Biomedical Engineering Reference
In-Depth Information
the vasculature. The pressure values discussed so far have been averaged across the car-
diac cycle, by taking multiple readings during the entire cardiac cycle. However, during
the cardiac cycle, the hydrostatic pressure changes within the microvascular beds and
oscillates on the order of 2 mmHg. We previously stated that the hydrostatic pressure
within the capillaries is approximately 25 mmHg. This means that the pressure would
vary between approximately 24 mmHg and 26 mmHg during the cardiac cycle. These tem-
poral changes are not that significant when compared with the relatively stable pressure
gradient across the blood vessels. Most of the data that has been discussed in this section
was collected by B. Zweifach and his research group during the 1970s.
6.5 VELOCITY DISTRIBUTION THROUGHOUT THE
MICROVASCULAR BEDS
In large blood vessels, where the vessel diameter exceeds the diameter of cells within
the blood by at least 3 times, the mean blood velocity is approximately equal to the mean
velocity of the cells. This is because the cells tend to spread evenly throughout the cross-
section of the blood vessel (imagine the cell distribution across a cross section of the aorta).
However, as the blood vessel diameter approaches the diameter of the suspended cells,
this is not necessarily true anymore. The mean blood cell velocity within blood vessels
that have a diameter in the range of 15 to 25
m is higher than the mean blood velocity.
This is caused by a phenomenon where the blood cells are pushed to the centerline of the
blood vessel. As discussed previously, under single-phase flow the fully developed para-
bolic flow will have a centerline velocity that is greater than the mean blood velocity.
However, in these small blood vessels the flow profile will be blunted (similar to the
Casson model velocity profile) because the blood cells account for a significant portion of
the cross-sectional area (this will be discussed further in Section 6.7 ). As the blood vessel
diameter reduces to less than 15
μ
μ
m, the mean blood cell velocity and the mean blood
velocity approach the same value again. The blood vessels that have different mean cell
velocities and mean plasma velocities are found within the microvascular beds.
The change in the relationship between the velocity of blood cells and the velocity of
the blood occurs because the blood cells account for the majority of the cross-sectional
area of the blood vessel. In other words, the blood vessel diameter reduces to the cellular
diameter. When this occurs, the blood cells tend to plug the vessels and dominate the
entire velocity profile. What typically occurs is a packet of red cells (rouleaux) groups
together and passes through the capillary as one. This is then followed by a packet of
plasma, which passes through the capillary at the same velocity as the cells. This again
would be followed by a red cell packet (which would more than likely be of a different
size than the first packet). However, independent of red blood cell group size, the velocity
of each cell/fluid packet will be the same. When the blood vessel diameter reduces to
about 15
m, the red blood cells tend to move toward the centerline, as will be discussed
in Section 6.7 . In these vessels, plasma flows along the walls of the blood vessel with a
slow velocity. As the blood cells enter the capillaries, the plasma is squeezed against the
wall, and eventually, it leaks back through this narrow gap between the cells and the ves-
sel wall. This leak-back plasma is collected at one location, leaving a gap between the
μ
Search WWH ::




Custom Search