Lecture 21: The Digestive System, Part 1
1. Overview and Anatomy of the Digestive System
The digestive system can be split into two 'compartments'. The digestive tract is the tube that runs
from the mouth, down the esophagus, to the stomach, and then through the intestines to the rectum and
anus. The accessory glands are organs that secrete substances crucial for digestion, like enzymes, acids
and hormones - such organs include the salivary glands, the gall bladder, the pancreas and the liver.
The mouth leads to the esophagus which connects the mouth to the stomach. There are two
esophageal sphincters; the upper esophageal sphincter at the top of the esophagus and the lower
esophageal sphincter at the border of the esophagus and stomach. They control food movement into the
esophagus and stomach, respectively and the lower sphincter also prevents backflow of the stomach
contents into the esophagus. The stomach empties into the small intestine, with the pyloric sphincter
regulating the flow of partially-digested food (chyme) from the stomach into the first part of the small
intestine. The pyloric sphincter is controlled by the contraction of the stomach muscles, and by the
products of food breakdown in the initial section of the small intestine, the duodenum. The absorption
of nutrients from the digestion of food occurs across the gut wall of the small intestines. The small
intestine eventually enters into the large intestine, by which time most of the nutrients have been taken
out of the food - however, the large intestine still plays an important role in water and ion reabsorption.
Finally, fully digested foodstuffs (now feces) moves into the rectum and are voided through the anus.
Waves of mass peristalsis (organized muscle contractions designed to push something down a
muscular tube) trigger the defecation reflex at the very end of the digestive process. 2
2. Major Processes within the Digestive System
There are four major general categories of processes that occur within the digestive system, in order to
digest foods, absorb nutrients and void wastes. These are motility, digestion, secretion and
absorption. The digestive tract is very motile, surrounded by two layers of smooth muscle. This
muscle is autonomic - it contracts without any nervous input to keep the contents of the digestive tract
moving. However, there can also be nervous input that can enhance or slow muscle contraction.
Digestive processes are aided by both chemical and mechanical action. There are a number of secretory
processes and various substances are secreted by the accessory glands to aid digestion. Finally,
absorption occurs, as nutrients diffuse or are transported across the gut wall and into the blood.
3. Layers of the Digestive System
The innermost layer of the digestive system is the mucosa; just above it is the sub-mucosa which
contains blood vessels and some secretory cells. The sub-mucosa is connective tissue that links the
mucosa with a layer of smooth muscle, which itself is made up of an inner circular layer and an outer
longitudinal layer. The difference is that the muscles of the former are wrapped around the tube of the
gut, whilst the latter muscles run parallel along its length. In between the two layers of muscle lies a
dense network of nerves called the myenteric nerve plexis - the second nerve plexis, the sub-mucosal
nerve plexis, lies between the sub-mucosa and the muscle layer. The nerve plexes play an important
role in regulating the secretion of substances into the digestive tract (sub-mucosal plexis) and muscle
contraction (myenteric plexis). Finally, the outermost layer is called the serosa. 3
The smooth muscle contracts in what are called waves of peristalsis (successive contractions of the
muscle) that serve to move substances (food in the upper digestive tract, feces in the lower tract)
through the gut tube. During these waves of peristalsis, the circular muscle contracts behind the food
forcing it forward. The longitudinal muscle then contracts from back to front (from anus to mouth)
causing the food to move forward along the gut. Waves of peristalsis play an important role in moving
food through the digestive tract, particularly in the intestines. In the esophagus, gravity is able to do
most of the work (unless a person is eating while lying down) although muscular contraction helps to
force food down that otherwise might get stuck.
5. The Salivary Glands and Saliva
The salivary glands produce saliva. The sublingual gland is located underneath the tongue (lingual
means tongue), the submandibular gland (the mandible is the lower jaw) underneath the lower jaw
and the two parotid glands (which are the largest) are within the cheeks. All of these glands contain
secretory cells which produce the components of the saliva and ducts to move the saliva into the mouth.
Saliva has four main components: bicarbonate, which neutralizes acid; mucus, which lubricates;
salivary amylase, an enzyme which breaks down starches and glycogen; and lysozyme, which lyses 4
bacteria. The alkaline nature of saliva is crucial given that salivary amylase will not function in an
acidic environment. Many enzymes will only function in environments that are very acidic or very
alkaline, and because the digestive system changes in pH over its course (for example, the stomach is
very acidic), this means that many enzymes only work in small sections of the tract. The salivary
glands can become infected: an infection of the parotid glands is commonly known as mumps, which
was (and continues to be) a common childhood disease. For children mumps not particularly
dangerous, but in adults it can lead to conditions as serious as encephalitis, meningitis, and orchitis
(swelling in the brain, spinal cord and testicles, respectively).
6. The Esophagus, GERD, Esophagitis and Barrett’s Esophagus
Food from the mouth, lubricated by the saliva, travels into the esophagus. The esophagus is a pliant,
thin walled tube that is normally collapsed. It is normally collapsed to prevent the esophagus from
pressing on, and potentially constricting, the trachea which runs parallel to it. The upper esophageal
sphincter controls the entry of food into the esophagus. The lower esophageal sphincter prevents the
stomach contents from flowing back up into the esophagus.
If the lower sphincter isn’t functioning properly or opens inappropriately (due to disease or exess
pressure in the abdomen or some other factor which causes it to relax), acid reflux (or heartburn) may
occur. The highly acidic contents of the stomach can be forced up into the esophagus, damaging tissue
and causing pain that is often mistaken, due to its location and severity, for a heart attack. Chronic acid
reflux, a condition called gastroesophageal reflux disease (GERD), can be caused by smoking
(nicotine relaxes the lower esophageal sphincter), tight clothing, large meals and bending. Pregnancy or
obesity can also cause GERD due to increases in abdominal pressure than force stomach contents
upward into the esophagus. 5
Any infection or inflammation of the esophagus is referred to as esophagitis. Acid reflux is one
potential cause; as are viral (such as herpes or HIV), fungal, or yeast (such as candida) infections. A
hiatal hernia can prevent proper stomach emptying, and the increase in pressure can also open the
lower esophageal sphincter, causing esophagitis. We'll look more at hiatal hernias in a bit.
A more severe form of esophagitis is Barrett's Esophagus which is caused by years of damage to the
esophageal cells. This leads to the abnormal growth in the esophagus of cells that normally exist in the
intestines or stomach. These cells are acid-resistant and so the growth of these cells in the esophagus
can be seen as a protective mechanism in reaction to chronic acid reflux damage. However, because
these cells are abnormal for the esophagus they almost inevitably become malformed and cancerous -
and so Barrett's Esophagus is a very common precursor to adenocarcinoma.
7. Hiatial Hern