Biomedical Engineering Reference
In-Depth Information
Anderson [41] noted that high arterial pressure in the bone marrow probably
correlates with an absence of lymphatics in the bone marrow and cortex. We note
that lymphatic circulation is unlikely to play a role in bone fluid transport in a
normal bone, because lymphatic vessels are absent in the bone. A chapter on the
physiology of blood circulation in a book entitled
Blood Vessels and Lymphatics in
Organ Systems
[42] contains no description of the bone lymphatic system. It has
been shown using immunohistochemistry that lymphatics were not present in the
normal bone [43, 44].
9.7
The Levels of Bone Porosity and their Bone Interfaces
There are three levels of bone porosity containing blood or interstitial fluid within
the cortical bone and within the trabeculae of the cancellous bone. A section of
a long bone indicating the vascular structure is shown in Figure 9.1 and more
detailed views of the local bone structure are shown in Figures 9.2 and 9.5. The
three levels of bone porosity include the PV associated with the Volkmann canals
(Figures 9.2 and 9.5) and the Haversian or osteonal canals (Figures 9.2 and 9.5),
which are of the order of 20
m in radii; the PLC associated with the fluid space
surrounding the osteocytes and their processes (Figure 9.7), which is of the order
0
µ
m in radii; and the collagen-hydroxyapatite porosity (PCA) associated with
the spaces between the crystallites of the mineral hydroxyapatite (order: 0
.
1
µ
m
radius). The total volume of the bone fluid PV is about one-half or less than that of
the PLC [45-47].
.
01
µ
9.7.1
The Vascular Porosity (PV)
The PV occupied by bone fluid is the space outside the blood vessels and nerves
in the Volkmann and Haversian canals. The typical pore size (20
m in radii) of
the PV channels is not the blood vessel pore size; rather it is the size of the tubular
tunnels (Haversian systems or osteons and Volkmann canals) containing the blood
vessels, the arterioles, and the venules, with the actual dimensions of these vessels
subtracted from the volume of the tubular tunnels.
The PV is a low-pressure reservoir that can interchange fluid with the PLC. This
is the case because the lineal dimension associated with the bone fluid PV is 2
orders of magnitude larger than the lineal dimension associated with the PLC, and
the PV is typically at blood pressure, which is low in bones. The total volume of the
PV is, however, considerably less than that of PLC [45-47].
The measurement of the permeability of the PV has not been accomplished with
sufficient accuracy to date, primarily because of the topological intertwining of the
PV with the PLC. This difficulty and others are discussed in some detail in [48],
where it is noted that experiments reported [49] bovine cortical bone permeabilities
on the order of 10
−
14
µ
m
2
. Factors are present in [48] to suggest that the actual
