C H A P T E R 5 6
Introduction to the gastrointestinal system
889
T
he gastrointestinal (GI) system is the only system in the
body that is open to the external environment. It begins
at the mouth and ends at the anus. The GI system is
responsible for only a very small part of waste excre-
tion. The kidneys and lungs are responsible for excreting
most of the waste products of normal metabolism.
STRUCTURE AND FUNCTION OF THE
GASTROINTESTINAL SYSTEM
The GI system is composed of one continuous tube that
begins at the mouth, progresses through the oesopha-
gus, stomach and small and large intestines and ends at
the anus. The pancreas, liver and gallbladder are acces-
sory organs that support the functions of the GI system
(see Figure 56.1).
Structures
The tube that comprises the GI tract is continuous
with the external environment, opening at the mouth
and again at the anus. Because of this, the GI tract
contains many foreign agents and bacteria that are not
found in the rest of the body. The tube begins in the
mouth, which has salivary glands that secrete digestive
enzymes and lubricants to facilitate swallowing. The
mouth leads to the oesophagus, which connects to the
stomach. The stomach is responsible for mechanical
and chemical breakdown of foods into usable nutrients.
The stomach empties into the small intestine, where
absorption of nutrients occurs. The pancreas deposits
digestive enzymes and sodium bicarbonate into the
beginning of the small intestine to neutralise the acid
from the stomach and to further facilitate digestion. The
liver produces bile, which is stored in the gallbladder.
The bile is very important in the digestion of fats and is
deposited into the small intestine when the gallbladder
is stimulated to contract by the presence of fats. All of
the nutrients absorbed from the small intestine pass into
the liver, which is responsible for processing, storing or
clearing them from the system. The small intestine leads
to the large intestine, which is responsible for excret-
ing any waste products that are in the gastrointestinal
system. The excretion occurs through the rectum and is
an activity that one learns to control.
The peritoneum lines the abdominal wall and also
the viscera, with a small “free space” between the two
layers. It helps to keep the GI tract in place and prevents a
build-up of friction with movement. The greater and lesser
omenta hang from the stomach over the lower GI tract
and are full of lymph nodes, lymphocytes, monocytes and
Tongue
Parotid
gland
Pharynx
Epiglottis
Oesophagus
(peristalsis)
Salivary glands
Liver
Stomach
(churning)
Duodenum
Small intestine
(segmentation)
Rectum
Pancreas
Gall bladder
Large intestine
(mass movement)
FIGURE 56.1
The gastrointestinal tract.
