(3) Formation of lymphocytes. The lymphatic system is one of the main
sources for the manufacture of lymphocytes. Lymphocytes are almost the only cells
found in lymph. Occasionally monocytes are seen. Lymphocytes are a type of white
blood cell formed in the lymph nodes and other structures of the lymphatic system. The
number of lymphocytes found in the lymph varies with the time and the location.
Between 2,000 and 20,000 lymphocytes per microliter have been found in the lymph of
man (1 microliter = 1 cubic millimeter = 0.001 mL).
(4) Draining the tissue spaces. The lymphatic system provides an
accessory route by which fluids can travel from the interstitial spaces into the blood.
The lymphatics can carry proteins and even larger particulate matter away from the
tissue spaces. Neither can be removed by direct reabsorption into a blood capillary.
The removal of proteins from the tissue spaces is an absolutely essential function
performed by the lymphatic system.
b. Anatomical Distribution. Very small lymphatic vessels originate in the
intercellular spaces. They have endothelial walls that are extremely thin and very
permeable to fluid from the tissues. Their contents are not in direct contact with the
cells, nor do they mix with the tissue fluids.
c. Formation of Lymph. Lymph is formed by the filtration of tissue fluid into the
smallest lymphatic vessels. Lymph is formed from the excess of tissue fluid not
reabsorbed by the capillaries of the blood. In some tissues such as the intestinal tract,
liver, and heart, lymph formation is almost continuous. In some other tissues the
formation of lymph may be quite considerable only at certain times. The amount of
lymph formed varies with different parts of the body at different times. Lymph,
therefore, originates as interstitial fluid that filters from the blood capillaries through the
tissues into the lymphatics.
d. Composition and Properties. The chemical composition of lymph may vary
considerably from one lymphatic area to another. Lymph in the right lymphatic duct
varies considerably from lymph in the thoracic duct; lymph in the thoracic duct is high in
fat content due to the large amount of fat absorbed into the lymph in the area of the
small intestine. Except for its lower protein concentration, lymph is composed of a
watery solution of the same substances as those of blood plasma. The only cell
normally found in lymph is the lymphocyte, which is manufactured in the lymphatics.
e. Factors That Influence Lymph Flow.
(1) Valves. As mentioned previously, the lymphatic vessels have valves like
those of the veins that prevent lymph from flowing backward. Lymphatic flow is largely
due to external pressure variations, resulting from arterial pulsations, skeletal muscle
contractions, and respiration. The presence of many valves in the lymphatic vessels
ensures that the lymph always moves in the correct direction.