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PHYSIOLOGY
OF BLOOD CIRCULATION
In:
Blood Vessles and Lymphatics In Organ Systems
by
D.W. Lennox and D.S. Hungerford
This
material has been published in Blood Vessels and Lymphatics in Organ
Systems, edited by David I. Abramson and Philip B. Dorbin, the only
definitive repository of the content that has been certified and
accepted after peer review. Copyright and all rights therein are
retained by Elsevier Science (USA). This material may not be copied
or reposted without explicit permission.
Copyright
1984, Elsevier Science (USA). All rights reserved.
Besides biomechanically providing a rigid framework
for protecting vital organs and for the attachment of muscle and
ligament, bone also possesses a remarkable capacity to modify
its structure and function in response to injury, growth, aging,
neoplasm, metabolic derangement, infection, surgery, and neurohumoral
factors. Of utmost importance in maintaining the varied responsiveness
of bone is the existence of a highly responsive system of bone
blood flow to deliver necessary nutrients, cells, and possibly
chemical mediators.
Although at the present time there is no clinically
useful technique for the direct measurement of bone blood flow
in human subjects, a number of procedures are available for this
purpose in experimental animals. Several of these are considered
in the following discussion.
1. METHODS OF STUDYING
BONE BLOOD FLOW
a. Direct method:
Direct measurement of total bone blood flow is nearly impossible
due to the multiple afferent and efferent channels involved. Cumming
(1960) used a direct technique to study flow to the rabbit femur,
by cannulating the superficial femoral vein (having previously
ligated the deep femoral and circumflex veins) and measuring flow
from the cannulated vessel, considering this to represent femoral
flow. A similar methodology was reported earlier by Drinker
and Drinker (1916). Such an approach has more utility
in investigating gross changes in circulation in response to any
of a number of interventions than in determining precise values
for blood flow.
b. Indirect methods:
A number of indirect procedures exist for the study of blood flow
in bone, one of which is red blood cell (RBC) labeling. White
et al. (1964) utilized 51Cr-tagged red cells
for this purpose. The technique consists of injecting tagged red
cells intravenously, and, after an appropriate time to permit
mixing, blood is sampled to establish a reference radioactivity
level per volume collected. The animal is then sacrificed and
the bones under investigation are fixed, weighted, sectioned,
and measured for radioactivity. Circulating red cell volume is
then calculated and expressed as volume per net weight of specimen.
This method assumes that the hematocrit in bone blood is the same
as that in the blood sample.
A second approach, the radioisotope clearance method,
consists of the use of both non-bone-seeking isotopes to assess
bone blood flow. With the non-bone-seeking isotopic method, a
given amount of radioisotope is injected into the bone to be studied
and the radioactivity level is followed over time. Rapid clearance
is evidenced by a rapid diminution in radioactivity, which is
considered to be proportional to blood flow. Isotopes such as
Na131I have been employed for this purpose. Since the
procedure is an invasive one, it is subject to artefacts produced
by the injecting trocar. With the bone-seeking isotopic method,
either 86Rb (Kane, 1968), 45Ca, or 85Sr
is employed to estimate lower limb blood flow.
With the use of the substances mentioned above,
flow estimates are based upon the Fick principle. For example,
in the case of 85Sr the amount of the isotope fixed
in bone in a given time is divided by the arteriovenous concentration
difference in order to calculate flow. The flow value generated
by such an approach is less then the true value since the extraction
ratio for 85SR is less than unity (Shim et al.,
1971).
The use of tracer microspheres is a relatively new
method for measuring bone blood flow in experimental animals,
the value of which has not been fully established. It employs
15 µm microspheres labeled with any of a number of different
isotopes, including 125I, 85Sr, 46Sc,
141Ce, and 95Nb. The availability of several
different isotopes and their separability by spectroscopy create
the possibility for repeated measurements in one animal. Arterial
reference samples are obtained just prior to and for several minutes
following the intracardiac microsphere injection. At the conclusion
of the experiment, the animal is sacrificed and the bone cleaned
and counted in a gamma counter. Microspheres are trapped in the
bone microcirculation in numbers proportional to flow. The technique
allows for analysis of flow to various anatomic regions of bone,
as well as for overall bone blood flow. Although the method appears
to be applicable to many studies with experimental animals, its
validity in states of fracture healing, growth, or necrosis has
not been established (Gross et al., 1981). Tothill and
MacPherson (1980) have emphasized that due to removal of some
microspheres in preosseous capillaries, the microsphere technique
has limited applicability as standard in determining extraction
ratios. (For details of the technique, its assumptions, and discussion
of the validity and applicability of the method, see Gross et
al., 1979, 1981.)
2. BLOOD FLOW TO BONE
Recent measurements made with the microsphere technique indicate
a heterogeneous pattern of flow to bone and bone marrow. In the
anesthetized dog, low flow rates were reported for compact bone
(2 ml/min/100 gm) in humeral and femoral diaphysis) and much higher
values for hematopoietic cancellous bone and hematopoietic marrow
(18-30 ml/min/100 gm) (Gross et al., 1981). Morris and
Kelly (1980) found similar values for compact and cancellous blood
flow in the dog but recommended that, for optimal microsphere
technique, the animal should be conscious with the reference catheter
in the aorta. Whiteside et al. (1977a), utilizing the hydrogen
washout technique, measured epiphyseal cancellous bone blood flow
in the rabbit and found that for cancellous bone, the flow was
0.129 + 0.015 ml/min/ml, with the corresponding value for
cortical bone being 0.069 ± 0.002 ml/min/ml.
3. SKEKETAL BLOOD FLOW
Estimates of total skeletal blood flow vary widely. Figures as
high as 27.5% of cardiac output have been proposed (Brookes, 1967).
Wootton et al. (1976b) found bone blood flow to be 4.1
ml/min/100 gm in eight normal male volunteers using an 18F
isotope technique. Shim et al. (1971) reported a reading
of 2.4 ml/min/100 gm for man with an 85Sr clearance
method, a value similar to that previously found by Van Dvke et
al. (1965). With the microsphere technique, Gross et al.
(1981) estimated total skeletal blood flow in the dog to represent
11% of cardiac output. An earlier report by Shim et al.
(1967) noted a rate of 7.3 ± 3.0% of cardiac output. Morris
and Kelly (1980) estimated the percentage of cardiac output to
bone tissue in the conscious dog to be 9.6% in the mature animal
and 10.3% in the immature one. Of interest was their finding that
in the mature animal, approximately 2% was flow to cortical bone
and 8% to cancellous bone, whereas, in the immature dog, 7% was
flow to cortical bone and 4% to cancellous bone. Such results
were attributed to the fact that although cortical bone represents
80% of the skeleton by weight, its surface area is roughly equivalent
to less voluminous cancellous bone; moreover, in the immature
animal, appositional growth is greater, with possible shunting
to cortical bone.
4. ALTERED PHYSIOLOGIC PARATMETERS AND
BONE BLOOD FLOW
Although it seems reasonable to assume that bone blood flow,
constituting a significant portion of overall cardiac output,
should respond and be sensitive to altered physiologic parameters,
it is only recently that this expectation has gained experimental
confirmation.
a. Response to hemorrhage:
Syftestad and Boelkins (1980) subjected conscious rabbits to nonfatal
reversible hemorrhage and analyzed the effects on marrow, bone,
and a number of other tissues, utilizing the radioactive microsphere
technique. There was no evidence for immediate shunting of blood
from bone to marrow, but an increase in marrow blood flow did
occur 16 hr after hemorrhage. This response was interpreted as
a possible preparatory mechanism for increased erythropoietic
activity.
In a related experiment, Gross et al. (1979) induced hypotension
in the dog by arterial hemorrhage and followed changes in bone
blood flow. The response to this state was a marked increase in
vascular resistance and a decreased bone and marrow blood flow.
b. Response to exercise:
In the dog, exercise induced by treadmill markedly increased blood
flow to exercising skeletal muscles (Gross et al., 1979),
and, at the same time, vascular resistance in bone rose significantly
and flow, both to bone and marrow, diminished. During exercise,
nonexercising muscle (temporalis) exhibited increased vascular
resistance.
c. Response to hypoxia: In
the dog, systemic arterial hypoxia was found to reduce blood flow
to bone and marrow and raise vascular resistance in skeletal muscle
(Gross et al., 1979). However, Adachi et al. (1976)
had previously found no change under similar conditions. The differences
in results may reflect variation in anesthesia technique or level
of hypoxia.
d. Response to aging: MacPherson
and Tothill (1978) presented evidence for increased bone blood
flow to the tibia, fibula, femur, pelvis, humerus, radius, ulna,
and scapula with increase in age and weight of rats.
e. Response to growth: McInnis
et al. (1977) reported a positive correlation between bone
blood flow and percent of new bone formation in a standardized
tibial defect in the dog. The finding that cortical bone blood
flow is higher in the immature than in the mature animal (Pasternak
et al., 1966; Morris and Kelly, 1980) may be related to
greater appositional growth and more extensive bone remodeling
(Lee, 1964; Vanderhoeft et al., 1962). Whiteside et
al. (1977b) noted a positive correlation between bone blood
flow and osteoblastic activity in the rabbit tibia.
f. Summary: Blood flow to
bone and marrow is responsive to altered physiology. Vessels to
bone and marrow appear to be involved in the overall circulatory
adjustment to hypotension, hypoxia, and exercise. Neurohumoral
factors also produce alterations in blood flow to bone. (For a
discussion of neurohumoral factors, see Section C.)
References
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The effect of hypoxia on the regional distribution of cardiac
output in the dog. Circ. Res. 39, 314-319.
- Brookes, M. (1967). Blood flow rates in compact and cancellous
bone, and bone marrow. J. Anat. 101, 533-541.
- Cumming, J.D. (1960). A method for studying the rate of blood
flow through the bone marrow of a rabbit's femur. J. Physiol.
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- Drinker, C.K., and Drinker, K.R. (1916). A method for maintaining
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- Gross, P.M., Heistad, D.D., and Marcus, M.L. (1979). Neurohumeral
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PHYSIOLOGY OF BLOOD CIRCULATION
In: Blood Vessels And Lymphatics In Organ Systems
by D.W. Lennox and D.S. Hungerford This material
has been published in Blood Vessels and Lymphatics in Organ Systems,
edited by David I. Abramson and Philip B. Dorbin, the only definitive
repository of the content that has been certified and accepted after
peer review. Copyright and all rights therein are retained by Elsevier
Science (USA). This material may not be copied or reposted without
explicit permission.
Copyright 1984, Elsevier Science (USA). All rights reserved.
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