402
Chapter 12
Lymph
node
Valve
Lymphatic
vessel
Lymphatic
vessel
Lymph
node
Lymph capillaries
Systemic
blood
capillaries
Artery
Blood
capillaries
in lungs
Lymph capillaries
Vein
Heart
(a)
Thymus
Right lymphatic
duct
Intestinal
lymph node
Lymphatic vessel
(transports lymph)
Bone
marrow
Tonsils
Cervical
lymph node
Thoracic duct
Axillary
lymph node
Mammary
plexus
Subclavian
veins
Thoracic
duct
Spleen
Lacteals in
intestinal wall
Inguinal
lymph node
(b)
Figure 12–47
The lymphatic system (green) in relation to the cardiovascular system (blue and red). (a) The lymphatic system is a one-way system from
interstitial fl uid to the cardiovascular system. (b) Prior to re-entering the blood at the subclavian veins, lymph fl ows through lymph nodes in the
neck, armpits, groin, and around the intestines.
As lymph formation increases, however, like when there is
increased fl uid fi ltration out of blood vessel capillaries, the
increased fl uid entering the lymphatics stretches the walls and
triggers rhythmical contractions of the smooth muscle. This
constitutes a negative feedback mechanism for adjusting the
rate of lymph fl ow to the rate of lymph formation and thereby
preventing edema.
In addition, the smooth muscle of the lymphatic ves-
sels is innervated by sympathetic neurons, and excitation of
these neurons in various physiological states such as exercise
may contribute to increased lymph fl ow. Forces external to the
lymphatic vessels also enhance lymph fl ow. These include the
same external forces we described for veins—the skeletal mus-
cle pump and respiratory pump.
Figure 12–48
Elephantiasis is a disease resulting when mosquito-borne fi laria
worms block the return of lymph to the vascular system.
Figure 12–47
physiological
inquiry
How might periodic ingestion of extra fl uids be expected to
increase the fl ow of lymph?
Answer can be found at end of chapter.
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