Section VI. SOME PROTECTIVE MECHANISMS ASSOCIATED WITH
THE HUMAN DIGESTIVE SYSTEM
The human digestive system is essentially a continuous tube. It is open at both
ends. Therefore, the lumen (cavity) connects directly with the surrounding environment.
a. Along with the ingested food, almost anything can pass through the mouth
into the digestive system. Almost anything does enter the digestive system.
b. The digestive tract is open to the surrounding environment also at the other
end, the anus.
6-24. COMMENT ABOUT THE RETICULOENDOTHELIAL SYSTEM
As indicated above, a variety of toxic materials and/or microorganisms may be in-
cluded with the ingested foods. To protect against these undesired materials, special
protective mechanisms are associated with the human digestive system. Such
protective mechanisms are said to belong to the reticuloendothelial system. This term
refers to the association of such mechanisms with a particular layer of epithelial cells.
6-25. COMMENT ABOUT LYMPHOID TISSUES
a. The lymphocyte is an important type of white blood cell that is also found in
the interspaces of a tissue called lymphoid (or lymphatic) tissue. Lymphocytes signal
other types of white blood cells to phagocytize (engulf) foreign materials found within
the body. The lymphoid tissues are particularly important in individuals from birth until
about 15 years of age. The mass of lymphoid tissue found in the body of a 12-year-old
is about twice the mass found in a full-grown adult. (Between 6 and 15 years of age,
the immune system of the blood becomes the primary protector of the body from
b. The lymphoid tissues are a primary component of the reticuloendothelial
Tonsils are aggregates of lymphoid tissue found at the beginning of the
pharynx. There are three pairs of tonsils. Together, they form a ring of lymphoid tissue
at the beginning of the pharynx. This ring, called Waldeyer's ring, completely surrounds
the entrance to the pharynx from both the mouth (digestive entrance) and the nose and
nasal chambers (respiratory entrance).
a. In the upper recess of the pharynx is the pair of pharyngeal tonsils (commonly
known as the adenoids).