Functions and Features of the Large Intestine

Anatomy and Physiology of the Large Intestine

Overview of the Large Intestine

• Length: Approximately 5 feet; Diameter: 2.5 inches

• Main Functions:

  • Propulsion: Movement of contents through the intestines.

  • Water and Nutrient Absorption: Absorbs water and nutrients produced by intestinal flora.

  • Mucus Secretion: Lubricates contents for easier movement.

  • Waste Storage and Compaction: Stores and compacts digestive wastes.

  • Defecation: Elimination of waste from the body.

Parts of the Large Intestine

1. Cecum:

   • Function: Collects and stores material from the ileum.

   • Appendix: A lymphatic organ; serves as storage for beneficial gut bacteria.

2. Colon:

   • Functions: Absorption of water and vitamins; compaction of feces.

   • Sections:

     • Ascending Colon

     • Hepatic Flexure

     • Transverse Colon

     • Splenic Flexure

     • Descending Colon

     • Sigmoid Colon

3. Rectum:

   • Function: Temporary storage of feces.

4. Anus:

   • Function: Site of defecation.

Distinctive Features of the Large Intestine

• Haustra:

  • Definition: Series of pouches that allow the colon to expand and elongate.

• Teniae Coli:

  • Definition: Longitudinal bands of smooth muscle; remnants of the muscular layer.

  • Function: Muscle tone creates the haustra structure.

• Gut Microbiome:

  • Role: Breaks down remaining nutrients, producing gases (flatus), and synthesizes vitamins (e.g., Vitamin K for blood clotting).

Histology of the Large Intestine

• Epithelial Lining:

  • Type: Simple columnar epithelium.

  • Characteristics: Lacks villi; contains abundant goblet cells and mucous glands for lubrication.

• Muscle Layer:

  • Structure: Circular smooth muscle with remnants of longitudinal smooth muscle (teniae coli).

Defecation Reflexes

1. Stimulus: Rectal distention stimulates stretch receptors.

2. Types of Reflexes:

   • Intrinsic Defecation Reflex (Short Reflex):

     • Function: Stimulates local peristalsis moving feces toward the anus, relaxes internal anal sphincter.

   • Parasympathetic Defecation Reflex (Long Reflex):

     • Function: Stimulates parasympathetic motor neurons to increase peristalsis in descending/sigmoid colon; increases rectal pressure, relaxes internal and external anal sphincters.

   • Control: Internal and external anal sphincters must relax for defecation to occur; external sphincter is under conscious control.

Nutrient and Water Absorption

Carbohydrate Digestion

• Initiation: Begins in the oral cavity with salivary amylase.

• Process:

  • Carbohydrates are broken down into disaccharides by salivary amylase and pancreatic alpha-amylase.

  • Monosaccharides absorbed via facilitated diffusion and cotransport mechanisms into the bloodstream.

Protein Digestion

• Mechanism: Proteolytic enzymes secreted as inactive precursors (zymogens).

• Activation:

  • Low pH activates pepsinogen to pepsin.

  • Enteropeptidase activates trypsinogen to trypsin in the brush border of the intestinal mucosa.

Fat Digestion and Absorption

• Chylomicrons:

  • Formation: Triglycerides combined with cholesterol and phospholipids form chylomicrons.

  • Transport: Chylomicrons are too large for capillaries, transported via lymph to the thoracic duct, entering the bloodstream at the left subclavian vein.

Water Reabsorption

• Mechanism: Passive movement of water occurs via osmosis from areas of low to high solute concentration.

• Relation to Nutrient Absorption: Water follows nutrient absorption, highlighting osmosis' crucial role in digestive processes.