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
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.
Elephantiasis is a disease resulting when mosquito-borne ﬁ laria
worms block the return of lymph to the vascular system.
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.