Digestive System: An Overview

• Module 10 covers the digestive system.

Components of the Digestive System

• The digestive tract includes the mouth/oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, and anus.
• Accessory digestive organs are the tongue, teeth, salivary glands, liver, biliary ducts, gallbladder, and pancreas.

Functions of the Digestive Tract

• Digestion reduces food into smaller pieces, facilitating chemical processing by enzymes.
• Absorption occurs when nutrients, water, vitamins, etc., are absorbed in the small intestine and the large intestine after digestion.
• Secretion occurs when cells in the stomach's mucosal wall release gastric acid to perform chemical digestion.
• Motility occurs when muscles in the esophageal wall contract and relax to push food through the esophagus down to the stomach after food is swallowed.
• Elimination of waste occurs as leftover materials not absorbed or utilized are eliminated through defecation.

Histology of the Digestive Tract Wall

• The mucosa is the innermost layer, consisting of surface epithelium, lamina propria, and muscularis mucosa.
• Surface epithelium reflects the organ's function, such as secretion, absorption, and protection.
• Lamina propria is a layer of loose connective tissue under the surface epithelium.
• Muscularis mucosa is a layer of smooth muscle fibers under the lamina propria.
• The submucosa is composed of dense irregular connective tissue and contains blood vessels, lymphatics, glands, and nerve plexuses.
• Muscularis externa consists of circular and longitudinal layers of smooth muscle with nerve plexuses in between.
• The plexuses within the muscularis externa are responsible for muscular contraction, propelling food through the digestive tract.
• Serosa/adventitia is the outermost layer; when a serous membrane, it is known as serosa.
• Serosa is a single layer of thin flat cells that form a membranous sheet and secrete lubricating fluid.
• When the outermost layer consists of loose connective tissue, it is known as an adventitia.

Oral Cavity

• The vestibule is the space between the cheeks and lips and the gums and teeth.
• The oral cavity proper includes the remaining areas of the mouth.
• The palate forms the superior border of the oral cavity, divided into the hard palate (bone) and soft palate (muscle).
• The uvula is the posterior extension of the soft palate.
• Teeth are necessary for mechanical digestion, where humans have two sets in a lifetime.
• Deciduous teeth, also known as "baby teeth," include 2 incisors, 1 canine, and 2 molars in each quadrant, totaling 20 teeth.
• Permanent teeth include 2 incisors, 1 canine, 2 premolars, and 3 molars in each quadrant, totaling 32 teeth.
• The tongue is connected to speech, taste, and mechanical manipulation of food.
• Intrinsic muscles control the shape of the tongue itself.
• Extrinsic muscles control its movement.
• On the inferior surface is the frenulum, which anchors the tongue to the floor of the mouth.
• Superior and lateral surfaces are covered in papillae (bumps/projections).
• There are four types of papillae, some containing taste buds.
• The pharynx is part of both the digestive and respiratory systems.
• Salivary glands secrete digestive enzymes and mucus to help break down food while chewing.
• Parotid glands secrete serous (watery) fluid, located anterior and inferior to the external ear opening.
• Submandibular glands secrete serous and mucous (viscous) fluid, located inferior to the mandible.
• Sublingual glands secrete mucus and are located inferior to the tongue.
• Tonsils are collections of lymphoid tissue in the pharynx, contributing to the immune system.
• There are three tonsils: palatine, pharyngeal, and lingual.

Esophagus and Stomach

• Esophagus is a muscular tube about 25cm long, extending from the pharynx.
• The mucosa of the esophagus is a stratified squamous epithelium protecting the esophagus from friction when food travels through it.
• The submucosa contains mucus-secreting glands, and mucus is transported through a duct to the lumen, allowing food to pass more easily.
• Muscularis externa consists of smooth and skeletal muscle
• The upper 1/3 is only skeletal muscle, the middle 1/3 is a mix of both, and the lower is only smooth.
• Serosa/adventitia of the esophagus is mostly adventitia (loose connective tissue).
• The stomach is a J-shaped organ divided into three parts: fundus, body, and atrium.
• The antrum is continuous with the first aspect of the small intestine, the duodenum.
• The shape of the stomach creates the lesser and greater curvatures.
• The greater omentum hangs off the greater curvature, protecting the abdominal viscera.
• Mucosa is folded into ridges/folds called rugae, which are non-permanent folds that allow the stomach to expand following the ingestion of food or liquid.
• The surface consists of simple columnar epithelium.
• Cells secrete mucus to protect the stomach from acidic conditions.
• Inward folds extend into the lamina propria to form gastric glands.
• These glands secrete stomach acids that facilitate the chemical digestion of food.
• Submucosa contains blood vessels, lymphatics, glands, and nerve plexuses that supply the stomach tissue with oxygen and control the contraction of the musculature.
• Muscularis externa has three layers of smooth muscle arranged in different directions: an outer longitudinal, middle circular, and inner oblique layer.
• serosa/adventitra of the stomach is covered by serosa.

The Intestines

• The small intestine is attached to the posterior abdominal wall and is ~6m long.
• It is divided into three parts: duodenum, jejunum, ileum.
• The duodenum is 25cm, C-shaped, and encloses the head of the pancreas.
• The jejunum is 2.9m, mostly in the upper left quadrant of the abdomen.
• The ileum is 3.5m, mostly in the right lower quadrant of the abdomen.
• Intestinal histology: mucosa
• Villi are fingerlike projections that extend into the lumen.
• The epithelium on the villi is simple columnar with many absorptive cells whose apical surfaces have microvilli (brush borders).
• The brush border greatly increases the surface area for absorption of nutrients.
• Goblet cells between absorptive cells secrete mucus to lubricate the passage.
• Enteroendocrine cells secrete hormones involved in the regulation of satiety, blood sugar level, and growth of epithelial cells.
• Lamina propria forms the core of each villus and contains blood lymphatic capillaries.
• Collections of lymphatic tissue known as Peyer's patches are present, predominantly in the ileum.
• Intestinal glands, or crypts of Lieberkuhn, are deep folds of mucosa between the villi that secrete intestinal juices.
• Plicae circulares, formed from the mucosa and submucosa, are permanent transverse folds that help increase SA for absorption and causes the material to spiral through the small intestine.
• Duodenal/Brunner's glands are in the submucosa of the duodenum, secreting alkaline mucus to protect the small intestine from stomach acid.
• Muscularis externa has two smooth muscle layers, organized into circular (inner) and longitudinal (outer) layers with nerve plexuses in between.
• The serosa is the outermost layer.
• The large intestine is the terminal portion of the digestive tract.
• Is split into three sections: the cecum, colon, and rectum.
• The cecum is the first portion with an extension called the vermiform appendix.
• The colon is the second portion, divided into the ascending colon, transverse colon, descending colon, and sigmoid colon.
• The ascending colon goes up to the right side of the abdomen
• The transverse colon extends from below the liver to the descending colon.
• The descending colon goes down the left side of the abdomen to the sigmoid colon.
• The sigmoid colon leads to the rectum.
• The rectum is the last portion of the large intestine, temporarily storing fecal matter.
• Mucosa is similar to the small intestine, though without villi or plicae circulares.
• The surface consists of simple columnar epithelium, containing an increasing amount of goblet cells towards the anus.
• A switch to stratified squamous epithelium in the anal canal.
• Intestinal glands and accumulations of lymphatic tissues are present in the lamina propria.
• Submucosa contains blood vessels, lymphatics, glands, and nerve plexuses.
• Muscularis external has a unique arrangement depending on the location.
• In the cecum and colon, the outer longitudinal layer forms three longitudinal bands (teniae coli) that contract and bunch up the wall of the large intestine forming sac-like structures called haustra.
• In the rectum, the three taeniae coli merge to form the continuous longitudinal muscular layer.
• In the anal canal the circular muscle layer thickens into the internal anal sphincter, used in waste excretion
• Can be covered in either serosa or Adventitia
• The anal canal is the last portion of the digestive tract that leads to the external world.
• The Anal Column are longitudinal ridges
• The external and Internal Anal Sphincters maintain continence and relax to enable evacuation during defecation