Biomedical Engineering Reference
In-Depth Information
probes, and the pressure readings obtained from the probes, a pressure gradient can be
calculated. As the diameter of the blood vessel decreases through the vascular network
(e.g., arterioles to capillaries) the pressure gradient significantly increases. As the diameter
of the blood vessel increases (capillaries to venules) the pressure gradient reduces again.
This can be explained by the rapid changes in hydrostatic pressure within the precapillary
arterioles, caused by constriction or dilation of the precapillary sphincter.
In contrast, the hydrostatic pressure within the arteriolar side is relatively constant until
the blood vessels approach a diameter of approximately 40
m. Therefore, the pressure gra-
dient is low within these vessels. Vessels of this size would typically be found one to two
bifurcations upstream of the metarteriole/precapillary sphincters. For arterioles in the range
of 15 to 40
μ
m in diameter, there is a rapid decrease in pressure to approximately 30 mmHg,
which is associated with a rapid increase in the pressure gradient. This rapid decrease
occurs so that the blood velocity is slow enough for nutrient and waste exchange to occur
within the capillaries at the same time that blood is directed rapidly into the capillaries. The
pressure variation in capillaries and venules is much lower than that seen in the precapil-
lary arterioles. Within capillaries (diameter ranging from 5 to 10
μ
m) the pressure decreases
from approximately 25 mmHg to at most 20 mmHg, under normal conditions. However,
the pressure gradient driving blood into the capillaries is relatively large so that blood
movement through these vessels is efficient. Within the venous circulation, the pressure
continually drops (to approximately 0 mmHg in the right atrium), but again, it is much
more gradual, taking the entire length of the venous system. Therefore, the pressure gradi-
ent is much lower within the venules/venous system. In post-capillary venules (diameter as
large as 50
μ
m), the pressure is no more than 15 mmHg under normal conditions.
To continue the discussion of the pressure gradient throughout the microvascular bed,
there is approximately an 8-fold increase in the pressure gradient within small capillary seg-
ments (100 to 300
μ
μ
μ
m length) as compared to arterioles and venules (approximately 2000
m
μ
length, approximately 40
m diameter). For metarterioles and post-capillary venules
(approximately 15
m diameter) the pressure gradient is 50% of the pressure gradient in the
capillaries. This suggests that the flow is directed into the capillaries and then it slows to
allow sufficient time for nutrient exchange. Recall that the gradient may be large, but flow
will be diverted into many small capillaries to perfuse the entire vascular bed.
The pressure variation in microvascular beds under hypertensive and hypotensive con-
ditions has also been investigated. Interestingly, under both conditions, the mean hydro-
static pressure within the capillaries as well as the pressure gradient across the capillaries
was equivalent to that seen under normal conditions. Also, the pressure (hydrostatic and
pressure gradient) within the post-capillary venules was the same under these conditions
as under normal conditions. The major change was observed within the arteriolar vascula-
ture where the pressure gradient was significantly greater under hypertensive conditions
and significantly lower under hypotensive conditions. This suggests that metarterioles
(and the precapillary sphincters) regulate capillary blood flow to maintain it at its normal
levels, so that nutrient exchange is maintained at the optimal level. This also suggests that
the circulatory system is designed to maintain a constant flow through the microcircula-
tion, independent of the mean arterial pressure.
The last major variation in pressure throughout the microvascular beds is based on the
temporal changes from the cardiac pressure pulse and the wave propagation throughout
μ
Search WWH ::
Custom Search