The Digestive System
The digestive system consists of a muscular tube, called the digestive tract (i.e., mouth, pharynx,
esophagus, stomach, small intestine, and large intestine), and various accessory organs (i.e., teeth,
tongue, and various glandular organs salivary glands, liver, pancreas).
o Digestion is basically the mechanical and chemical breakdown of complex food into small
molecules that can be absorbed by the epithelium that lines the digestive tract for transfer into
the circulating blood.
Muscle layers and movement
o The digestive tract contains visceral smooth muscle tissue. Being comprised of smooth muscle,
the digestive system is under involuntary muscle control. The initial activation stimulus can be
from a neighbouring motor neuron, local response to chemicals, hormones, the concentration
of O2and CO ,2or physical factors such as extreme stretching or irritation. However, smooth
muscle can maintain a stretched position without contraction (or until a contraction signal is
sent) this ability to tolerate extreme stretching is called plasticity.
These smooth muscle cells are electrically connected to one another via gap junctions.
Thus, when one visceral smooth muscle cell contracts, the contraction spreads in a
wave that travels throughout the tissue.
Muscle contractions that create waves along the digestive tract so as to move the
bolus (mass of food) along its length this occurs in the esophagus, as well as in the
small and large intestines.
Longitudinal muscles contract, shortening the adjacent segments of the
Circular muscles contract next causing the materials (i.e., bolus) to move in the
Circular muscle The Digestive System
Muscle contractions that occur in most areas of the small intestine and some regions
of the large intestine.
These movements churn and fragment the digestive materials, mixing the contents
with intestinal secretions. They do not produce any net movement in a particular
direction they are just for mixing.
o Segmentation and peristalsis may be triggered by pacesetter cells (found in the muscularis
mucosae and externa; undergo spontaneous depolarization triggering contractions).
Segmentation and peristalsis may also be triggered by hormones, chemicals, physical
stimulation, or by afferent and efferent fibers within the glossopharyngeal, vagus, and/or
pelvic nerves that synapse on the myenteric plexus.
o The peritoneum is the serous membrane that forms the lining of the abdominal cavity. The
peritoneal membrane is a semi-permeable membrane that lines the abdominal wall (parietal
peritoneum) and covers the abdominal organs (visceral peritoneum).
o The peritoneal lining continually produces a watery peritoneal fluid that lubricates the
peritoneal surfaces. The Digestive System
Fused, double sheets of serous membrane that supports and stabilizes the position of
an organ in the abdominopelvic cavity and provides a route for the associated blood
vessels, nerves, and lymphatics.
The mesentery, within the peritoneal cavity, is separated into two regions:
Dorsal mesentery AKA greater omentum
- Located along the greater curvature of the stomach and extends
inferiorly between the body wall and the anterior surface of the small
- The loose connective tissue within the mesentery of the greater
omentum usually contains a thick layer of adipose tissue. The lipids in
this adipose tissue are an important energy reserve as well as an
important insulator that reduces heat loss across the anterior abdominal
Ventral mesentery AKA lesser omentum
- Located on the lesser curvature of the stomach, between the stomach
and the liver.
Omentum literally means fat skin.
o The oral cavity (or buccal cavity), is lined by oral mucosa (non-keratinized stratified squamous
epithelium) that protects the mouth from abrasion during the ingestion of food.
o The roof of the oral cavity is formed by the hard and soft palates, while the tongue dominates
the floor of the oral cavity.