J. exp. BioL (1978), 76, 149-165
With 10 figures
Printed in Great Britain
149
STATIC PRESSURE-VOLUME CURVES FOR THE LUNG
OF THE FROG (RANA PIPIENS)
BY G. M. HUGHES AND G. A. VERGARA*
Research Unit for Comparative Animal Respiration,
University of Bristol, Woodland Road, Bristol BS8 1UG
(Received 20 January 1978)
SUMMARY
1. Static pressure/volume curves have been determined for isolated
frog lungs inflated with either air or saline. In both cases a hysteresis was
present: the pressure required to produce unit change of volume being
greater during inflation than deflation.
2. The pressure necessary for a given volume change was less for the
saline-filled than the air-filled lungs. The difference between these curves
is due to the surface tension at the air/lung interface.
3. Pressure/volume curves for air-filled lungs in situ were similar to curves
for isolated lungs. However, a greater pressure was required for the same
volume change during both inflation and deflation.
4. Compliance was calculated from different parts of air pressure/volume
curves and gave values greater than those obtained using similar calculations
for higher vertebrates.
5. These observations support other evidence for the presence of a surfactant in the lung lining of frogs in spite of the relatively large diameter
of their 'alveoli.' The precise role of such a lining is uncertain and it is
concluded that similar forces may be involved during the inflation and deflation of lungs of frogs and higher vertebrates in spite of differences in
gross morphology.
INTRODUCTION
During the past 30 years evidence has accumulated supporting the view that the
lungs of mammals are lined with a layer of lipids and proteins, probably as lipoprotein, whose main function is to reduce the surface tension of the curved alveolar
surfaces. The first indication of such a lining was probably that obtained by von
Neergaard in 1929 when he demonstrated that the pressure necessary to enlarge the
lungs to a given volume was less when they were filled with liquid than when they
were inflated with air. Replacement of the air-alveolar interface by a liquid-alveolar
interface evidently produced a significant reduction in the retractive forces operating
on the lung, von Neergaard concluded that surface tension forces were important
in the lungs. From pressure/volume experiments approximate calculations were
made which suggested that a considerable surface tension (about 40-50 dyn/cmf) is
• Present address: Department of Pharmacology, The Medical School, University of Bristol,
Bristol BS8 iTD.
t 1 dyn/cm = 1 mN/m.
G. M. HUGHES AND G. A. VERGARA
present at such air-alveolar interfaces (Radford, 1954; Brown, 1957). Brown (1957)
who knew about the lining film, calculated a much lower surface tension in deflated
lungs. Since these initial experiments this hypothesis has been supported by several
other lines of evidence including electron-microscopic studies of the lung lining
layers, chemical analyses of lung tissue and of washings from the lungs and also from
direct measurements of the surface tension using a Wilhelmy balance (Clements,
1957), or a bubble technique (Pattle, 1958). Corresponding studies with the lungs
of lower vertebrates are as yet fragmentary, but in general support the view that
similar lining layers are present in lungfish, amphibians, reptiles and birds (Brooks,
1970; Clements, 1962; Clements, Nellengoben & Trahan, 1970; Hallman & Gluck,
1976; Hughes, 1967, 1970, 1973; Pattle, Schock, Creasey & Hughes 1977). However,
there have been few studies (Cragg, 1975) of pressure-volume relationships corresponding to the initial work of von Neergaard. The purpose of the present investigations
was to determine these relationships for lungs filled with air and saline and as a result
to make estimates of the surface tension of the lung lining. A preliminary report of this
work has been given (Vergara & Hughes, 1977).
MATERIALS AND METHODS
Experiments were carried out on frogs (Rana pipiens) of 15-35 g body weight.
The animals were maintained in the laboratory at 13-14 °C and before cannulation
of the lung they were anaesthetized by an intra-peritoneal injection of a o*i ml
saline solution containing 10% MS 222. A cannula (PP120) was flared at one end
and inserted into the glottis where it was tied firmly. In some experiments inflation
and deflation of the lung was carried out using the intact frog. In most cases, however,
the frog was dissected carefully from the ventral side and one of the lungs isolated
together with its connexion to the cannula. With most preparations, it was only
possible effectively to cannulate a single lung in any given specimen. The isolated
lung was connected into the apparatus shown in Figs. 1 and 2 and usually was used
for air inflation and deflation experiments before being filled with saline. The apparatus and procedures were similar to those used by Young, Tierney & Clements (1970)
and by Cragg (1975). Results obtained in the last of the air P/V curves were compared with those obtained for the second of the saline curves in order to calculate
surface tension. Many lungs contained parasites and in no case were such lungs used
for the experiments.
In the air experiments the cannulated lung was at its normal volume when connected to the apparatus (Fig. 1) and all taps were open to atmospheric pressure.
When the taps were closed, air was injected via a syringe (I/D). Then the pressure
between the two water manometers was adjusted (syringe P) so that there was no
difference in the water levels of manometer II. The pressure necessary to maintain
constant volume (PTU) of the tubes connecting manometer II to the lung was read on
manometer I. Volumes of o*i or 0-2 ml were used for each step of a given inflation/
deflation cycle and the pressure reading was made 15 s after the injection or withdrawal of air. A correction was made for the compression or rarefaction of the air
by means of the following equation:
Frog lung pressure-volume curves
M II
Fig. 1. Diagram of apparatus used for the inflation and deflation experiments using air. SP
is the syringe used for adjusting the levels of M II; S I / D is used to change the volume of
the frog lung. Pressure is read on M I.
M I
Fig. 2. Diagram of apparatus used for inflation and deflation of a frog lung with saline.
Changes in volume during inflation and deflation are measured in S I / D ; SP is syringe
for adjusting pressures.
where VLi = lung volume of any step, PB = barometric pressure, PH>O = water
vapour pressure, Px = pressure for the minimum lung volume, Ps = pressure for
any volume step, VL1 = minimum lung volume, P^i = volume of syringe I/D for
the minimum lung volume, f£2 = volume of syringe I/D for any volume step,
Vra = volume of tubing between lung and manometer II.
Inflation with saline (0-9 % NaCI) was carried out using the apparatus in Fig. 2.
First of all the pressure in the isolated lung was reduced to — 20 cm H,0 using the
air apparatus (Fig. 1). It was then attached to the saline apparatus at this reduced
G. M. HUGHES AND G. A. VERGARA
152
I
10 -
0-5 -
10
Pressure (cm H2O)
Fig. 3. Plots of pressures in a balloon when inflated and then deflated
to different volumes with air (O) or saline (A).
pressure with the lung at a minimal volume. After adjusting (syringe P) the pressure
within the saline apparatus to the same level (— 20 cm H2O) connexion was made to
the lung. Inflation of the lung with saline (syringe I/D) was once more carried out at
steps of o-i-o-2 ml, 3 min being taken for each step. As saline was injected into the
lung the pressure was read off on MI, as it approached atmospheric pressure and
the condition indicated in Fig. 2. Inflation above atmospheric pressure was continued
and followed by deflation. During the saline inflation/deflation experiments the lung
remained submerged in saline.
Before the apparatus was used for experiments with frog lungs some control studies
were made to determine the pressure/volume curves of rubber balloons using air
and saline (Fig. 3). The results indicated that there was no hysteresis in the pressure/
volume relationship because during both inflation and deflation with either air or
saline, the sigmoid-shaped curves were perfectly matched. Similar curves could be
obtained provided the balloons were used on no more than three or four occasions.
Lungs were fixed at different inflation volumes by immersion in a 2-5 % glutaraldehyde solution in a cacodylate buffer (pH 7-2) before embedding in an Epon-Araldite
Frog lung pressure-volume curves
1
2
3
4
5
6
7
Pressure (cm H2O)
8
153
9
Fig. 4. Typical pressure/volume curves during inflation and deflation of a lung isolated from
a frog of 27 g. Separate curves are plotted during inflation/deflation with air and saline.
mixture. Large sections, covering the whole cross-section, were cut with a LKB
ultratome and stained with toludine blue. These sections were analysed morphometrically (Hughes & Weibel, 1976) by means of intersection and point counting
using a projection microscope.
RESULTS
1. Air pressure/volume relationships
During inflation of the lung from atmospheric pressure, the increment in pressure
per unit volume change decreased rapidly until there was very little change in pressure
although lung volume increased very significantly. Above this level the curve showed
its sigmoid nature as the pressure rose more steeply during the final 1 ml of inflation.
The deflation curve was similar in general shape but was shifted to the left relative
to the inflation curve (Fig. 4). Consequently the pressure at any given volume is less
during deflation than during inflation. This marked hysteresis was found in all preparations. Differences in the precise nature of the curves were observed when repeated inflation and deflation cycles were carried out.
G. M. HUGHES AND G. A. VERGARA
Pressure (cm H2O)
Fig. 5. Curve showing the relationship between pressure and volume during inflation of a
frog lung with air and saline. The dashed Line shows the pressure (PJ used for calculating the
constant K and the surface tension. Weight of frog, 27 g.
2. Pressure/volume relationships of saline-filled lungs
The general form of the curves is similar to that obtained with air but during both
inflation and deflation the pressure changes were very much less for any given volume
change. The sigmoid shape of these curves was less pronounced with saline-filled
lungs as also was the hysteresis (Fig. 4). Although variations were found in the precise
nature of these curves according to the size of the frog, the same qualitative differences
were always observed. There was no significant change in the shape or the hysteresis
of the P/V curve when the rate of change of volume was altered. In Fig. 5 the inflation of the curves are plotted and a line drawn to indicate the difference in pressure
at one particular volume.
3. Pressure/volume curves from, lungs in situ
Pressure/volume curves showing hysteresis for lungs in the whole animal are
shown in Fig. 6 for comparison with those obtained for isolated preparations. The
curves obtained from the intact animal (Fig. 6 a) did not change very markedly when
the frog was opened up and the air pressure/volume relationship once more determined (Fig. 6 b). But when the heart, pericardium and liver were removed the resting
Frog lung pressure-volume curves
50 -
40 /
30 -
it
20
10
1
-3
-2
|
-I
ff
1
• Intact animal
± In situ, intact
• In situ, without
liver and heart
i
0
i
i
i
i
1 2
3 4
Pressure (cm H2O)
Fig. 6. Plots showing P/V loops for a frog during progressive isolation
of the lung. For explanation see text.
volume of the lung at atmospheric pressure was reduced (Fig. 6 c). Thus in the whole
animal a greater pressure was required for the same volume change - presumably
because of the limitations imposed by the viscera and body wall.
4. Lung compliance
From the pressure/volume curve described above it is possible to obtain values for
lung compliance. It is evident that the compliance during saline filling is greater
than during filling with air, the pressure changes for a given volume of saline injection
being much less than those following a comparable volume change produced by air
injection. The compliance and area of the hysteresis loop during air filling were almost
identical both before and after the lung had been inflated with saline.
Estimates of compliance usually indicate the slope of some part of the pressure/
volume curve. In the frog little data is available concerning the range of volume
changes during normal ventilation. Calculations were therefore made on all parts
of the pressure/volume curves for the isolated and in situ lungs. Fig. 7 shows plots
of the compliance in relation to the relative volume; each point represents the compliance at intervals of 1 cm H2O pressure change. The maximum compliance was
found when the volume of the lung was about half its maximum inflation, i.e. 50%
156
G. M. HUGHES AND G. A. VERGARA
-Inflation
^ — h
6
Deflation-
100 100
200
100
200
•a Human
Humana
100
• Cat "
10
Rabbit1
Frog
Lizard
01
B Mouse
0
10 20 30 40 50 60 70 80 90 100 100 90 80 70 60 50 40
Relative volume (%)
30 20 10
0
Fig. 7. Compliance at different volumes (% maximum inflation) obtained from P/V curves
of isolated lungs from a variety of vertebrates (references 2,3, 5, 7, 8, 11 in Table 1). In order
to clarify the change in compliance which occurs between the end of inflation and the beginning of deflation, the region of maximum volume (i.e. 100% relative volume) ha» been
expanded in this figure.
of the volume at which an increase in pressure gave scarcely any increase in volume.
The minimum values occurred at the end of inflation and the beginning of deflation
(i.e. 100% relative volume). During deflation the value rose to approximately the
same maximum as during inflation. An approximately tenfold range of compliance
was found during inflation for any frog lung. Analysis of the pressure/volume curves
from intact preparations (Fig. 8) gave essentially the same results, but the whole range
of values was slightly lower. It is evident from the plots given in Fig. 7 that values
for the frog are about ten times greater than corresponding values for the mouse or
lizard of approximately the same body mass.
DISCUSSION
(a) Lung compliance
As a result of the investigations on the frog lung an interest developed in the range
of compliance values obtained in different parts of the pressure/volume curve, and a
comparison with results obtained for mammals. Values normally given for the
mammal are in relation to the functional residual capacity (shown as the first point
plotted in Fig. 7), but for comparative purposes a number of published pressure/
volume curves have been analysed. Maximum and minimum values for compliance
derived from such curves are summarized in Table 1. In making comparative studies
scaling factors must always be taken into account and for mammals (Stahl, 1967)
the relationship has been shown to be:
lung compliance (at F.R.C.) = 0-0021 W1'08 (W in g).
:
Frog lung pressure-volume curves
200 -
- 200
100:
..-.
:
" • <s.
.
. •' •
~
-
.
"Z.
o
01b
.
0
.
.
.
I
I
I
I
^
\ \ // /
A
I
I
dio
Frog
\f
^
Protopterus ~
/
^ ^ y \ S _ /
.2
\
Cat
.-•
1=
. . ^ ^ — - ^
Frog Lizard ^r
-^^^
Froi
r~^ Frog*^^
•*•
=100
' • € Human
/
' " •
\
e
r
2
.
^
<Cat*--^\
atir
iob
. • • • ' " ' • • • • .
x
Human • . " • '
Protopterus
s
57
_
I
'Lizard
dl
!
^
I
I
1
dOl
1
I
1
I
I
I
10 20 30 40 50 60 70 80 90 100 100 90 80 70 60 50 40 30 20 10 0
Relative volume (%)
Fig. 8. Compliance at different volumes (% maximum inflation) obtained from P/V curves of
in situ lungs of a variety of vertebrates (references a, 4, 8, 9, 10 and 11 in Table 1).
During the present survey a similar relationship has been shown for maximum
compliance ( = 0-00244 W10*9) determined from the P/V curves of mammals and
a relationship having a similar slope ( = 0*036 W0*9*) was obtained for lower vertebrates (Fig. 9). The tenfold difference in the intercept values for these regression
lines indicates that at almost all body sizes a lower vertebrate lung would be expected
to have a compliance that is approximately ten times that for a mammal of comparable
body size. Thus the deduction based upon the relationships between frog and mouse
lungs (Fig. 7) is given further support.
It is also possible to compare the result obtained from these static pressure/volume
relationships with the P/V curves obtained by West & Jones (1975) for the dynamic
relationships recorded during pulmonary ventilation. Calculations of compliance for
different parts of their curves suggest a similar range of values (Fig. 8). Thus it can
be concluded that the values of compliance obtained in this study give a good indication of the type of compliance relationship which operates in the normal pulmonary
ventilation of Rana pipiens. Presumably the greater compliance of the frog lung reduces the work of breathing.
(b) Surface tension
The general form of the pressure/volume curves obtained using both air and saline
was very similar and showed a hysteresis in all cases. The pressure required for
inflation to a given volume using saline was always less than with air. These observations support the view that the lung lining contains a substance which reduces
surface tension forces within the lung. The finding that no such differences in
6
EXB 76
158
G. M. HUGHES AND G. A. VERGARA
Table 2. Summary of measurements made on the surface tension
of material obtained from vertebrate lungs
Species
Material
Method
Surface
tension
(mN/m)
Reference
deLemos et al. (1969)
Lamb
Lung extract
W.B.*
Max: 36
Min: 6
(Area i S %)
Dog
Lung washings
W.B.
Max: 30
Min: 0-3
(Areai 7 %)
Finley et al. (1968)
Dog
Lung extract
WJ.
Max: 35
Min: 13
(Area 20%)
Modes & Vergara (1973)
Dog
Cat
Rat
'Lung'
Brown, Johnson & Clements (1959)
P/V curves Max: 50
Min: 5
(Area 10-20%)
Cat
'Lung'
P/V curves Max: 50
Min: o-a
(Area ao %)
Rat
Cat
Lung extract
W.B.
Rat
Compression
fresh lung
W.B.
Guinea pig
Lung extract
and washings
W.B.
Pigeon
Lung extract
W.B.
Chicken
Compression
fresh lung
W.B.
Turtle
Compression
fresh lung
WJ.
Frog
Compression
fresh lung
W.B.
Dog
Max: 40-30
Min: 10-15
(Area 30%)
Max: 40
Min: 19
(Area 20%)
Max: 40
Min: 8
(Area 20%)
Min: 18
(Area 20%)
Max: 52
Min: 29
(Area 20%)
Max: 52
Min: 28
(Area 20%)
Max: 55
Min: 30
(Areaao%)
Fisher, Wilson & Weber (1970)
Brown et al. (1959)
Miller & Bondurant (1961)
Motles et al. (1971)
Klaus et al. (1962)
Miller & Bondurant (1961)
Miller & Bondurant (1961)
Miller & Bondurant (1961)
Frog (R. pipieni) 'Lung'
P/V curves Max: 50
Min: 0-3
(Area 25 %)
This study
Squeezed
Clawed toad
(Xenopus laevit) fresh lung
Bubble
Pattle & Hopkinson (1063)
Chicken (Gailus) Squeezed
fresh lung
Mammal
Squeezed
fresh lung
Lung extract
Toad
Pattle & Hopkinson (1963)
Stability
ratio: 0-76
Pattle & Hopkinson (1963)
Stability
Bubble
ratio: o-6-o-87
Klaus et al. (1962)
W.B.
Min: 18
(Area 20%)
• Wilhelmy Balance.
Bubble
Stability
ratio: o-8i
Frog lung pressure-volume curves
^i 1000
lOOOp
Human
100
o
K=0036WOW4
r=0-957
X
u
8
10
10
D.
O
u
K=10ml
\Frog.
Fr
°g"/V=10ml
Lizard•
X
i (t Mouse i
= 00024 W 1 0 4 »
r=0-969
i i i 11 in
103
Body weight (g)
10*
i
ii
i 11n
103
Fig. 9. Log-log plot of maximum compliance of the lungs of vertebrates against body weight.
Regression line* are shown for mammals and lower vertebrates. A dashed line shows the
relationship obtained by Stahl (1967) for mammalian lung compliance at resting volume
(i.e.
F.R-C).
pressure/volume curves were observed in balloons inflated with air or saline is also
in agreement with this interpretation.
Assuming that the forces due to tissue elasticity remain the same when the lung
is filled with air or saline and that there are no changes in the basic geometry of the
alveoli and other air spaces, then it is probable that the only mechanisms responsible
for the difference in pressure/volume relationships are due to the surface forces at
the air-liquid interface.
From each pair of air and saline pressure/volume curves calculations were made of
the surface tension using a method based upon that of Fisher, Wilson & Weber
(1970) and Bachofen, Hildebrandt & Bachofen (1970). In this method it is necessary
to assume a maximum value for the surface tension, and for this purpose the
value used (50 mN/m) was based upon measurements of the surface tension of
washings from mammalian lung using a Wilhelmy surface tension balance (Table 2).
It has often been assumed that the internal surface area of a vertebrate lung (mammal)
is proportional to the two-thirds power of the lung volume. In a lung containing
large numbers of alveoli approximately spherical in shape, such an assumption is
reasonable, but for a lung so different in basic organization as that of the frog it was
decided to investigate this relationship in more detail. Morphometric estimates of the
air volume were obtained from point counts of sectioned material and an estimate of
the internal surface area was obtained from intersection counting. Results obtained
6-2
i6o
G. M. HUGHES AND G. A. VERGARA
100-
0
10
20
30
40
50
Surface tension (dyn/cm)
Fig. io. Plot of surface tension against relative surface area for a frog lung.
For detailed explanation see text
for lungs of different inflation volumes showed that the surface area is more closely
related to the one-half power of the volume (SA oc V°~a).
(c) Calculation
P aU and Pganne are the pressures of the air and saline-filled lungs; Pair/water *8 the
pressure due to surface forces = PB (Fig. 5), i.e. pressure differences between air
and saline curves at any volume.
The work done on the surface lining can be expressed as
W=Ps.dV or
W=y.dA,
where dV = increment in volume; dA = increment in area, and y = surface tension.
From morphometric analysis:
A = K.Vi,
which on differentiating:
. . K.dV
dA =
-
(1)
Frog king pressure^volume
tiTable i . Maximum
curves
161
and minimum values for lung compliance from
pressure/volume
curves of vertebrates: volumes and pressures at maximum inflation are also given
Species
Human
Tot. reap. syst.
Human
Lungs alone
Tot. resp. syst.
Dog
Both lungs
Monkey*
Dog*
Cat
Tot. resp. syst.
Cat*
Rabbit*
Rat*
Mouse*
Lizard
Both lungs
(Lacerta ticula)
Tot. pulm. syst.
{Lacerta viridit)
Frog (R. pipiens)
Tot. pulm. syst.
Frog (JR. pipiens)
Both lungs
Tot. pulm. syBt
Fish (Protopterw)
Tot. pulm. syst.
Maximum
compliance
(ml/cm H,O)
*
Inn.
Deft.
Minimum
compliance
(ml/cm H,O)
Inn.
Den.
Maximum
inflation
Volume Pressure
(ml) (cm H,O)
Body
weightf
(g)
Ref.
250
310
40
10
5300
25
(76400)
1
170
180
142
no
58
80
4a
2700
3000
136
136
(44650)
(44650)
a
203
44
35
4«
2000
400
243
17-5
as-a
(30460)
(6670)
(6570)
3
3
(5S6s)
1-9
34-0
ai-5
31
19-8
3O-O
"3
201
33
31
193
aa
6-5
o-35
16
2
1
12
7
489
57
4
40
98
0-47
390
i-7
33o
1
300
(1375)
285
28
25
5
8
a3S
39
8
O'll
O IO
0-21
0-38
o-io
o-oa
05a
O-75
i-oo
O-O5
o-oa
71
x-78
140
0-16
0-06
a-8
60
80
a-65
300
028
038
0-05
0-03
5-5
56
5-8
84
26
36
50-0
9-0
500
1-37
33-8
I-I
o-io
o-oi
240
75
7-3
1
4
3
5
6
7
(5090)
3-3
O-I3
OO2
-
3
11
11
• Probably both lungs.
t Weights in parentheses have been estimated from lung volumes given by original authors, using
the relationship: total lung capacity = 53-5 W1"0* (Stahl, 1067).
t References: 1, Mead, Whittenberger & Radford (1957); 2, Butler (1957); 3, Bachofen et al. (1970);
4, Radford (1964); 5, Lempert& Macklem (1971); 6, Weiss & Jumia(i97i); 7, Morstatter et al. (1976);
8, Cragg (1975); 9, West & Jones (197s); 10, G. M. Hughes (unpublished); n , this study.
Then
2.
Hence
r=
K
'
and
P
K
(3)
To calculate K, the maximum pressure difference (P,,m»x) between the air and
saline inflation curves was taken together with the volume at which this occurred
(Fig. 5). The mean value of K for five preparations was 261; S.E. ± 18. The corresponding value for cat lung is 1225; S.E. ± 100 (Fisher et al. 1970).
162
G. M. HUGHES AND G. A. VERGARA
It is evident from the calculations summarized above that insertion of values feu
K and appropriate volumes and Ps in equation (3) gives the surface tension at differerl
volumes. Some of the values obtained are given in Table 2, together with the results
of surface tension measurements on a variety of tetrapod lungs using different
methods.
Changes in surface tension associated with changing surface area plotted out as
a change in relative area, calculated on the basis of the J-power relationship to lung
volume are given in Fig. 10. This plot shows a hysteresis loop which encloses a
smaller area than that obtained for a mammalian lung (Fisher et al. 1970).
When interpreting the curves relating surface tension to surface area for both the
frog and mammalian lungs it is apparent that the fall in surface tension is very rapid
following a decrease of only 15-20 % in surface area. This contrasts with the results
obtained with the surface-tension balance, using washings or extracts of mammalian
lungs, where a much greater reduction in area, by about 40-50 % of its original area,
is necessary to obtain a comparable reduction in surface tension, i.e. to about 5-10 mN/m.
When the lung volume, and consequently surface area, is reduced to about 50%
of the total lung capacity, the surface tension falls to about 5 % of its maximum value.
At small volumes, however, the differences in pressure between the air and saline
deflation curves approach zero and consequently the surface tension must also
approach zero. During re-expansion of the lung the surface tension rises once more
but at any given surface area is always less during deflation than during inflation.
Miller & Bondurant (1961) studied the surface tension/area loop of extracts of frog
lung with a Wilhelmy balance. They found a minimum value of 30 dyn/cm when
the surface was reduced to 20 % of its original area. The area of the hysteresis loop
was half that found for rat lungs under comparable conditions. No mention was
made of the temperature at which these experiments were carried out. This parameter has a very important effect on surface activity and has been shown to be
especially relevant in relation to the properties of frog lung surfactant (Pattle et al.
1977). The experiments described in the present paper were carried out at 13 °C.
However, it is difficult to compare satisfactorily the loops obtained with a surface
balance by Miller and Bondurant and those described here using P/V curves, as
many authors have found a discrepancy between the results of the two methods for
mammalian lungs.
One advantage of the present study is that it is based on a morphometric determination of the relationship between surface area and volume of the lung without any
assumption concerned with the shape of the basic units.
In conclusion it may be stated that evidence for the existence of a surfactant lining
in the frog lung has now been increased by the results of these pressure/volume
studies. Added to the known evidence derived from electron microscopy, and measurements on bubbles squeezed from frog lungs, it seems safe to conclude that such a
lining is present. The possible function of such a lining is not very clear at first sight.
The function usually applicable to the mammalian lung does not seem ideal because
of the much larger dimensions of the alveolar units. Nevertheless, it is very probable
that the presence of a surface tension reducing film would play an important role in
the early stage of inflation of these lungs, especially in small frogs, for they are almost
completely emptied during expiration.
Frog lung pressure-volume curves
163
It was noticeable during the inflation experiments that the lung tended to fill first
In its more anterior regions, i.e. those regions closest to the glottis, and perhaps
during such a stage of the ventilatory cycle the lining film would be important.
The range of the pressure and volume changes employed in the present study are
clearly in excess of those of the normal ventilatory cycle. However, it is well known
that the lungs of frogs are inflated to volumes which greatly exceed the normal dimensions at full inspiration in certain stages of the life-cycle, e.g. vocalization during
the breeding season. The lungs are also inflated supramaximally to serve a buoyancy
function. Thus it can be concluded that the total range of pressure/volume relationships shown here approximates to that which normally occurs in such structures.
From comparison of washings obtained from frog lungs with washings from
mammalian lungs (Hughes & Vergara, 1978), it appears that the lining layers are
more similar in their composition than was at first supposed and this becomes especially apparent when the concentrations of the different phospholipid components
are compared in relation to the area of their internal surfaces. Thus, in spite of the
differences in alveolar dimensions, these similarities suggest that perhaps the lining
layers may perform similar functions. Consequently the supposed role of such layers
in reducing the surface tension forces may have been over-emphasized. An 'antiglue' function of a surfactant lining is clearly a possibility, as has been suggested for
the mammalian lung (Sanderson et al. 1976). In the case of the mammalian lung,
some morphometric and physiological investigations (Weibel et al. 1973; Hills, 1971)
have suggested that many alveoli collapse at different stages during the ventilatory
cycle and the alveolar linings slide over one another as they open and close. Similar
geometric modifications might also take place in lungs of the frog type (Hughes,
1978); in fact material fixed at different levels of inflation has confirmed this possibility. Until further evidence is forthcoming such changes in overall lung morphology
must be speculative, but at least such an hypothesis has the advantage of enabling
us to understand similarities between lungs which may have wide differences in their
surface properties because of their dimensions alone.
We wish to thank the Medical Research Council for providing a research grant
which made it possible to carry out this work.
We are grateful to Professor Robert Bils for skilfully cutting the large sections of
frog lungs.
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