Transvascular and intravascular fluid transport in the rainbow trout: revisiting Starling's forces, the secondary circulation and interstitial compliance
Indiana University School of Medicine, South Bend Center for Medical Education, University of Notre Dame, Notre Dame IN 46556, USA
* Author for correspondence (e-mail: olson.1{at}nd.edu)
Accepted 7 October 2002
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Summary |
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Key words: fish cardiovascular system, transcapillary fluid filtration, oncotic pressure, hemorrhage, microcirculation, rainbow trout, Oncorhynchus mykiss
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Introduction |
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Invasion of the terrestrial environment, however, added a gravitational
force on body fluids and this created problems that were not anticipated in an
aquatic environment. Anti-gravity responses of the mammalian cardiovascular
system are well known and include an elevated arterial blood pressure to move
blood above the heart and venous reflexes to prevent blood from pooling below
it (Rowell, 1993). Satchell
(1991
) realized this and he
suggested that, because fish are neutrally buoyant relative to their
environment, they needed neither high blood pressure nor active control of
venous tone. With the exception of tunas, blood pressure in fish is indeed low
(Olson, 1997
); however, it is
also evident that active regulation of venous capacitance is necessary to
regulate cardiac output and to respond to alterations in blood volume and the
effects of acceleration (Olson,
1997
).
Attendant with an elevated arterial blood pressure and increased
intravascular pressure in dependent vessels is the need to minimize fluid
extravasation across the capillaries. To achieve this, mammalian capillaries
act as a barrier to plasma protein and maintain a sufficient protein
concentration gradient between plasma and interstitial fluid to offset the
hydraulic tendency for transvascular fluid filtration. Collectively these are
the well-known Starling's forces. In addition, lymph vessels return the small
amount of extravasated protein back to the circulation along with excess
filtered fluid (Renkin and Tucker,
1995; Aukland and Reed,
1993
).
Considerable controversy surrounds our understanding of transvascular fluid
balance in fish. In a review of the literature, Olson
(1992) suggested that the full
compliment of Starling's forces may not be applicable in fish because fish
capillaries appear to be relatively permeable to protein. This conclusion was
based on observations that protein concentration in peritoneal and
subcutaneous fluid was similar to that in plasma
(Turner, 1937
;
Hargens et al., 1974
) and that
blood volumes measured with protein indicators consistently exceeded those
determined from tagged red blood cells. More recently, Bushnell et al.
(1998
) measured the time course
of appearance of 125I-albumin in trout tissues following
intra-arterial injection and observed a rapid and sustained accumulation of
radiolabel in many tissues, including gut and skeletal muscle, which they
attributed to protein extravasation.
The extent of the so-called secondary circulation and an apparent lack of a
true lymphatic system in fish is a second point of contention. There is
adequate anatomical evidence for a secondary vascular system
(Vogel, 1985;
Steffensen and Lomholt, 1992
;
Olson, 1996
), but estimates of
the volume of the secondary circulation have been based on kinetic
measurements of 125I-albumin disappearance from the primary
circulation (dorsal aorta), and the conclusion that the volume of the
secondary circulation in rainbow trout is 1.5 times that of the primary
circulation (Steffensen and Lomholt,
1992
) has been questioned
(Bushnell et al., 1998
). Two
assumptions were made in the study by Steffensen and Lomholt
(1992
): (1) that fish
capillaries are relatively impermeable to protein, and (2) the kinetic
components can accurately separate mixing within and between the different
vascular compartments. To date there is little evidence to support or refute
either of these assumptions. Obviously, if fish capillaries were permeable to
protein and the kinetics of protein extravasation were near those attributed
to mixing into the secondary circulation, the estimates of Steffensen and
Lomholt (1992
) would be
erroneous.
In the present study we used a different approach to estimate fluid compartments and protein permeability in fish. Hematocrit was monitored in spleenectomized rainbow trout at 5 min intervals following volume loading with either saline or trout plasma, or after hemorrhage. In another group of spleenectomized fish, hematocrits were measured continuously during and after hemorrhage. These methods (1) eliminated the kinetic mixing-in of exogenous indicators, (2) provided an estimate of both the volume and rate of fluid movement within the vasculature and between intravascular and extravascular compartments, and (3) allowed us to assess the contribution of plasma protein to Starling's forces. In addition, we obtained the first estimates of interstitial compliance in any fish and found evidence for a red-blood-cell-poor vascular compartment that could be rapidly mobilized in response to hypovolemia.
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Materials and methods |
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Methods for cannulation of the dorsal aorta (DA) and caudal vein (CV) have
been described in detail (Olson et al.,
1997; Perry et al.,
1999
). Trout were anesthetized in benzocaine
(ethyl-p-aminobenzoate; 1:12 000 w:v) prior to surgery. The DA was
cannulated percutaneously through the roof of the buccal cavity with
heat-tapered polyethylene tubing (PE 60); the gills were not irrigated during
this brief (<1 min) procedure. The gills were then continuously irrigated
with 10°C aerated water containing 1:24 000 w:v benzocaine, during CV
cannulation and spleenectomy. The CV was cannulated with beveled PE 50 tubing
via a lateral incision at the level of the caudal peduncle to expose
the hemal arch. The wound was closed with interrupted silk sutures and the
cannula was affixed to the caudal peduncle with an additional suture. Both DA
and CV cannulae were filled with heparinized [100 USP (United States
Pharmacopeia) ml-1] saline (0.9% NaCl). The spleen was exteriorized
via a 1 cm incision slightly right-lateral to the ventral midline and
anterior to the pelvic fin. The splenic arteries were ligated with heavy silk,
the spleen was removed, and the wound closed with interrupted silk sutures.
The fish was then revived and placed in a black plastic tube in a 100 liter
tank with free-flowing, aerated tapwater.
The following day the cannulae were connected to a peristaltic pump and
blood was pumped from the DA to the CV at a rate of 5 ml min-1.
Duplicate blood samples were withdrawn into 25 µl microhematocrit tubes,
centrifuged and read on a hematocrit reader. Samples were collected 5 min
prior to and at 5 min intervals for 1 h after volume expansion or depletion.
In one group of fish, blood volume was expanded by infusion of a volume of
saline equivalent to 40% of the estimated plasma volume
(VPE). The second group of trout were injected with an
equivalent volume of trout plasma previously obtained (within 1 h prior to
infusion) from donor fish. VPE was estimated from an
assumed blood volume (VB) of 35 ml kg-1
(Duff et al., 1987) and the
hematocrit of the experimental fish prior to volume expansion
(Hct0);
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Hematocrit in the fifth group of trout was continuously monitored during
hemorrhage using the impedance flow-cell method developed by Tanaka et al.
(1976) and modified by S. S.
Hillman (personal communication). The flow cell was placed on the aspiration
(DA) side of the peristaltic pump and impedance was recorded at 100 ms
intervals using Labtech notebook software. 1 s block averages were archived on
a computer for 10 min before and 1 h after hemorrhage of 35% of the estimated
blood volume. Hemorrhage was achieved by diverting blood on the discharge side
of the pump away from the fish and this usually took approx. 4 min. It was not
possible to draw blood directly from the DA because this interrupted flow
through the flow-cell and affected the impedance. Hematocrits were also
measured manually by collecting samples from the pump discharge at 10-15 min
intervals throughout the experiment, and these were used to calibrate the
impedance cell. The dead space of the flow-cell and arterial cannula was 0.5
ml; the dead space of the entire extra-corporeal loop was 1.2 ml.
A sixth group of fish served as controls to ensure that the extra-corporeal pump did not affect blood volume or red cell integrity. These fish were instrumented as above and hematocrits were measured at 5 min intervals for 1 h. Hematocrit remained constant for the 60 min sampling period (Fig. 1).
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Calculations
A curve-fitting program (Jandel) was applied, assuming a mono-exponential
recovery, to determine the rate constants of the averaged data for all
experiments in which hematocrit was manually determined. The equation used for
volume recovery following infusion was:
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It was not possible to apply the curve-fit analysis to data from individual fish because we could only read hematocrits down to 0.5%, and this frequently created enough variation to preclude a good fit of the curve. All curve-fitting was done on averaged volumes, except for the instantaneous hematocrit measurements (see below). If the curves for volume expansion (Equation 2) were forced through the actual volume expansion at t=0, the r2<0.8; however, when the curves were derived at t=5-60 min, r2=0.99. Similarly, hemorrhage curves (Equation 3) inadequately described the data points when they were forced through t=5 min. Failure to fit the curves around t=0-5 min has physiological significance and this is described in detail in the Discussion. Other values are presented as mean ± S.E.M.
The instantaneous hematocrit response to hemorrhage clearly showed that two processes were involved. The first response appeared to occur only while blood was being withdrawn and ended within 15-30 s after the end of the hemorrhage period. This rapid response was not amenable to curve-fitting. The second response became evident after the first had ended and followed a mono-exponential relationship. In order to avoid artifact from the transition between these two events, the starting point for curve-fit analysis of the second response was 1 min after hemorrhage was completed. The resultant curve was extrapolated back to the onset of hemorrhage to predict the volume contribution of each process (see Fig. 4).
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Statistics
Comparisons were made with Student's t-test; significance was
assumed at P0.05. Values were expressed as mean ± standard
error of the mean (S.E.M.), or mean + S.E.M. in Figs
1,2,3.
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Results |
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Volume expansion
The kinetics of volume restoration after saline or trout plasma infusion
are shown in Fig. 2, and in
Table 1. 60 min after infusion
of 12 trout with 10.0±1.0 ml kg-1 saline (40% of the
estimated plasma volume), 7.3±0.8 ml kg-1 of fluid had been
transferred out of the vasculature and 28.1±3.6% of the injected volume
remained in the circulation. In the same period after 9.6±0.5 ml
kg-1 of trout plasma infusion (also 40% of the estimated plasma
volume) into 15 trout, 5.9±0.6 ml kg-1 of fluid had been
transferred out of the vasculature while 37.9±6.0% of the injected
volume remained in the circulation. The actual volume expansion (sum of
pre-expansion plasma volume and infusion volume) at t=0 after the
onset of infusion was consistently less than the plasma volume predicted by
Equation 2 at the same time period for both saline infusion (35.3±1.1
ml kg-1 versus 39.2 ml kg-1, respectively), and
after plasma infusion (33.9±1.0 ml kg-1 versus 36.5
ml kg-1, respectively).
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The rate constant for volume recovery following saline infusion was twice that for plasma infusion and the half-time for recovery from saline infusion was half that of plasma infusion (Table 1). However, in spite of the slower rate of plasma efflux from the vasculature, the predicted equilibrium plasma volumes (the plasma volume after fluid efflux from the vasculature had stabilized; estimated from Equation 2) for saline and plasma infusion were essentially the same (Table 1). Thus volume restoration was slower after trout plasma infusion, but the presence of trout plasma proteins did not quantitatively affect the actual volume restored.
Hemorrhage
The kinetics of volume restoration after 20% or 35% hemorrhage are shown in
Fig. 3 and in
Table 2. Within 5 min after
either 20% or 35% hemorrhage, the plasma volume estimated from the change in
hematocrit appeared to be essentially back to normal
(Fig. 3). By 60 min after 20%
and 35% hemorrhage, the estimated blood volumes were 41.2±1.7 and
40.9±1.8 ml kg-1, respectively, or 117.7±5.0 and
116.8±5.2% of the initial blood volume. The estimated blood volume
(based on hematocrit) greatly exceeded the predicted blood volume (from
Equation 3) at 5 min after hemorrhage; following 20% hemorrhage the estimated
blood volume was 34.5±1 ml kg-1, and blood volume predicted
from Equation 3 was 31.4 ml kg-1; after 35% hemorrhage the
estimated blood volume was 31.2±1.0 and the predicted blood volume was
27.1 ml kg-1. Despite the fact that considerably more blood was
removed by 35% hemorrhage compared to 20% hemorrhage (12.3 versus 7
ml kg-1, respectively), the predicted blood volumes after recovery
from 20% or 35% hemorrhage were essentially identical, as were the rate
constants for recovery and the half-times
(Table 2).
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Instantaneous hematocrit
When hematocrit was measured continuously, it was evident that removal of
35% of the blood volume produced a two-phase change in hematocrit
(Fig. 4;
Table 3). Within seconds after
the onset of hemorrhage, hematocrit began to fall rapidly and it continued its
rapid descent until shortly after the blood was withdrawn. Thereafter, there
was an abrupt transition to the second, slower phase. The time course of the
rapid phase appeared to be directly coupled to the duration of hemorrhage and
the faster blood was withdrawn, the faster hematocrit fell. The slow phase
followed a mono-exponential time course with a rate constant (0.052;
Table 3) similar to those
determined for plasma volume loading (0.045;
Table 1) and 20% and 35%
hemorrhage (0.052 and 0.046, respectively;
Table 2). The rapid response
accounted for approximately 25% of the total volume response and the predicted
blood volume at equilibrium (40.7 ml kg-1) was 16% greater than the
estimated blood volume prior to hemorrhage (35 ml kg-1).
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Discussion |
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Vascular and interstitial compliance and transvascular fluid
filtration
The rate and degree of recovery of hematocrit following volume expansion
with either saline or plasma indicate that fluid rapidly leaves the primary
circulation, and that at equilibrium only 30% of the injected fluid remains in
the primary circulation. As will be demonstrated below, we feel that there is
sufficient justification to omit the hematocrit responses during the first 5
min after infusion when estimating vascular and interstitial compliance and
transvascular filtration.
Our estimates of compliance and transvascular fluid filtration rate are
based on two assumptions. First, we assume that fluid lost from the primary
circulation enters the interstitial space and not the secondary circulation.
Steffensen and Lomholt (1992)
have shown that the flow between the primary and secondary circulations in
trout is 6 ml kg-1 h-1, which is less than our observed
saline loss in 25 min (6.4±ml kg-1, or 15.36 ml
kg-1 h-1). It could be argued that volume expansion
increased the flow of saline into the secondary circulation; however,
comparison of the kinetics following volume expansion with saline
versus plasma argue against this. If saline and plasma entered the
secondary circulation, the kinetics of plasma volume recovery following volume
loading with saline and plasma (Fig.
2) should be identical, but they are not. In fact, the rate
constant for volume recovery following volume loading with plasma is less than
half the rate constant following saline loading
(Table 1). It seems unlikely
that the presence of plasma proteins could have such a substantial affect on
the rate of fluid entry into the secondary circulation. The most plausible way
to account for the delayed loss of volume from the circulation following
plasma infusion is by a barrier that delays protein transit. Capillary
endothelia and the interstitial matrix have this barrier, the secondary
circulation does not. It is possible that transcapillary fluid efflux and
entry into the secondary circulation are occurring simultaneously, although
this also seems unlikely because both conditions are described by a
monoexponential recovery (except for the instantaneous phase, discussed
below). Furthermore, our observation that the rate constants for fluid efflux
from the circulation following plasma infusion and those for fluid recovery
following hemorrhage are virtually identical indicates that essentially all of
the fluid influx following hemorrhage follows the same pathway as fluid efflux
following volume expansion with plasma. While our experiments do not allow
quantitative differentiation between transcapillary fluid movement and fluid
flux between the primary and secondary compartments, we feel that the data
strongly support the transcapillary route and that the contribution by the
secondary circulation may be quite small.
Our second assumption is that the change in central venous pressure
following volume expansion is an accurate indicator of simultaneous changes in
capillary hydraulic pressure. Determination of the fluid filtration
coefficient, vascular compliance and interstitial compliance are all based on
a relative change in pressure; it is not necessary to know the absolute
capillary or interstitial pressures. If we assume that arterial resistance is
greater than venous resistance, then changes in venous pressure will most
nearly reflect changes in capillary pressure during volume expansion.
Furthermore, the net change in capillary pressure at equilibrium will reflect
a similar change in interstitial pressure at equilibrium. Hoagland
(2001) measured the rapid
(within seconds) change in arterial and central venous pressure following
volume expansion with whole blood up to 150% of blood volume in trout with an
intact pericardium. These data, 2.41 mmHg at rest, 5.35 mmHg after 40% volume
expansion and 3.35 mmHg at 12% volume expansion (the latter is equivalent to
the equilibrium period after volume loading when only 30% of expansion fluid
remains in the vasculature), can be used to predict the effect of volume
expansion on the change in central venous pressure (and, therefore, the change
in capillary pressure) in the present experiments. In addition, because
central venous pressure is linearly related to volume over the expansion
volumes employed in this study (100-150% blood volume) and because changes in
volume and central venous pressure are temporally coupled within seconds
(Zhang et al., 1995
;
Hoagland, 2001
), it can be
assumed that the fall in capillary pressure during the recovery period
following volume expansion has the same rate constant as the volume curve
(Fig. 2) and can be described
by Equation 2, after substituting the appropriate pressures for volumes. Using
this relationship, the vascular compliance following saline or plasma infusion
is 5.5 and 4.6 ml mmHg-1 kg-1, respectively. These
values are close to those reported previously for unanesthetized, intact trout
and in perfused whole fish (Zhang et al.,
1995
; Hoagland,
2001
).
The rise in interstitial volume following volume expansion is inversely
related to the loss of fluid from the vasculature and can be described by
Equation 3, substituting interstitial volume (VI) for
blood volume and the time-dependent volumes,
VI0 for pre-volume loading (control period),
and VIinc for the net gain in interstitial
volume at equilibrium:
![]() | (4) |
The time course for the rise in interstitial fluid pressure as
intravascular pressure falls can be estimated by substituting pressures for
volumes in Equation 4 and by using the rate constants determined for volume
distribution:
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Because we used a dynamic analysis to determine vascular compliance with our time scale in minutes, the calculated vascular compliance is also equal to the transvascular fluid filtration coefficient. Thus for saline infusion, this coefficient is 5.5 ml mmHg-1 kg-1 min-1, and for plasma infusion it is 4.6 ml mmHg-1 kg-1 min-1. To our knowledge, this is the first estimate of the transvascular fluid filtration coefficient in an intact fish.
Although the information is quite limited (we are unaware of filtration
coefficients in reptiles and birds), there appears to be a phylogenetic trend
toward lower interstitial compliance and lower transvascular filtration rates
in the higher vertebrates. Using a method essentially identical to ours,
Tanaka (1979) found that
interstitial compliance in the dog was 5.9 ml mmHg-1
kg-1, nearly half our value for the trout. Aukland and Reed
(1993
) summarized more recent
compliance measurements in mammals obtained through a variety of methods and
in general these values were approximately one-third of those in trout. Our
estimate of the transvascular fluid filtration coefficient in trout is only
50% higher than that reported by Hancock et al.
(2000
) for two anurans,
Bufo marinus (3.6 ml mmHg-1 kg-1
min-1) and Rana catesbeiana (3.2 ml mmHg-1
kg-1 min-1), whereas it is five times greater than that
found by the same authors for the rat (1.0 ml mmHg-1
kg-1 min-1) and 15 times greater than that reported in
the dog (0.3 ml mmHg-1 kg-1 min-1) by Tanaka
(1979
).
It is tempting to speculate that the reduction in transvascular filtration
coefficient and interstitial compliance in mammals was necessitated by
elevated intravascular pressures, the latter attendant with adopting a
terrestrial lifestyle. It may also help explain why mammals are less readily
able to mobilize extravascular fluid after constant-volume or
constant-pressure hemorrhage than trout
(Duff and Olson, 1989).
However, birds are paradoxical. They have arterial blood pressures in the
mammalian range, yet respond like trout in their ability to mobilize fluid
following hemorrhage (Djojosugito et al.,
1968
; Kovach et al.,
1969
; Ploucha and Fink,
1986
). Clearly, it is necessary to examine these parameters in
other vertebrates, especially birds and fish with high blood pressure, such as
tunas (Brill and Bushnell, 2001).
Theoretically, we could have also calculated compliance and filtration
coefficients from the hemorrhage studies. We did not do this because the trout
vascular capacitance curve becomes non-linear as blood volume is lowered below
resting levels (Zhang et al.,
1995; Olson et al.,
1997
; Hoagland
2001
), and it becomes difficult to estimate dynamic changes in
venous pressure.
Rapid hematocrit responses and the role of the microcirculation
A mono-exponential curve quite accurately fits the experimental data
following either volume expansion (Fig.
2), or hemorrhage (Fig.
3), except for the first 5 min after volume perturbation. Within
this 5 min period, the curve appears to overestimate plasma volume (it
predicted that hematocrit should have fallen to a lower level) after volume
expansion and under-estimate volume (it predicted that hematocrit should have
been higher) after hemorrhage. By continually measuring hematocrit during
hemorrhage (Fig. 4) it became
apparent that the initial decrease in hematocrit was nearly instantaneously
coupled to the reduction in blood volume. There are three possible
explanations for these observations: (1) transvascular fluid flux is unusually
high during volume perturbation; (2) plasma is skimmed off, or returned from,
the secondary circulation during expansion or hemorrhage; (3) plasma is stored
in, or mobilized from, an intravascular compartment. We feel that the third
explanation is the most plausible.
It seems unlikely that a transient increase in transvascular fluid flux could account for the observed changes. First, it would have to be triggered by both volume expansion and depletion. Second, it would have to be turned on and off within seconds of the start and end of addition or removal of fluid. Third, once turned off, it would have to remain off, even though part of the original volume perturbation remains. Fourth, the process would require an unrealistically high capillary permeability.
The secondary circulation could be a site of plasma storage or
mobilization. The narrow-bore vessels that form the entrance into the
secondary circulation restrict red blood cell access
(Steffensen and Lomholt, 1992;
Olson, 1996
), thus forming a
low-hematocrit vascular reserve that could conceivably inflate or deflate with
concomitant pressure changes in the primary circulation. However, it would
seem that a vasculature as capacious as the secondary circulation is reported
to be (Steffensen and Lomholt,
1992
) would be able to provide all the post-hemorrhage fluid
without need of capillary resorption, and that it would continue to serve as a
fluid depot or reserve as along as fluid imbalance remained in the primary
circulation. As described above, the distinctly different kinetics observed
after volume expansion with saline compared to plasma indicate that this is
apparently not the case.
Plasma storage or mobilization from an intravascular compartment (whose
hematocrit is lower than systemic hematocrit) in response to volume expansion
or depletion, respectively, appears to be the most likely scenario, and it has
a precedent in mammalian studies. It is well known that in mammals the
hematocrit in capillary-size vessels is considerably less than that in large
vessels (Johnson, 1971), and
that the ratio of the hematocrit in the whole cardiovascular system to
hematocrit in large systemic vessels (Hctw/Hctsys), also
called the Fcell ratio, is approximately 0.9
(Albert, 1971
). In a recent
review, Lee (2000
) defined the
mammalian microcirculation as all vessels with a diameter smaller than 250
µm, i.e. arterioles, capillaries and venules. Lee
(2000
) proposed that this
microcirculation contains 40-50% of the total blood volume and collectively
contributes to the low Fcell ratio. Because the
microcirculation is compliant, when blood is withdrawn from the
macrocirculation of an intact animal, there is a nearly simultaneous shift of
blood from the microcirculation into the macrocirculation
(Lee, 2000
). The amount of
fluid mobilized from the microcirculation depends on the relative compliance,
volume and pressure drop in both the microcirculation and macrocirculation;
however, because of the lower hematocrit in the microcirculation, the outcome
is always the same, large-vessel hematocrit falls. Lee described the
relationship between these parameters and the volume of the microcirculation
(Vmic) and total blood volume (Vtotal)
as:
![]() | (6) |
The Fcell ratio in fish has not been accurately
measured, but it appears to be 0.8 or less, which is lower than that reported
in mammals (Olson, 1992). Many
estimates in fish are unreliable because they are compromised by
methodological problems, especially those that use albumins as plasma volume
indicators. For example, the Fcell ratio falls from 0.63
to 0.39 between 4 and 16 h after injection of radio-labeled albumin into trout
(Bushnell et al., 1998
).
Failure to account for splenic sequestration of tagged red blood cells can
also be problematic (Duff et al.,
1987
). Probably the best estimate can be derived from the report
by Duff and Olson (1989
), in
which labeled red cell and albumin spaces were determined within 30 min after
injecting the indicators. This is long enough for mixing, yet short enough to
minimize albumin extravasation. The Fcell calculated from
this study (Duff and Olson,
1989
) is 0.8.
In Fig. 5, the relationship between Hctmic/Hctsys and Vmic/Vtotal is calculated from Equation 6 for Fcell ratios of 0.75, 0.8 and 0.85. With an Fcell ratio of 0.8, it is evident that the volume of the microcirculation must be at least 20% of the total blood volume because Hctmic/Hctsys cannot fall below zero. Similarly, Hctmic cannot exceed 80% of Hctsys because Vmic cannot be greater than Vtotal. Realistically, both Hctmic and Vmic are probably mid-way between these extremes, i.e. 40-50% of Hctsys and Vtotal. If fish Fcell ratios turn out to be lower than 0.8, which seems quite possible, then either Hctmic will be lower and/or Vmic will be proportionally greater than the above estimates.
|
Equation 6 also has implications for the volume of fluid transferred from
the microcirculation to the macrocirculation. Our estimate of the volume
transferred from the microcirculation after 35% hemorrhage (4.7 ml
kg-1; Table 3) was
based on the assumption that the hematocrit of the fluid transferred from the
microcirculation was zero, which is highly unlikely. From the relationship
between the pre-hemorrhage (Hctpre) and post-hemorrhage
(Hctpost) systemic hematocrits
(Table 3) and estimated
pre-hemorrhage systemic red cell (RCSpre) and plasma
(PSpre) spaces (assuming total blood volume is 35 ml
kg-1), and the post-hemorrhage contribution of red cells
(RCSmic) and plasma (PSmic) from the microcirculation to
the macrocirculation, shown in Equation 7:
![]() | (7) |
|
It is evident from Fig. 6
that if Hctmic exceeds 20, then the volume recruited from the
microcirculation approaches the total blood volume, which is impossible. If we
assume that 40-50% of the blood volume is in the microcirculation, as it is in
mammals (Lee, 2000), and if
all of it (14-17.5 ml) was transferred to the macrocirculation, then with a
macrocirculatory hematocrit of 28, Hctmic could not exceed 16-18.
It is unrealistic to assume that the entire microcirculatory volume would be
transferred to the macrocirculation, but these calculations put an upper limit
on Hctmic. It is also evident from
Fig. 6 that as
Hctmic falls below approx. 14 (half that of the macrocirculation),
there is less of an effect of Hctmic on the volume transferred from
the microcirculation.
Within the limits of Hctmic (between 0 and 16), we can use Figs
5 and
6 to predict the magnitude of
rapid fluid transfer from the microcirculation to the macrocirculation during
hemorrhage of 35% of the blood volume, and this is summarized in
Table 4. As Hctmic
increases, the predicted Vmic and the volume that is
transferred from the microcirculation into the macrocirculation during
hemorrhage increases. A modest decrease in the Fcell from
0.8 to 0.75 increases Vmic by 25% and lessens the impact
of hemorrhage on Vmic, although it is clear that at least
half, and more likely two-thirds, of the volume of Vmic is
transferred to the macrocirculation, irrespective of Hctmic or
Fcell. The actual Hctmic in fish is unknown. If
we assume it is between 8 and 12, which seems reasonable based on mammalian
studies (Johnson, 1971), then
between half and two-thirds of the volume lost from the macrocirculation
during 35% hemorrhage can be nearly instantaneously replaced by elastic recoil
of the microcirculation. This is of obvious benefit in maintaining central
venous pressure and it provides a safety factor until volume can be restored
from the interstitium.
|
Capillary permeability to protein and the significance of Starling's
forces in trout
We previously hypothesized that trout capillaries are quite permeable to
plasma protein based on the distribution volumes and kinetics of labeled
albumin and the appearance of labeled albumin in tissues that do not appear to
have a secondary circulation (Duff and
Olson, 1989; Olson,
1992
; Bushnell et al.,
1998
). The present experiments offer a novel methodological route
to the same conclusion.
Three pieces of evidence support our conclusion that plasma proteins
readily cross trout capillaries. (1) Volume recovery after volume expansion
with plasma is rapid, equivalent to 4.5% of the plasma volume
min-1. By comparison, plasma protein turnover in mammals is approx.
2% h-1 (Aukland and Reed,
1993); this is less than one-hundredth of the rate we observed in
trout. (2) In less than 90 min the volume lost from the circulation is the
same, irrespective of whether the trout were volume-expanded with saline or
plasma. If trout capillaries were an effective protein barrier, then after
volume expansion with plasma, the amount of intravascular protein would
increase as water was hydraulically driven across the capillaries. The
resultant increase in plasma oncotic pressure would cause retention of fluid
in the vasculature compartment, and at equilibrium the plasma volume after
plasma injection would be greater than plasma volume after saline injection.
Clearly this is not the case. Because volume recovery after plasma loading is
somewhat slower than recovery after saline loading, it appears that the
capillaries do indeed slow the rate of fluid extravasation, albeit minimally.
This is unlike the situation in the rat skin and muscle where albumin and
volume flux are not coupled (Renkin et
al., 1988
) and volume expansion (equal to the total blood volume)
with lactated Ringer does not affect the rate of bloodtissue albumin
transport in intact rats (Renkin et al.,
1989
). (The fact that saline does not isovolumetrically replace
whole blood, and neither does plasma, should also be kept in mind during
experiments requiring serial blood samples and fluid replenishment.) (3) The
similarity between the kinetics of volume expansion with plasma and hemorrhage
indicate that similar processes, i.e. movement of fluid and protein, are
involved both situations. Furthermore, because the rates are the same, it is
likely that the same amount of protein is being translocated in each
experiment. This implies that the interstitial protein concentration is very
close to that in the plasma and that whole-body reflection coefficients in
trout may approach 0, even though the vessels in certain tissues, especially
gill and brain, may be relatively protein-impermeable
(Bushnell et al., 1998
).
Note that when 125I-albumin is injected into normovolemic trout,
the rate of albumin efflux from the vasculature is considerably slower
(Bushnell et al., 1998) than
the apparent rate of plasma protein efflux observed when trout were
volume-expanded with trout plasma in the present experiments. It seems likely
that in the experiments by Bushnell et al.
(1998
), protein extravasation
was diffusion-limited, whereas in the present experiments the convective
transport of protein predominated. This is different from mammalian vessels
where transcapillary flux of albumin is not coupled to solvent flow
(Renkin et al., 1988
).
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Acknowledgments |
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References |
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---|
Albert, S. N. (1971). Blood Volume and Extracellular Fluid Volume. 273p. Springfield: Charles C. Thomas.
Aukland, K. and Reed, R. K. (1993).
Interstitial-lymphatic mechanisms in the control of extracellular fluid
volume. Physiol. Rev.
73, 1-78.
Brill, R. W. and Bushnell, P. G. (2002). Cardiovascular system of tunas. In Fish Physiology Vol. 19, Tuna Physiology, Ecology and Evolution (ed. B. A. Block and E. D. Stevens), pp. 79-120. San Diego: Academic Press, Inc.
Bushnell, P. G., Conklin, D. J., Duff, D. W. and Olson, K.
R. (1998). Tissue and whole-body extracellular, red blood
cell, and albumin spaces in the rainbow trout as a function of time: a
reappraisal of the volume of the secondary circulation. J. Exp.
Biol. 201,1381
-1391.
Conlon, J. M. (1999). Bradykinin and its receptors in non-mammalian vertebrates. Regul. Pept. 79, 71-91.[CrossRef][Medline]
Djojosugito, M., Folkow, B. and Kovach, G. B. (1968). The mechanism behind the rapid blood volume restoration after hemorrhage in birds. Acta. Physiol. Scand. 74,114 -122.[Medline]
Duff, D. W., Fitzgerald, D., Kullman, D., Lipke, D. W., Ward, J. and Olson, K. R. (1987). Blood volume and red cell space in tissues of the rainbow trout, Salmo gairdneri. Comp. Biochem. Physiol. 87A,393 -398.
Duff, D. W. and Olson, K. R. (1989). Response
of rainbow trout to constant-pressure and constant-volume hemorrhage.
Am. J. Physiol. Regul. Integr. Comp. Physiol.
257,R1307
-R1314.
Farrell, A. P. and Jones, D. R. (1992). The heart. In Fish Physiology Vol. XII, Part A. The Cardiovascular System (ed. W. S. Hoar, D. J. Randall, and A. P. Farrell), pp. 1-73. San Diego: Academic Press, Inc.
Farrell, A. P. and Olson, K. R. (2000). Cardiac natriuretic peptides: a physiological lineage of cardioprotective hormones? Physiol. Biochem. Zool. 73, 1-11.[CrossRef][Medline]
Hancock, T. V., Hoagland, T. M. and Hillman, S. S. (2000). Whole-body systemic transcapillary filtration rates, coefficients, and isogravimetric capillary pressures in Bufo marinus and Rana catesbeiana. Physiol. Biochem. Zool. 73,161 -168.[CrossRef][Medline]
Hargens, A. R., Millard, R. W. and Johansen, K. (1974). High capillary permeability in fishes. Comp. Biochem. Physiol. 48A,675 -680.
Hoagland, T. M. (2001). Blood volume perturbations affect venous function and cardiovascular homeostasis in the rainbow trout (Oncorhynchys mykiss).239 p. PhD thesis, University of Notre Dame, Indiana, USA.
Hoagland, T. M., Weaver, L., Jr., Conlon, J. M., Wang, Y. and
Olson, K. R. (2000). Effects of endothelin-1 and homologous
trout endothelin on cardiovascular function in rainbow trout. Am.
J. Physiol. Regul. Integr. Comp. Physiol.
278,R460
-R468.
Johnson, P. C. (1971). Red cell separation in
the mesenteric capillary network. Am. J. Physiol.
221,105
-111.
Kovach, A. G. B., Szasz, E. and Pilmayer, N. (1969). Mortality of various avian and mammalian species following blood loss. Physiol. Acad. Sci. Hung. 35,109 -116.
Lee, J.-S. (2000). 1998 Distinguished lecture: biomechanics of the microcirculation, an integrative and therepeutic perspective. Ann. Biomed. Eng. 28, 1-13.[CrossRef][Medline]
Loretz, C. A. and Pollina, C. (2000). Natriuretic peptides in fish physiology. Comp. Biochem. Physiol. 125A,169 -187.
Nilsson, S. (1984). Adrenergic control systems in fish. Mar. Biol. Lett. 5, 127-146.
Olson, K. R. (1992). Blood and extracellular fluid volume regulation: role of the renin-angiotensin system, kallikrein-kinin system, and atrial natriuretic peptides. In Fish Physiology Vol. XII, Part B. The Cardiovascular System (ed. W. S. Hoar, D. J. Randall and A. P. Farrell), pp.136 -232. San Diego: Academic Press, Inc.
Olson, K. R. (1996). The secondary circulation in fish: anatomical organization and physiological significance. J. Exp. Zool. 275,172 -185.[CrossRef]
Olson, K. R. (1997). The cardiovascular system. In The Physiology of Fishes (ed. D. H. Evans), pp.129 -154. Boca Raton: CRC Press LLC.
Olson, K. R., Conklin, D. J., Farrell, A. P., Keen, J. E.,
Takei, Y., Weaver, L., Jr, Smith, M. P. and Zhang, Y. (1997).
Effects of natriuretic peptides and nitroprusside on venous function in trout.
Am. J. Physiol. Regul. Integr. Comp. Physiol.
273,R527
-R539.
Perry, S. F., Fritsche, R., Hoagland, T. M., Duff, D. W. and
Olson, K. R. (1999). The control of blood pressure during
external hypercapnia in the rainbow trout (Oncorhynchus mykiss).
J. Exp. Biol. 202,2177
-2190.
Ploucha, J. H. and Fink, G. D. (1986).
Hemodynamics of hemorrhage in the conscious rat and chicken. Am. J.
Physiol. Regul. Integr. Comp. Physiol.
251,R846
-R850.
Renkin, E. M., Gustafson-Sgro, M. and Sibley, L.
(1988). Coupling of albumin flux to volume flow in skin and
muscles of anesthetized rats. Am. J. Physiol. Heart Circ.
Physiol. 255,H458
-H466.
Renkin, E. M., Rew, K., Wong, M., O'Loughlin, D. and Sibley,
L. (1989). Influence of saline infusion on blood-tissue
albumin transport. Am. J. Physiol. Heart Circ.
Physiol. 257,H525
-H533.
Renkin, E. M. and Tucker, V. L. (1995). Integration of capillary, interstitial and lymphatic function. In Interstitium, Connective Tissue, and Lymphatics (ed. R. K. Reed, N. G. McHale, J. L. Bert, C. P. Winlove and G. A. Laine), pp.250 -270. London; Portland Press.
Rowell, L. B. (1993). Human Cardiovascular Control. 500p. New York: Oxford University Press, Inc.
Russell, M. J., Klemmer, A. M. and Olson, K. R. (2001). Angiotensin signaling and receptor types in teleost fish. Comp. Biochem. Physiol. 128A,41 -51.
Satchell, G. H. (1991). Physiology and Form of Fish Circulation. 235p. New York: Cambridge University Press.
Steffensen, J. F. and Lomholt, J. P. (1992). The Secondary Vascular System. In Fish Physiology Vol. XII, Part A. The Cardiovascular System (ed. W. S. Hoar, D. J. Randall and A. P. Farrell), pp. 185-213. San Diego: Academic Press, Inc.
Takei, Y. (2000). Structural and functional evolution of the natriuretic peptide system in vertebrates. Int. Rev. Cytol. 194,1 -66.[Medline]
Tanaka, Y. (1979). Whole body transvascular filtration coefficient and interstitial space capacitance. Jap. J. Physiol. 29,181 -193.[Medline]
Tanaka, Y., Morimoto, T., Watari, H. and Miyazaki, M. (1976). continuous monitoring of circulating blood hematocrit. Jap. J. Physiol. 26,345 -353.[Medline]
Turner, A. H. (1937). Serum protein measurements in the lower vertebrates. II. In marine teleosts and elasmobranchs. Biol. Bull. 73, 511.
Vogel, W. O. P. (1985). Systemic vascular anastomoses, primary and secondary vessels in fish, and the phylogeny of lymphatics. In Cardiovascular Shunts: Phylogenetic, ontogenetic, and clinical aspects (ed. K. Johansen and W. Burggren), pp.143 -159. Copenhagen: Munksgaard.
Wang, Y., Olson, K. R., Smith, M. P., Russell, M. J. and Conlon,
J. M. (1999). Purification, structural characterization, and
myotropic activity of endothelin from the trout, Oncorhynchus mykiss.Am. J. Physiol. Regul. Integr. Comp. Physiol.
277,R1605
-R1611.
Zhang, Y., Jenkinson, E. and Olson, K. R. (1995). Vascular compliance and mean circulatory filling pressure in trout: effects of angiotensin converting enzyme inhibition. Am. J. Physiol. Regul. Integr. Comp. Physiol. 268,H1814 -H1820.
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