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Drugs acting on the gastrointestinal system
other components of the mononuclear phagocyte system.
This barrier provides rapid protection for the rest of the
body if any of the bacteria or other foreign agents in
the GI tract should be absorbed into the body.
Layers of the GI tract
The GI tube is composed of four layers: the mucosa, the
muscularis mucosa, the
nerve plexus
(a network of nerve
fibres running through the wall of the GI tract that
allows local reflexes and control) and the adventitia.
Mucosal layer
The mucosal layer provides the inner lining of the GI
tract. It can be seen in the mouth and is fairly consistent
throughout the tube. It is important to remember when
assessing a person that if the mouth is very dry or full
of lesions, that is a reflection of the state of the entire
GI tract and may indicate that the person has difficulty
digesting or absorbing nutrients. This layer has an epi-
thelial component and a connective tissue component.
Muscularis mucosa layer
The muscularis mucosa layer is made up of muscles.
Most of the GI tract has two muscle layers. One layer
runs circularly around the tube, helping keep the tube
open and squeezing the tube to aid digestion and
motility. The other layer runs horizontally, which helps
propel the gastrointestinal contents down the tract. The
stomach has a third layer of muscle, which runs obliquely
and gives the stomach the ability to move contents in a
churning motion.
Nerve plexus layer
The nerve plexus has two layers of nerves—one submu-
cosal layer and one myenteric layer. These nerves allow
the GI tract local control over movement, secretions and
digestion. The nerves respond to local stimuli and act
on the contents of the GI tract accordingly. The GI tract
is also innervated by the sympathetic and parasympa-
thetic nervous systems. These systems can slow down or
speed up the activity in the GI tract but cannot initiate
local activity. The sympathetic system is stimulated
during times of stress (“fight-or-flight” response) when
digestion is not a priority. To slow the GI tract, the sym-
pathetic system decreases muscle tone, secretions and
contractions, and increases sphincter tone. By shutting
down the GI activity, the body saves energy for other
activities. In contrast, the parasympathetic system (“rest-
and-digest” response) stimulates the GI tract, increasing
muscle tone, secretions and contractions and decreasing
sphincter tone, allowing easy movement.
Adventitia layer
The adventitia is the outer layer of the GI tract. It serves
as a supportive layer and helps the tube maintain its
shape and position (see Figure 56.2).
Gastrointestinal activities
The GI system has four major activities:
•
Secretion
of enzymes, acid, bicarbonate and mucus
•
Absorption
of water and almost all of the essential
nutrients needed by the body
•
Digestion
of food into usable and absorbable
components
•
Motility
(movement) of food and secretions through
the system (what is not used is excreted in the form of
faeces)
These functions are discussed in detail in the following
sections.
Secretion
The GI tract secretes various compounds to aid the
movement of the food bolus through the GI tube, to
protect the inner layer of the GI tract from injury and
to facilitate the digestion and absorption of nutri-
ents (see Figure 56.1). Secretions begin in the mouth.
Saliva
, which contains water and digestive enzymes, is
secreted from the salivary glands to begin the digestive
process and to facilitate swallowing by making the bolus
slippery.
Mucus is also produced in the mouth to protect the
epithelial lining and to aid in swallowing. The oesopha-
gus produces mucus to protect the inner lining of the GI
tract and to further facilitate the movement of the bolus
down the tube.
The stomach produces acid and digestive enzymes.
In addition, it generates a large amount of mucus
to protect the stomach lining from the acid and the
enzymes. In the stomach, secretion begins with what is
called the cephalic phase of digestion. The sight, smell or
taste of food stimulates the stomach to begin secreting
before any food reaches the stomach. Once the bolus of
food arrives at the stomach,
gastrin
is secreted. Gastrin
stimulates the stomach muscles to contract, the parietal
Lumen of the
GI tract
Mucosal layer
Adventitia
Circular
muscularis mucosa
Longitudinal
muscularis mucosa
Nerve plexus
FIGURE 56.2
Layers of the gastrointestinal tract.
