The alimentary or gastrointestinal tract is not an internal organ of the
body but a tube passing through it from the mouth to the anus (Figure
6.6). Its principal functions are the digestion and absorption of food and
the excretion of waste. Unlike most of the body’s other external surfaces,
it is not lined with a dry protective skin and so, although it possesses
some protective features, it offers a more congenial environment for
micro-organisms and an easier route by which they can penetrate the
body.
In the mouth, food is mixed with saliva and broken down mechani-
cally to increase the surface area available for attack by digestive
enzymes. Saliva is an alkaline fluid containing starch-degrading (am-
ylase) enzyme and the antimicrobial factors immunoglobulin (IgA),
lysozyme, lactoferrin and lactoperoxidase. It provides lubrication to
assist chewing and swallowing and performs a cleansing function, rinsing
the teeth and mouth to remove debris. On average, an adult secretes and
swallows about 1.5 l of saliva each day.
Figure 6.6 The gastrointestinal tract. Inset: expanded view of inner surface of small
intestine
Chapter 6 173