752
UNIT 4
Maintenance of the Body
20
Lymphatic System
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
transport.
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
the
lymphatic vessels
, or
lymphatics
, 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
lymph
(
lymph
5
clear water).
Distribution and Structure
of Lymphatic Vessels
Te lymphatic vessels form a one-way system in which lymph
flows only toward the heart.
Lymphatic Capillaries
Te transport of lymph begins in microscopic blind-ended
lymphatic capillaries
(Figure 20.1a)
. 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
minivalves
(Figure 20.1b).
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
Tissue
fluid
Tissue cell
Lymphatic system:
Lymph duct
Lymph trunk
Lymph node
Lymphatic capillary
Blood capillaries
Blood
capillaries
Lymphatic
capillaries
Collecting lymphatic
vessels, with valves
Filaments anchored
to connective tissue
Fibroblast in loose
connective tissue
Endothelial cell
Flaplike minivalve
(a) Structural relationship between a capillary
bed of the blood vascular system and
lymphatic capillaries.
(b) Lymphatic capillaries are blind-ended tubes in which
adjacent endothelial cells overlap each other,
forming flaplike minivalves.
Heart
Arterial system
Venous system
Figure 20.1
Distribution and special features of lymphatic
capillaries.
Arrows in (a) indicate direction of fluid movement.
previous page 786 Human Anatomy and Physiology (9th ed ) 2012 read online next page 788 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off