressively more is required. This occurs because equilibration occurs at higher and greater saturation of maternal Hb; therefore less O2 is removed from each milliliter of maternal blood. Similarly, fetal blood undergoes greatest changes in [O2] with high ratios; therefore less fetal flow is required. Sum of both curves equals total maternal and fetal flow (dotted line). Optimum ratio for O2 transport in terms of total flow is ∼0.9.

Adapted from Power and Longo
Figure 35. Figure 35.

Effects of changes in various factors on transient placental O2 exchange rate. Results are normalized by plotting percent change in ( ) as function of percent change in assumed normal value of given factor. As in Fig. , slope of each curve at 100% of normal value indicates relative sensitivity of O2 exchange rate to that factor. ( ) is most sensitive to umbilical arterial , followed closely by total, maternal, and fetal flow rates and O2 affinity of maternal and fetal blood. Curves for maternal and fetal [Hb] are superimposed on and curves. Pm and Pf, maternal and fetal values; Dp, placental diffusing capacity.

Adapted from Longo et al.


Figure 1.

A. Near‐term human placental villus showing well‐differentiated syncytiotrophoblast with its border folded with tufts of microvilli facing maternal intervillous space. Large cytotrophoblast cell is on left and fetal capillary contains several erythrocytes. × 7,350. B. Scanning electron micrograph of 28‐day rhesus monkey villus with fetal capillary containing erythrocytes. × 2,250.

Courtesy of B. F. King


Figure 2.

Schematic illustration of functional anatomy and morphology of placental barrier of several species. Numerical values indicate typical O2 tensions ( , Torr) for inflowing and outflowing maternal and fetal blood.



Figure 3.

Cascade of from air to alveolar gas, arterial and uterine venous blood of maternal organism, fetal umbilical venous and arterial blood, and cristae of fetal mitochondria.



Figure 4.

Changes in human maternal (line 2) and fetal (line 1) hemoglobin (Hb) concentrations and fetal mean corpuscular volume (line 3) during course of gestation.



Figure 5.

HbO2 saturation curves for human maternal and near‐term fetal blood. Maternal and fetal Hb‐O2 affinities (P50) are 26.5 and 20 Torr, respectively. A and V, maternal arterial and venous values, respectively, under standard conditions; a and v, umbilical arterial and venous values, respectively; V′, a′, and v′, probable in vivo maternal venous, umbilical arterial, and umbilical venous values, respectively.



Figure 6.

Blood O2 content as function of for maternal ([Hb]m = 12 g · dl−1 and Pm50 = 26.5 Torr) and near‐term fetal ([Hb]f = 16.5 g · dl−1 and Pf50 = 20 Torr) blood. Asterisks, maternal and fetal ; A and V, maternal arterial and venous values; a and v, umbilical arterial and venous values.



Figure 7.

Relation of blood O2 capacity of fetus to that of mother in various species. □, Camel; ☆, cat; , chicken; ▪, cow; ♦, elephant; +, goat; , guinea pig; , hamster; ▴, human; , llama; , pig; ⋄, rabbit; ⊖, rat; Δ and , rhesus monkey; ○, seal; , sheep. Line of identity is shown.



Figure 8.

Relation of blood O2 affinity of fetus to that of mother in various species. Symbols are as in Fig. , with the addition of × for baboon and for dog.



Figure 9.

Diagrammatic representation of time course of change in in maternal and fetal blood during single transit in placental exchange vessels. Dashed lines, values assuming that values in uterine and umbilical veins are the same as those of maternal and fetal end capillaries, which is probably incorrect; long arrow, average maternal‐to‐fetal difference of ∼30 Torr calculated on this basis. Solid lines, time courses of changes in values in maternal and fetal placental exchange vessels calculated from measurements of placental CO diffusing capacity; short arrow, probable true maternal‐to‐fetal difference of 5–7 Torr based on calculations of placental diffusing capacity for CO ( ); under these conditions, values in maternal and fetal blood probably reach near equilibrium in end‐capillary vessels.



Figure 10.

Schematic representation of maternal‐to‐fetal partial pressure differences and resistance to diffusion. Total pressure difference (Pm — Pf) consists of 1) difference between interior of maternal erythrocyte and plasma (Pm), 2) difference across membrane between maternal and fetal plasma ( ), and 3) difference between fetal plasma and interior of fetal erythrocyte ( , ‐ Pt). Overall resistance to diffusion (1/DP) is sum of individual resistances: 1) effective resistance of maternal blood to diffusion (1/θmVm), 2) resistance of placental membrane per se (1/DM), and 3) resistance of fetal blood (1/θfVf).



Figure 11.

Calculated changes in transient O2 exchange rate and maternal and fetal placental end‐capillary values (dashed curves) and steady‐state maternal and fetal end‐capillary (solid curves) as functions of Dp. Vertical dashed lines, assumed normal value of Dp. During steady‐state conditions, a normal ( ) may be maintained within limits by decreases of fetal arterial (bottom panel).



Figure 12.

Theoretical effects of maternal exercise on placental O2 transfer. Top, contribution of individual factors to net ( ). Bottom, net effect of these changes on transient and steady‐statetransfer rates. Tf, fetal temperature; Tm, maternal temperature; Hbm, maternal Hb concentration; , maternal arterial ; pHf, fetal arterial pH; pHm, maternal arterial pH; , uteroplacental blood flow.



Figure 13.

Calculated effects of changes in maternal arterial ( ) on transient maternal and fetal end‐capillary values and O2 exchange rate (dashed curves) and steady‐state end‐capillary values and fetal arterial ( ) (solid curves). Moderate increases in above normal values (95 Torr, vertical dashed lines) increase end‐capillary and transient ( ) only slightly, whereas decreases in produce substantial decreases in ( ) and end‐capillary values. Decreases in alone cannot maintain normal ( ) when is below ∼40 Torr.



Figure 14.

Calculated effects of changes in on end‐capillary and O2 exchange rate. Changes in from normal values (20 Torr, vertical dashed lines) result in increases in either end‐capillary or ( ) but not both and may serve to restore to a normal value.



Figure 15.

Effects of experimentally varying on O2 transfer rate and . Solid lines, theoretical curves.

Adapted from Power and Jenkins


Figure 16.

Effects of varying on ( ) and outflowing umbilical . Solid lines, theoretical curves.

Adapted from Power and Jenkins


Figure 17.

Relation of to that of mother in chronically catheterized sheep (□) and monkeys (Δ) and acutely anesthetized sheep (○).



Figure 18.

values in uterine arterial (•) and venous (○) blood and in umbilical arterial (Δ) and venous (▴) blood as function of . When Pmvo2 was >200 Torr, uterine venous HbO2 was completely saturated and O2 crossing the placenta was almost entirely from that physically dissolved in maternal plasma. In all cases, was < .



Figure 19.

Cascade of from ambient air to maternal and fetal blood at ∼5,000 m (dotted line) as compared with that at sea level (solid line).



Figure 20.

Calculated maternal and fetal blood O2 contents as function of when relative positions of maternal and fetal HbO2 saturation curves are identical; i.e., both Pm50 and Pf50 are 26.5 Torr. Asterisks, and ; A and V, maternal arterial and venous values; a and v, umbilical arterial and venous values.



Figure 21.

Maternal and fetal blood O2 contents as function of when relative positions of maternal and fetal HbO2 saturation curves are reversed. In this instance, affinity of maternal blood is normal (Pm50 = 26.5 Torr), whereas that of fetus is increased (Pf50 = 35 Torr) due to Hb Kansas replacing Hb A. Symbols are as in Fig. .



Figure 22.

Maternal and fetal blood O2 contents as function of when maternal and fetal HbO2 saturation curves are reversed. In this instance, Pm50 = 12 Torr and [Hb]m = 15 g · dl−1, whereas Pf50 = 20 Torr. Symbols are as in Fig. .



Figure 23.

Human maternal and fetal HbO2 saturation curves showing CO effects. The HbO2 saturation ([HbO2]) is that percentage of Hb not bound as HbCO.



Figure 24.

Calculated O2 content vs. of human blood with [HbCO]f = 0% and 10% and [HbCO]m = 0% and 9.4%. [Hb]m and [Hb]f are assumed to be 12 and 16.5 g · dl−1, respectively. Normal arteriovenous O2 difference of 5 ml · dl−1 is assumed for both the uterus and its contents and the fetus. Figure depicts mechanism accounting for reduction of umbilical artery and vein O2 tensions and contents resulting from elevated [HbCO].



Figure 25.

Dissociation curves for CO2 in human maternal and fetal blood showing effects of O2 saturation and base deficit, a, Adult deoxygenated blood; b, adult oxygenated blood; c, pregnant women and fetuses; d, fetal blood. Heavy lines, positions expected for maternal and fetal blood based on degree of oxygenation.

Adapted from Eastman et al.


Figure 26.

Effect of changes in various factors on transient placental CO2 transfer rate. To facilitate comparison, values are plotted as percent changes from assumed normal values. Slope of each curve at 100% may be used as index of relative sensitivity of CO2 transfer to that factor.

Adapted from Hill et al.


Figure 27.

Time course of during single transit through exchange vessels. Solid lines, values within erythrocytes; broken lines, values in plasma. Changes occurring after blood leaves exchange area are plotted on right with scale compressed 40 times.

Adapted from Hill et al.


Figure 28.

Calculated effects of changes in maternal placental blood flow, [Hb], Hb flow, and O2 flow on transient maternal and fetal placental end‐capillary values and ( ) (dashed lines) and on steady‐state end‐capillary values and (solid lines).



Figure 29.

Calculated effects of changes in fetal placental blood flow, [Hb], Hb flow, and O2 flow on transient maternal and fetal placental end‐capillary values and ( ) (dashed lines) and on steady‐state end‐capillary values and (solid lines).



Figure 30.

Relations between venous outflow , placental O2 transfer, and fetal cotyledonary flow in perfused isolated sheep and rabbit placentas. Solid lines, predicted relations using mathematical model.

Adapted from Power and Jenkins


Figure 31.

Calculated blood O2 content as function of in presence of maternal anemia ([Hb]m = 5.75 g · dl−1) with associated slight increase in O2 affinity (Pm50 = 28 Torr). A and V, maternal arterial and venous values; a and v, umbilical arterial and venous values.



Figure 32.

Calculated blood O2 content as function of in presence of fetal anemia ([Hb]f = 8.25 g · dl−1). Symbols are as in Fig. .



Figure 33.

Total uterine vein‐to‐umbilical vein difference at various values of (solid line). Contribution of distribution effect is shown over entire range of values (dashed line) assuming that distribution of maternal to fetal flow ( ) at higher values is similar to that during normal oxygenation. Diffusion effect (shaded area) is shown to be insignificant when exceeds 100 Torr. Dotted line, maternal‐to‐fetal venous difference produced by 15% maternal shunt. Sum of various factors must represent total of this difference.

Adapted from Longo and Power


Figure 34.

Volume of maternal ( ) and fetal ( ) placental flows required to exchange 1 ml of O2. As ratio of to increases above 1.0, progressively more is required. This occurs because equilibration occurs at higher and greater saturation of maternal Hb; therefore less O2 is removed from each milliliter of maternal blood. Similarly, fetal blood undergoes greatest changes in [O2] with high ratios; therefore less fetal flow is required. Sum of both curves equals total maternal and fetal flow (dotted line). Optimum ratio for O2 transport in terms of total flow is ∼0.9.

Adapted from Power and Longo


Figure 35.

Effects of changes in various factors on transient placental O2 exchange rate. Results are normalized by plotting percent change in ( ) as function of percent change in assumed normal value of given factor. As in Fig. , slope of each curve at 100% of normal value indicates relative sensitivity of O2 exchange rate to that factor. ( ) is most sensitive to umbilical arterial , followed closely by total, maternal, and fetal flow rates and O2 affinity of maternal and fetal blood. Curves for maternal and fetal [Hb] are superimposed on and curves. Pm and Pf, maternal and fetal values; Dp, placental diffusing capacity.

Adapted from Longo et al.
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Lawrence D. Longo. Respiratory Gas Exchange in the Placenta. Compr Physiol 2011, Supplement 13: Handbook of Physiology, The Respiratory System, Gas Exchange: 351-401. First published in print 1987. doi: 10.1002/cphy.cp030418