Study Notes on the Large Intestine

Chapter 24: The Digestive System (Part 8) - Large Intestine

Anatomy of the Large Intestine

  • Functions

    1. Absorb most of the remaining water from indigestible food while regulating the consistency of feces.

    2. Store feces.

    3. Eliminate feces.

Key Processes in the Large Intestine

  • Absorption (A)

    • Nutrients from the large intestine (LI) are transported to the blood and then to body cells.

  • Secretion (S)

    • Substances from the blood are secreted into the LI, leading to the excretion of waste products.

Appendicitis

  • There is no clearly defined cause of appendicitis.

  • Possible Causes of Obstruction:

    • Fecal material is believed to obstruct the appendix.

    • Infectious agents such as bacteria, viruses, fungi, and parasites can lead to infection.

  • Infection results in the swelling of the tissues of the appendix wall.

  • Complications:

    • A ruptured appendix can lead to serious conditions such as gangrene or peritonitis, which can be life-threatening.

Histology of the Large Intestine

  • The structure consists of the same four layers found in other parts of the digestive system but differs in specific features.

  • No circular folds or villi are present; hence, more absorption occurs in the small intestine.

  • Microvilli: Present in absorptive cells to enhance absorption.

  • Absorptive Cells: Specialized cells that absorb water from waste material.

  • Goblet Cells: Most abundant cell type in the large intestine, secreting mucus to lubricate the passage of colonic contents.

  • Mucosa: The innermost layer consists of epithelial cells.

Submucosa and Muscular Layers

  • Submucosa: Composed of areolar connective tissue.

  • Muscularis Externa:

    • Longitudinal muscle modified to form teniae coli (three long strips).

    • Contracts to gather the colon into a series of pouches known as haustra.

    • Haustra: Pouches formed by the contraction of the teniae coli.

  • Serosa: A part of the visceral peritoneum covering the outer surface.

Mechanical Digestion in the Large Intestine

  • Haustral Churning:

    • Occurs when distension reaches a certain point, causing the walls of the haustra to contract and move contents onward.

  • Peristalsis:

    • Propulsive contractions that move contents along the digestive tract.

  • Mass Peristalsis:

    • A strong peristaltic wave that begins in the transverse colon, quickly driving contents toward the rectum.

Chemical Digestion in the Large Intestine

  • Final stages of chemical digestion occur through bacterial action.

  • Bacterial Action:

    • More than 1,000 different types of bacteria inhabit the colon.

  • Certain vitamins, including Vitamin B12 and Vitamin K, are synthesized by bacterial action within the colon.

Sphincters of the Large Intestine

  • The opening of the anal canal (anus) is controlled by two sphincters:

    1. Internal Anal Sphincter: Composed of smooth muscle.

    2. External Anal Sphincter: Composed of skeletal muscle.

Functions of Sphincters

  • Control the entrance of food into the gastrointestinal (GI) tract.

  • Regulate the progression of material along the digestive tract.

  • Prevent reflux of material back into the tract.

  • Control the elimination of feces.

Identifying Sphincters

  • Upper Esophageal Sphincter (UES)

  • Lower Esophageal Sphincter (LES)

  • Pyloric Sphincter

  • Ileocecal Sphincter

  • Anal Sphincters

Structure of Anal Canal

  • The epithelium lining the distal anal canal differs from the lining of the stomach, small intestine, and colon.

  • Types of Epithelium:

    • Simple columnar epithelium.

    • Stratified squamous epithelium.

    • Transitional epithelium.

    • Correct Answer: Stratified squamous epithelium is present in the anus.

    • Purpose: Provides protection against abrasion during the passage of feces.

Disorders of the Colon

  • Diarrhea:

    • Occurs when material passes too rapidly through the colon, resulting in insufficient water absorption.

  • Constipation:

    • Occurs when material passes through the colon too slowly, leading to excessive water absorption.

  • Colorectal Cancer:

    • Often begins with the formation of a polyp.

  • Diverticula:

    • Pouches that can form in the intestinal wall.

    • Diverticulosis:

    • Condition where diverticula do not cause problems.

    • Diverticulitis:

    • Condition where diverticula become infected and inflamed.