The Lymphatic System - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 248

pressure ( X -axis), within the range of given values. However, we know that lymph flow rate can-
not exceed approximately 20 times its normal flow rate, so outside of this range each approxima-
tion will fail (see Figure 8.2 ).

The lymphatic system has a pump mechanism similar to the one seen in venous flow.
As lymphatic vessels fill with lymph, they stretch. This stretch causes an automatic con-
traction of the smooth muscle cells that surround the lymphatic vessels. The contraction of
smooth muscle cells forces lymph through a small lymphatic section (divided by valves)
into a downstream lymphatic section. As the fluid fills the next section, the lymphatic ves-
sel stretches again, causing an automatic smooth muscle cell contraction. This pumping
mechanism acts to propel the lymph along the entire lymphatic vascular system. Pressure
within the lymphatic vessel, due to smooth muscle cell contraction, can be as high as
100 mmHg (in the thoracic duct), but is generally significantly lower than this. Smooth
muscle cells are not the only mechanism that contracts lymphatic vessels. A contraction of
neighboring skeletal muscle can propel lymph through the lymphatic system, similar to
the venous system. Compression of any tissues that transmits a compressive load to the
lymphatic system can also help to move lymph throughout

the lymphatic system.

Remember that the lymphatic system is a low-pressure system.
Lymph flow through the lymph nodes is also important to discuss ( Figure 8.3 ). Lymph
is delivered to the lymph nodes through a few afferent lymphatic vessels (typically in the
range of five) per node. Within the lymph node, there are hundreds of lymph node
sinuses, which are open passageways throughout the node. Lymph can flow freely
through these sinuses. The first set of sinuses that lymph flows through is termed the sub-
capsular space, which houses a large number of macrophages and other cells to initiate an
immune response if necessary. The subcapsular space is typically on the order of three to
five cells in thickness (50 to 100

m). After flowing through this space, lymph enters the
outer cortex of the lymph node, which contains a large concentration of B-cells. The outer
cortex also contains the locations for B-cell division, termed the germinal center. The outer

µ

FIGURE 8.2 Regression analysis of data presented
in the in-text example.

25

First-order polynomial fit
Second-order polynomial fit
Data points

20

15

10

5

-4

-3 -2 -1

Pressure (mmHg)

0 1 2