Maintenance of the Body
List the functions of the lymphatic vessels.
Describe the structure and distribution of lymphatic vessels.
Describe the source of lymph and mechanism(s) of lymph
As blood circulates through the body, nutrients, wastes, and
gases are exchanged between the blood and the interstitial
ﬂuid. As we explained in
Focus on Bulk Flow Across Capillaries
(Figure 19.17 on pp. 718–719), the hydrostatic and colloid os-
motic pressures operating at capillary beds force ﬂuid out of
the blood at the arterial ends of the beds (“upstream”) and
cause most of it to be reabsorbed at the venous ends (“down-
stream”). Te ﬂuid that remains behind in the tissue spaces, as
much as 3 L daily, becomes part of the interstitial ﬂuid.
Tis leaked ﬂuid, plus any plasma proteins that escape from the
bloodstream, must somehow be returned to the blood to ensure
that the cardiovascular system has suﬃcient blood volume to oper-
ate properly. Tis problem of circulatory dynamics is resolved by
, elaborate networks of drain-
age vessels that collect the excess protein-containing interstitial
ﬂuid and return it to the bloodstream. Once interstitial ﬂuid enters
the lymphatic vessels, it is called
Distribution and Structure
of Lymphatic Vessels
Te lymphatic vessels form a one-way system in which lymph
ﬂows only toward the heart.
Te transport of lymph begins in microscopic blind-ended
. Tese capillaries weave
between the tissue cells and blood capillaries in the loose con-
nective tissues of the body. Lymphatic capillaries are wide-
spread, but they are absent from bones and teeth, bone marrow,
and the entire central nervous system (where the excess tissue
ﬂuid drains into the cerebrospinal ﬂuid).
Although similar to blood capillaries, lymphatic capillaries
are so remarkably permeable that they were once thought to be
open at one end like a straw. We now know that they owe their
permeability to two unique structural modiﬁcations:
Te endothelial cells forming the walls of lymphatic capillar-
ies are not tightly joined. Instead, the edges of adjacent cells
overlap each other loosely, forming easily opened, ﬂaplike
Collagen ﬁlaments anchor the endothelial cells to surround-
ing structures so that any increase in interstitial ﬂuid volume
opens the minivalves, rather than causing the lymphatic cap-
illaries to collapse.
So, what we have is a system analogous to one-way swinging
doors in the lymphatic capillary wall. When ﬂuid pressure in the
interstitial space is greater than the pressure in the lymphatic
capillary, the minivalve ﬂaps gape open, allowing ﬂuid to enter
the lymphatic capillary. However, when the pressure is greater
vessels, with valves
to connective tissue
Fibroblast in loose
(a) Structural relationship between a capillary
bed of the blood vascular system and
(b) Lymphatic capillaries are blind-ended tubes in which
adjacent endothelial cells overlap each other,
forming flaplike minivalves.
Distribution and special features of lymphatic
Arrows in (a) indicate direction of ﬂuid movement.