HLTAP301A ANATOMY & PHYSIOLOGY
LECTURE 11 - THE DIGESTIVE SYSTEM
The digestive system takes in food, breaks it down into nutrient molecules and
absorbs them into the bloodstream, and then rids the body of the indigestible
remains.
Food choices can have an effect on our health. Biologic drives for foods high
in fat and simple carbohydrates (or sugars) played an important part in early
human survival. These foods, which are rare in nature, provide quick and
sustaining energy sources. In today’s society, we still have this biologic
craving for fats and sugars, even though we no longer expend the energy that
they provide. The result is an epidemic of obesity.
Emotional eating is one type of substitute for lack of touch stimulation and
loving relationships with others. Foods such as chocolate and other
fat/carbohydrate combinations generate serotonin and other ‘feel-good’ neurochemicals just as effectively as a hug does.
Eating food is often at the heart of social rituals – what would Christmas be
without all the food? Healing practices usually involve the ingestion of herbs.
Mating behaviours involve food and the act of eating stimulates our lips and
tongue and provides comfort and sensory stimulation.
The organs of the digestive system are broken in to two categories – those
forming the alimentary canal and the accessory digestive organs.
The alimentary canal breaks food down into smaller fragments and absorbs
those fragments through its lining into the blood. The accessory organs
(teeth, tongue, salivary glands, pancreas, liver and gallbladder) assist the
process of digestive breakdown in various ways.
The alimentary canal, which is also called the gastrointestinal [GI] tract
(GIT), is a continuous muscular tube that winds through the ventral body
cavity and is open at both ends. It’s organs are the mouth, pharynx,
esophagus, stomach, small intestine and large intestine. The terminal
opening is called the anus. In a cadaver, the GIT is approximately 9m long,
but in a living person it is much shorter because of its relatively constant
muscle tone.
Food enters the digestive tract through the mouth, or oral cavity. The process
is known as ingestion. The oral cavity is a mucous membrane-lined cavity,
with the lips protecting its anterior opening, the cheeks forming its lateral walls
and the hard and soft palate forming its roof. The area contained by the teeth
is the oral cavity proper.
The muscular tongue occupies the floor of the mouth and is anchored by the
frenulum, a fold of mucus membrane which limits posterior movement.
14/3/16
1 of 4
LECTURE 11
HLTAP301A ANATOMY & PHYSIOLOGY
Children born with an extremely short frenulum are referred to an ‘tongue-tied’
and usually have distorted speech because of the restricted movement of the
tongue. This condition can be repaired by surgically cutting the frenulum.
The paired palatine tonsils are located posterior to the soft palate and the
lingual tonsils cover the base of the tongue just beyond. They are part of the
body’s defense system and when they become inflamed and enlarged, the
partially block the entrance into the pharynx, making swallowing difficult and
painful.
As food enters the mouth, the closed lips and cheeks hold the food between
the teeth during chewing. The tongue constantly mixes food with saliva
during chewing and initiates swallowing. Papillae containing taste buds (taste
receptors) are found on the tongue surface, allowing use to enjoy and
appreciate food as it is eaten.
From the mouth, food passes posteriorly into the pharynx.
Actually
swallowing food is a complex process that requires the coordinated activity of
the tongue, soft palate, pharynx and esophagus – it is also the first step in
propulsion (step 2 in the digestive process). The buccal phase, which is
voluntary, occurs in the mouth – the food is chewed and mixed well with saliva
to form a bolus.
The second phase, the pharyngeal-esophageal phase, is involuntary. The
parasympathetic nervous system controls this phase – the tongue blocks off
the mouth, the soft palate closes off the nasal passages. The larynx rises and
the epiglottis folds back to close off the respiratory passageway. Food is
moved through the pharynx into the esophagus by wavelike peristaltic
contraction of the muscle walls – first the longitudinal muscles contract then
the circular muscles contract. The esophageal sphincter relaxes to allow food
past and then contracts again as the larynx and epiglottis return to their
original position.
The esophagus (also known as the gullet) runs from the pharynx through the
diaphragm to the stomach and is about 25cm long.
The entire lining of the GIT is a mucous membrane made up of three layers of
tissue – epithelium, connective tissue and smooth muscle. The walls of the
alimentary canal organs from the esophagus to the large intestine are made
up of the same four basic tissue layers – the mucosa, submucosa, muscularis
externa and the serosa.
The mucosa is the innermost layer that lines the cavity of the organ. The
submucosa is a soft connective tissue layer that contains blood vessels, nerve
endings, lymph nodules and lymphatic vessels. The muscularis externa is a
double layer of muscles – the inner layer is circular and the outer layer is
longitudinal smooth muscle cells. The serosa is also known as the visceral
peritoneum.
14/3/16
2 of 4
LECTURE 11
HLTAP301A ANATOMY & PHYSIOLOGY
The abdominal cavity (or ventral cavity) is lines with a mucous membrane
known as the peritoneum that prevents friction. The parietal peritoneum lies
against the body wall and the visceral peritoneum surrounds each organ.
Organs that are only covered with peritoneum on the anterior surface are
known as retroperitoneal.
The stomach, a sac-like organ that is actually an enlargement of the GIT, is
approximately 25cm long and can hold up to 4 litres of food when full. When
it is empty, it collapses inward on itself and it’s mucosa is thrown into large
folds called rugae. Food enters the stomach from the esophagus through the
cardioesophageal sphincter. Apart from the usual two muscle layers, the
stomach has a third oblique layer in the muscularis externa that allows food to
not only be moved along but to be churned, mixed and pummeled, physically
breaking it down to small fragments. This process is known as mechanical
digestion.
The lining of the stomach is dotted with deep gastric pits which lead into
gastric glands that secrete a fluid known as gastric juice – about 2-3 litres is
produced every day.
Asprin and alcohol are absorbed through the stomach, but everything else is
broken into smaller fragments and after the food has been processed in the
stomach, it looks like lumpy, thick cream, and is known as chyme. The chyme
enters the small intestine through the pyloric sphincter.
The small intestine is the major digestive organ of the body, a muscular tube
that is about 2m in length (in a living person). It is suspended from the
posterior abdominal wall by the mesentery. The small intestine is divided into
three parts – the duodenum, the jejunum and the ileum.
The duodenum, the shortest part of the small intestine at 25cm long, curves
around the head of the pancreas. Bile ducts and the pancreatic duct join at
the hepatopancreatic ampulla and bile (which is produced by the liver) and
pancreatic juice (obviously produced by the pancreas) enter the duodenum.
Almost all food absorption takes place in the small intestine, and the jejunum,
at 2.5m long, is the major structure where this absorption takes place. Glands
located in the walls of the jejunum provide secretions for the digestive
process. The ileum (3.6m long) connects the small intestine to the large
intestine at the ileocecal valve.
The walls of the small intestine have three structures that increase the
absorptive surface – microvilli, villi and circular folds. Food reaching the small
intestine is only partially digested.
This is where chemical digestion
intensifies.
The circular folds of the small intestine decrease in number towards the end
of the small intestine and local collections of lymphatic tissue (Peyer’s
patches) increase – this occurs because the remaining food residue that
reaches the end of the small intestine contains large numbers of bacteria,
which must be prevented from entering the bloodstream, where possible.
14/3/16
3 of 4
LECTURE 11
HLTAP301A ANATOMY & PHYSIOLOGY
The large intestine is about 1.5m in length ands it’s major function is to
absorb water from the indigestive food residue, thereby forming feces to be
eliminated from the body. The large intestine is broken into several parts –
the cecum, the appendix, the colon, the rectum and the anal canal. The
cecum is a blind pouch that receives the digested matter from the ileum. The
appendix is a potential trouble spot, because it is an ideal area for bacteria to
accumulate and multiply.
Inflammation of this area is called appendicitis and if untreated, can lead to
peritonitis, which can be fatal. The colon is divided into distinct regions – the
ascending colon, the transverse colon, the descending colon and the sigmoid
colon. The rectum leads to the anal canal, which has an internal involuntary
sphincter and an external voluntary sphincter which opens and closes the
anus during defecation.
Accessory Digestive Organs
The pancreas produces a wide spectrum of enzymes, responsible for food
breakdown which are secreted into the duodenum via the pancreatic ducts. It
is also an endocrine gland, which will be discussed in a later lecture.
The liver is the largest gland in the body and one of the body’s most
important organs. It has many metabolic and regulatory roles, but it’s
digestive role is to produce bile, which leaves the liver via the common bile
duct. The gallbladder stores and concentrates bile, releasing it as required.
If the bile becomes too concentrated, the cholesterol is contains may
crystallize, forming bilestones which are extremely painful.
The salivary glands excrete saliva, which is a mixture of mucus and serous
fluid which contains salivary amylase and substances that inhibit bacteria.
14/3/16
4 of 4
LECTURE 11