Physiology of Digestive System

Study Notes

The digestive system consists of the digestive tract (also known as alimentary tract or canal, gut, or gastrointestinal tract) and accessory digestive organs.
Accessory digestive organs include teeth, tongue, salivary glands, liver, gall bladder, and pancreas.

The GI tract is approximately 9 meters long in a cadaver and extends from the mouth to the anus.
The GI tract is divided into the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine.

The oral cavity consists of the vestibule, oral cavity proper, lips, cheeks, palate, and tongue.
The tongue is involved in speech, taste, mastication, and swallowing.

Salivary glands produce saliva and consist of three pairs: parotid, submandibular, and sublingual.
The parotid gland is the largest, while the sublingual gland is the smallest.

The pharynx is divided into the nasopharynx, oropharynx, and laryngopharynx.
The oropharynx transmits food, while the laryngopharynx transmits both food and air.

The esophagus is a straight muscular tube (22cm) that connects the pharynx to the stomach.
It transports food from the pharynx to the stomach and passes through the esophageal hiatus (opening) of the diaphragm.

Digestion is done by enzymatic hydrolysis.
Absorption is the process of the passage of small absorbable units that result from digestion into blood and lymph.
Storage and elimination (excretion) involve the temporary storage of undigested, unabsorbed food, which is then eliminated in feces by defecation.

Motility refers to the movement of food through the GI tract.
Motility involves ingestion, mastication, deglutition, and peristalsis.
Peristalsis is a rhythmic wave-like contraction that moves food through the GI tract.
Motility in the small intestine involves segmentation and migration contractions.
Motility in the large intestine involves mass movements and local reflexes in the enteric plexus.

Regulation of the GIT involves the enteric nervous system (ENS), which can affect the GIT directly or indirectly through extrinsic nerves and gastrointestinal hormones.

The layers of the GI tract include the mucosa, submucosa, muscularis externa, and serosa.
The mucosa lines the luminal surface of the digestive tract and is highly folded to increase surface area for absorption.
The mucosa consists of the mucous membrane, lamina propria, and muscularis mucosa.
The submucosa is a thick layer of connective tissue that contains larger blood and lymph vessels and the submucous nerve plexus.
The muscularis externa is the major smooth muscle coat of the digestive tract, consisting of inner circular and outer longitudinal layers.
The serosa is the outer covering of the digestive tract.