Physiology of Digestive System

Digestive System

• 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.

GI Tract

• 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.

Oral Cavity

• 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

• 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.

Pharynx

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

Esophagus

• 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.

Digestive Processes

• 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

• 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

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

Layers of the GI Tract

• 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.