Human Physiology, 14th edition (2016)

Blood, Heart, and Circulation 443

The smallest vessels of the lymphatic system are the lymphatic

capillaries ( fig.  13.36 ). Lymphatic capillaries are microscopic

closed-ended tubes that form vast networks in the intercellular

spaces within most organs. Because the walls of lymphatic cap-

illaries are composed of endothelial cells with porous junctions,

interstitial fluid, proteins, extravasated white blood cells, microor-

ganisms, and absorbed fat (in the intestine) can easily enter. Once

fluid enters the lymphatic capillaries, it is referred to as lymph.

From merging lymphatic capillaries, the lymph is carried

into larger lymphatic vessels called lymph ducts. The walls of

lymph ducts are similar to those of veins. They have the same

three layers and also contain valves to prevent backflow. Fluid

movement within these vessels occurs as a result of peristaltic

waves of contraction (chapter 12, section 12.6). The smooth

muscle within the lymph ducts contains a pacemaker that

initiates action potentials associated with the entry of Ca^2 1 ,

which stimulates contraction. The activity of the pacemaker,

and hence the peristaltic waves of contraction, are increased in

response to stretch of the vessel. The lymph ducts eventually

empty into one of two principal vessels: the thoracic duct or

the right lymphatic duct. These ducts drain the lymph into the

left and right subclavian veins, respectively. Thus interstitial

fluid, which is formed by filtration of plasma out of blood cap-

illaries (chapter 14, section 14.2), is ultimately returned to the

cardiovascular system ( fig. 13.37 ).

Before the lymph is returned to the cardiovascular system,

it is filtered through lymph nodes ( fig.  13.38 ). Lymph nodes

Figure 13.36 The relationship between blood
capillaries and lymphatic capillaries. Notice that lymphatic
capillaries are blind-ended. They are, however, highly permeable,
so that excess fluid and protein within the interstitial space can
drain into the lymphatic system.

Tissue cells

Lymph capillary

Interstitial space

Capillary

bed

Venule

Lymph duct

Arteriole

Lymph flow Lymphatic

capillaries

Pulmonary

capillary

network

Lymph node

Lymphatic

vessels

Lymph node

Systemic

capillary

network

Lymphatic

capillaries

Blood

flow

Figure 13.37 The relationship between the

circulatory and lymphatic systems. This schematic

illustrates that the lymphatic system transports fluid from the

interstitial space back to the blood through a system of lymphatic

vessels. Lymph is eventually returned to the vascular system at

the subclavian veins.

CLINICAL APPLICATION

Lymphedema is a swelling of an arm or leg caused by

excessive amounts of fluid and protein in the interstitial fluid.

This results from blockage or destruction of the lymphatic

drainage, usually because of surgery or radiation treatments

for breast and other cancers. There are presently no cures

for lymphedema, and the protein-rich interstitial fluid can

trigger inflammation that leads to degenerative changes in

the surrounding tissues. Lymphedema can also occur in the

tropical equatorial regions because of infection with a spe-

cies of nematode worm, which can block lymphatic vessels

and cause enormous swelling of a leg or scrotum in the dis-

ease elephantiasis (chapter 14; see fig. 14.10).