Biomedical Engineering Reference
In-Depth Information
Nutrient
artery
Radial branches of
nutrient artery
Volkmann's
canal
Arteriolar
branch
to sinusoid
Central
venous sinus
Marrow
sinusoids
Figure 9.3
The relationship between the marrow and cor-
tical bone circulations. The radial branches of the nutrient
artery form a leash of arterioles that penetrate the endosteal
surface to supply the bone capillary bed. Small arterioles
from these radial branches supply the marrow sinusoids
adjacent to the bone. (Modified from Williams
et al
.[9].)
pressure in the capillaries does not fall below that in the extravascular space, the
interstitial fluid pressure. Note that transmural pressure (blood pressure minus
the interstitial fluid pressure) must initially exceed osmotic pressure if filtration is
to occur. Absorption of tissue fluid depends upon the transmural pressure being
less than the osmotic pressure of the blood at the end of the sinusoid. Osmotic
pressure is generally held to be about 20 mmHg. It follows that the pressure in the
collecting sinuses of the diaphyseal marrow may be of the order of 55 mmHg. Note
that 1 mmHg is 133.3 Pa or that 3 mmHg is approximately 400 Pa, 60 mmHg is
approximately 8 kPa. Bone fluids are interesting in that they exhibit metabolically
produced differential diffusion gradients [15, 16]. They are sometimes limited in
range, but well documented. Thus, many ions, such as potassium, calcium, and
phosphorus, exist in very different concentrations between the blood and bone [17].
9.2.3
Transcortical Arterial Hemodynamics
Bridgeman and Brookes [18] have shown that aged bone cortex is supplied pre-
dominantly from the periosteum in contrast to the medullary supply in young
human and animal bones, based on cross sections through the mid-diaphyses.
They argue that this change is attributed to increasingly severe medullary ischemia
with age, brought on by arteriosclerosis of the marrow vessels. They note that
