9. Functions of the GIT
• The functions of the gastrointestinal tract are digestion and absorption of
nutrients. To serve these functions, there are four major activities of the
gastrointestinal tract.
(1) Motility; propels ingested food from the mouth toward the rectum and
mixes and reduces the size of the food. The rate at which food is propelled
through the gastrointestinal tract is regulated to optimize the time for
digestion and absorption.
(2) Secretions from the salivary glands, pancreas, and gallbladder add fluid,
electrolytes, enzymes, and mucus to the lumen of the gastrointestinal
tract. These secretions further aid in digestion and absorption.
(3) Ingested foods are digested into absorbable molecules.
(4) Nutrients, electrolytes, and water are absorbed from the intestinal lumen
into the bloodstream.
10. Structure of the Gastrointestinal Tract
• The gastrointestinal tract is arranged linearly in the following
sequence: mouth, esophagus, stomach, small intestine (including the
duodenum, jejunum, and ileum), large intestine, and anus.
• Other structures of the gastrointestinal tract are the salivary glands,
pancreas, liver, and gallbladder, all of which serve secretory functions.
11. • The wall of the gastrointestinal tract has two surfaces, mucosal and serosal.
• The mucosal surface faces the lumen, and the serosal surface faces the blood.
• The layers of the gastrointestinal wall are as follows, starting from the lumen and
moving toward the blood:
• A mucosal layer consists of a layer of epithelial cells, a lamina propria, and a muscularis
mucosae. The epithelial cells are specialized to carry out absorptive and secretory functions.
The lamina propria consists primarily of connective tissue, but it also includes blood and
lymph vessels. The muscularis mucosae consists of smooth muscle cells; contraction of the
muscularis mucosae changes the shape and surface area of the epithelial cell layer.
• Beneath the mucosal layer is a submucosal layer, which consists of collagen, elastin, glands,
and the blood vessels of the gastrointestinal tract.
• Motility of the gastrointestinal tract is provided by two layers of smooth muscle, circular
muscle and longitudinal muscle, which are interposed between the submucosa and the
serosa.
• Two plexuses, the submucosal plexus and the myenteric plexus, contain the nervous system
of the gastrointestinal tract. The submucosal plexus (Meissner's plexus) lies between the
submucosa and the circular muscle. The myenteric plexus lies between the circular muscle
and the longitudinal muscle.
12. Innervation of the Gastrointestinal
• The gastrointestinal tract is regulated, in part, by the autonomic
nervous system, which has an extrinsic component and an intrinsic
component;
• The extrinsic component is the sympathetic and parasympathetic innervation
of the gastrointestinal tract.
• The intrinsic component is called the enteric nervous system. The enteric
nervous system is wholly contained within the submucosal and myenteric
plexuses in the wall of the gastrointestinal tract; it communicates extensively
with the parasympathetic and sympathetic nervous systems.
