Biomedical Engineering Reference
In-Depth Information
directly enter the cardiovascular system. The remaining sections of this chapter will dis-
cuss how lymph is formed and its path through the lymphatic system.
Lymphocytes are the last component of the lymphatic system and are specialized cells
that can circulate within the blood or the lymphatic system. Lymphocytes account for
approximately 25% of the entire circulating leukocyte cell population. Under normal con-
ditions, only a small portion of the lymphocyte population is circulating throughout the
body and the vast majority of these cells are stored in lymphatic tissues/organs. The body
contains more than 10
12
lymphocytes. There are three major classes of lymphocytes circu-
lating within the blood, the T-cells (thymus-dependent), the B-cells (bone marrow-
derived), and the natural killer cells (NK cells). Approximately 75% of the lymphocytes
are T-cells, which can be sub-divided into the cytotoxic T-cells, the helper T-cells, and the
suppressor T-cells. The primary function of the cytotoxic T-cells is to attack foreign cells or
destroy the body's cells that have been infected by a virus. The helper T-cells stimulate the
activation and production of T-cells and B-cells during an inflammatory response.
Suppressor T-cells inhibit the activation of T-cells and B-cells. B-cells account for approxi-
mately 15% of the lymphocyte population. Activated B-cells differentiate into plasma cells,
which are responsible for mass production and secretion of antibodies in response to an
infection. Therefore, T-cells play a critical role during the cell-mediated immunity, while
B-cells play a critical role during antibody-mediated immunity. The natural killer cells
make up the remaining 10% of the lymphocyte population. These lymphocytes are respon-
sible for attacking foreign cells and innate cells infected with a virus. These cells continu-
ously monitor the quantity of foreign particles, and therefore, their function is termed
immunologic surveillance. The life span of lymphocytes varies, but the vast majority of
them last for at least 5 years (approximately 80%). Some lymphocytes can last as long as
20 to 30 years in circulation and therefore act as a memory for past foreign particle inva-
sions and can help to stem a secondary attack.
8.2 LYMPH FORMATION
Lymph is composed only of the particular compounds of the interstitial fluid that enters
the lymphatic system. Therefore, as stated before, the composition of lymph is very similar
to that of the interstitial fluid as it first enters the lymphatic vessels. However, under nor-
mal conditions, the protein concentration of lymph is higher than the interstitial fluid pro-
tein concentration because a large portion of lymph is derived from the liver. The liver has
a protein concentration in the range of 6 g/dL and lymph derived from the liver composes
approximately 50% of all lymph within the lymphatic system. The remaining portion of
lymph has a protein concentration that ranges from 2 g/dL to 4 g/dL, which is the aver-
age interstitial protein concentration (recall that the interstitial fluid protein concentration
will vary based on the tissue). This makes the total lymphatic protein concentration in the
range of 3 g/dL to 5 g/dL. A second difference between lymph and interstitial fluid is the
presence of large bacteria. Large bacterial cells can invade the lymphatic system after they
migrate into the interstitial space. These cells will typically be cleared after the lymph
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