Digestive System


Functions

mechanical digestion (breakdown of food to smaller size without chemical change)

chemical digestion (breakdown of food with chemical change; i.e. starch to sugar, protein to amino acids)

storage of food (stomach)

absorption of nutrients (primarily small intestine with some activity in stomach)

compaction of waste and reabsorption of water (colon)

Components

GI Tract (gastro-intestinal tract or alimentary canal): consist of any part of the digestive system through which the food being processed could pass. This includes, but is not limited to, the esophagus, stomach, and even the cecum (caecum).

Accessory Organs: includes all organs of the digestive system not a part of the GI tract such as the pancreas, liver, and salivary glands.


Cross-section of the GI tract

This cross sectional description starts from the innermost part of the small intestine. Please use your textbook diagram to accompany this.

Lumen: cavity within the GI tract

Rugae: the large inner folds of the stomach wall that aid in mechanical digestion and movement of digested material.

Villus (pl. Villi): "cylindrical" projections of the GI tract into the lumen. These are countless. The term microvilli refers to the cells of the epithelial layer of a villus so they are not the same thing! Villi are present in the small intestine only.

Tunica mucosa: layer of the GI tract exposed to the lumen

Epithelial cells: consists of simple columnar cells, except in the upper esophagus where the tissue type is stratified squamous. Goblet cells that secrete mucous make up much of this layer in the colon. The mucosa aids in the movement of material through the GI tract.

Lamina propria: this layer below the epithelial cells is highly vascularized. In the small intestine, nutrient absorption is accomplished by means of this tissue.

Muscularis mucosa: this is a layer of smooth circular muscle that, by contraction, can aid in the movement of material. Do not confuse this with the Tunica mucosa.

Tunica submucosa: this is a highly vascularized layer of connective tissue.

Tunica muscularis: this is a double layer of smooth muscle. The inner ring of muscle is circular and thus constricts the GI tract. The outer layer is longitudinal smooth muscle and by working in conjunction with other areas of the same muscle type can alternately shorten or lengthen (by relaxation) local segments of the GI tract.

Tunica serosa: This outermost layer is continuous with the mesenteries and is sometimes called the adventitia.


Organs and Structures of the Digestive System

  • Mouth
    • Vestibule: area between the teeth and gums
    • Hard palate: portion of the roof of the mouth formed by the palatine process of the maxilla and the palatine bone; aids in mechanical digestion and speech
    • Soft palate: separates oral cavity from nasopharynx
    • Tongue: aids in mechanical digestion, taste, and speech
    • Teeth: all aid in mechanical digestion
      • Incisors: biting
      • Canines: tearing
      • Bicuspids (pre-molars): grinding
      • Molars: grinding
    • Uvula: closes off the nasopharynx during swallowing
  • Pharynx (extends about 6-8 inches from the internal nares to the larynx)
    • Nasopharynx: from the internal nares to the oropharynx; contains the adenoids
    • Oropharynx: the portion of the pharynx that would be visible in a mirror; contains the tonsils
    • Laryngeopharynx: passage from the oropharynx to the upper esophagus and glottis
  • Esophagus
    • Proximal portion lined with stratified squamous epithelium
    • Proximal portion under voluntary control
    • Peristalsis: the sequential contraction of smooth muscle that moves a bolus toward the stomach
    • Bolus: a swallowed piece of food or other object
  • Stomach
    • Principle function is the storage of food
    • Both mechanical and chemical digestion occur here, the latter being the partial breakdown on protein to amino acids. Cells lining the gastric pits release pepsinogen and hydrochloric acid (HCl) that react to form pepsin which is an enzyme responsible for protein degradation. Pepsinogen is released by zymogenic cells and HCl is released by parietal cells, both in the gastric pits
    • Components of the stomach
      • Cardiac sphincter: controls the passage from the esophagus into the stomach; sphincter muscles normally limit passage to one direction
      • Pyloric sphincter: controls the passage from the stomach into the duodenum
      • Fundus: most superior part of the stomach, generally superior to the cardiac sphincter
      • Cardia: portion of the stomach inferior to the fundus and superior to the pylorus
      • Pylorus: most inferior part of the stomach
      • Body: the area of the stomach consisting of the cardia and the pylorus
      • Greater Curvature: the curved surface of the lateral part of the stomach
      • Lesser Curvature: the curved surace of the medial part of the stomach
  • Liver
    • Together with the brain, uses most of the energy of the resting body
    • Largest organ in the abdomen
    • Functions
      • Detoxification of the blood
      • Breakdown of RBCs
      • Production of bile for use in fat emulsification (bile exits via the Hepatic Duct)
      • Regulation of body temperature
      • Storage of glucose as glycogen
    • The Cystic and Hepatic Ducts unite to form the Common Bile Duct that exits into the duodenum via the ampula of Vader
  • Gall Bladder
    • Stores bile produced by the liver
    • Bile exits this organ via the Cystic Duct
    • Gallstones are solid cholesterol
    • The Cystic and Hepatic Ducts unite to form the Common Bile Duct that exits into the duodenum via the ampula of Vader
  • Pancreas
    • Produces pancreatic amylase for the breakdown of carbohydrates
    • Pancreatic Duct delivers pancreatic amylase to the ampula of Vader; from there it passes through the sphincter of Odi into the duodenum
    • also an endocrine gland producing, most notably, insulin and glucagon
  • Small Intestines
    • Principle site of digestion and absorption
      • Fats are emulsified here by bile delivered via the Common Bile Duct
      • Proteins are further digested
      • Carbohydrates are broken down to absorbable sugars, largely by Pancreatic Amylase
    • Three divisions
      • Duodenum: proximal end; a little over a foot in length; pancreas is located largely in the curve of this structure; begins at the pyloric sphincter
      • Jejenum
      • Ileum: terminates at the ileocecal valve which regulates passage from the ileum into the cecum (caecum)
  • Large Intestines (colon)
    • Functions
      • Reabsorption of water
      • Compaction of waste
    • Segments (proximal to distal)
      • Cecum
      • Ascending Colon
      • Transverse Colon
      • Descending Colon
      • Sigmoid Colon
    • Haustra: pouches of the colon formed due to differential contraction of the teniae coli (see below)
    • Teniae coli: the longitudinal smooth muscle of the tunica muscularis is not continuous around the colon; instead it is formed into three separate longitudinal bands; the differential contraction of these bands aids in the forward movement of the contents and the appearance of pouches or haustra.
    • The cross-sectional view shows many goblet cells in the epithelial layer of the tunica mucosa and no villi.
  • Rectum
    • Storage of waste
    • Terminates at the anal sphincter which regulates passage to the exterior