The Lymphatic System - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 242

side of the heart. Unlike the cardiovascular network, the first vessels in the lymphatic net-
work are the lymphatic capillaries. Lymphatic capillaries are present in most tissues of the
body and are only absent from tissues that do not have a direct blood supply. Lymphatic
capillaries tend to permeate microvascular beds, to aid in the absorption of interstitial
fluid that has not entered cells or returned to the cardiovascular system along the venous
side of the capillary ( Figure 8.1 ). If lymphatic capillaries did not absorb water, then the
interstitial space would swell due to continual diffusion of water out of the blood capillar-
ies (recall that overall there was a water loss from the capillaries into the interstitial space).
In this case, the hydrostatic pressures (capillary and interstitial) and osmotic pressures
(capillary and interstitial) would eventually reach equilibrium, so that there would be no
net movement of nutrients or water between the blood and the interstitial space. Similar to
the blood vessels of the cardiovascular network, lymphatic vessels are lined by endothelial
cells. However, the endothelial cells in lymphatic capillaries are not bound as tightly as
compared to those in blood capillaries. Instead, endothelial cells tend to overlap one
another, forming channels that allow for the uni-directional flow of fluids, large molecules
(such as proteins), and cells from the interstitial space into the lymphatic capillaries.
Lymphatic capillaries are generally larger than blood capillaries with a typical diameter in
the range of 20

m. Also, lymphatic vessels are not circular in cross section,
because they experience a very low hydrostatic pressure (gauge, approximately
2

mto30

µ µ

6 mmHg). In this regard, they mimic the venous blood vessels of the car-
diovascular system. As the reader can imagine, the lymphatic system is not a closed sys-
tem by itself because the lymphatic capillaries are open-ended.

4 mmHg to

2

FIGURE 8.1 The association of a
lymphatic capillary and a capillary
network. Arrows indicate the move-
ment of fluid from the blood vessel
into the interstitial space and then
into the lymphatic system. The lym-
phatic system is an open-ended sys-
tem that starts at the lymphatic
capillaries and progresses to larger
lymphatic

Arteriole

Lymphatic
capillary

Interstitial
fluid

vessels. Adapted from

Martini and Nath (2009).

Lymph
flow

Blood
flow