the interstitial fluid enters the lymphatic capillaries,
the fluid is called lymph.
Lymphatic Vessels
Lymphatic vessels are formed from the merging
of lymphatic capillaries (fig. 1-37). Lymphatic
vessels, also known simply as lymphatics, are similar
to veins in structure. The vessel walls are composed of
three layers: an inner layer of endothelial tissue, a
middle layer of smooth muscle and elastic fibers, and
an outer layer of connective tissue. Like a vein, the
lymphatic vessel has valves to prevent backflow of
lymph. The larger lymphatic vessels lead to
specialized organs called lymph nodes. After leaving
these structures, the vessels merge to form still larger
lymphatic trunks.
Lymphatic Trunks and Ducts
Lymphatic trunks drain lymph from large regions
in the body. The lymphatic trunks are usually named
after the region they serve, such as the subclavian
trunk that drains the arm. There are many lymphatic
trunks through out the body. These lymphatic trunks
then join one of two collecting ducts, the thoracic
duct and the right lymphatic duct (fig. 1-38).
Lymphatic trunks from the upper half of the right
side of the body converge to form the right lymphatic
duct, which empties into the right subclavian vein.
Drainage from the remainder of the body is by way of
the thoracic duct, which empties into the left
subclavian vein.
LYMPH NODES
Lymph nodes, which are frequently called glands
but are not true glands, are small bean-shaped bodies of
lymphatic tissue found in groups of two to fifteen
along the course of the lymph vessels (fig. 1-38).
Major locations of lymph nodes are in the following
regions: cervical, axillary, inguinal, pelvic cavity,
abdominal cavity, and thoracic cavity. Lymph nodes
vary in size and act as filters to remove bacteria and
particles from the lymph stream. Lymph nodes
produce lymphocytes, which help defend the body
against harmful foreign particles, such as bacteria,
cells, and viruses. Lymph nodes also contain
macrophages, which engulf and destroy foreign
substances, damaged cells, and cellular debris.
THE RESPIRATORY SYSTEM
LEARNING OBJECTIVE: Identify the
location and function of each part of the
respiratory system, and recall the process of
respiration.
Respiration is the exchange of oxygen and carbon
dioxide between the atmosphere and the cells of the
body. There are two phases of respiration:
Physical, or mechanical, respiration involves
the motion of the diaphragm and rib cage. The
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LYMPH
CAPILLARY
TISSUE
CELLS
VENULE
LYMPHATIC
VESSEL
ARTERIOLE
HM3F0137
Figure 1-37.—Lymphatic capillary and vessel.
