Biomedical Engineering Reference
In-Depth Information
who observed spaces of 175
˚
A between adjacent endothelial cells that were filled
with an amorphous material seen by electron microscopy.
9.5
Venous Drainage of Bone
The venous complexes draining a long bone parallel those of the arteries. Many
workers have commented [21, 24] on the extreme thinness of their walls. In the
marrow, the venous sinusoids drain into a large, single-cell-walled, central venous
sinus, which in turn drains into the ''nutrient'' veins of the diaphyses. In the adult
dog, this thin-walled ''nutrient'' vein accounts for only 10% of the drainage from
the diaphyses [27]. The multiple, penetrating, venous radicles in the metaphyses
and epiphysis are also thin walled and run a more tortuous course than the arteries
[21]. The major share of the venous blood leaving long bones has been shown by
phlebography to travel by this route [27]. The abundantly anastomosing periosteal
network of veins is considered by some workers to drain the diaphyseal bone
cortex completely under normal conditions [28]. Many of the veins leaving the long
bone pass through muscles, in particular, the calf muscle in the case of the lower
limb. The alternate contraction and release of the muscles containing the veins is
effectively a pump returning the blood toward the heart and away from the bone
and decreasing the intermedullary pressure. The intermedullary pressure can be
reduced by exercise of the muscles of the calf. This arrangement in the case of the
calf muscles is illustrated in Figure 9.6. It is clear that the long bone as a whole has
multiple venous pathways, the relative importance of which can vary with time and
circumstance.
Impaired venous circulation (venous stasis) has been shown to stimulate pe-
riosteal bone formation or increase bone mass in the young dog [29], the young
goat [30], and in a disuse, hind limb suspended, rat model [31]. Venous stasis was
induced in the experimental animals by applying tourniquets or vein ligation that
lasted from 10 days (with additional 30 days for recovery [32]) up to 42 days [29]
before the bones were examined. There are many other studies demonstrating sim-
ilar effects [32-35]. A hypothesis for the underlying mechanism of the periosteal
bone formation induced by venous stasis has been presented [36].
9.6
Bone Lymphatics and Blood Vessel Trans-Wall Transport
The purpose of this section is to indicate where the interstitial fluid can flux from
a blood vessel and where it may flux back into a blood vessel. Interstitial fluid can
flux from an arterial blood vessel with nutritional solutes that are used to nourish
the cells in the bone. Interstitial fluid may flux back into venous blood vessels
with wastes as these vessels leave the bone; this becomes a possibility because of
evidence (see below) of a lack of lymphatics in the periosteum.
