1.7 Large Intestine

The large intestine is the final part of the gastrointestinal tract. It receives undigested food from the small intestine, absorbs water and electrolytes, absorbs vitamins produced by bacteria, and forms and stores feces until elimination.

Definition

The large intestine is the last part of the digestive system, about 1.5 meters long, extending from the ileocecal junction to the anus. It consists of the cecum, colon (ascending, transverse, descending, and sigmoid), rectum, and anal canal. Its main functions are absorption of water and electrolytes, absorption of vitamins, and formation and storage of feces.

Main Idea

• Absorbs water and electrolytes — makes stool solid
• Absorbs vitamins — produced by bacteria (Vitamin K and Biotin)
• Reduces acidity — secretes bicarbonate
• Protects from infections — acts as a mucosal barrier
• Forms and stores feces — until elimination

Core Concept

• Cecum = proximal part; where small and large intestines meet
• Appendix = blind tube attached to cecum; contains lymph nodules
• Colon = 1.5 m long; four parts: ascending, transverse, descending, sigmoid
• Rectum = straight muscular tube; stores feces
• Anus = exit point; guarded by sphincter
• Ileocecal sphincter = guards junction between ileum and large intestine
• No further digestion occurs in the large intestine
• Water absorption makes stool solid
• Bacterial flora produce Vitamin K and Biotin

Detailed Explanation

Structure of the Large Intestine

• The proximal part of the large intestine.
• Where the large and small intestines meet.
• The junction between ileum and large intestine is called the ileocecal junction.
• Guarded by the ileocecal sphincter.

• A small blind tube about 9 cm long.
• Attached to the cecum.
• Called the vermiform appendix.
• Walls contain many lymph nodules.
• Has an undetermined role in immunity.
• Important in fetal life as it contains endocrine cells that release biogenic amines and peptide hormones.
• Appendicitis = inflammation of the appendix.
• Appendectomy = removal of the appendix.

• About 1.5 m long.
• Consists of four parts:

• A straight, muscular tube.
• Begins at the termination of the sigmoid colon.
• Ends at the anus.
• Guarded by a sphincter.
• Stores feces temporarily.

• The final part of the large intestine.
• Opens to the exterior through the anus.
• Guarded by internal and external anal sphincters.

Functions of the Large Intestine

The large intestine performs several important functions:

• Further digestion or breaking down of nutrients does not take place in the large intestine.
• The proximal half of the large intestine functions to reabsorb some of the water and electrolytes.
• This makes the stool solid.
• The substances that remain in the tube become feces.
• Feces are stored for a time in the distal portion (rectum) of the large intestine.

• The large intestine helps in absorption of vitamins made by bacteria that normally live in the large intestine.
• These bacteria also produce large amounts of vitamins.
• The most important of these are Vitamin K and Biotin (a B vitamin).

• The mucosa of the large intestine secretes bicarbonate to neutralize the increased acidity resulting from the formation of fatty acids and other digestive components at earlier parts of the intestines.
• It acts as a mucosal barrier and protects from microbial infections and invasions.

Bacterial Flora of the Large Intestine

The large intestine contains a large population of bacteria (about 100 trillion).

• Ferment undigested carbohydrates — produce short-chain fatty acids
• Synthesize Vitamin K — essential for blood clotting
• Synthesize Biotin — a B vitamin
• Produce gases — methane, hydrogen, carbon dioxide
• Compete with pathogens — prevent infection
• Metabolize bilirubin — give feces its brown color

Feces Formation

Composition of Feces:

• Due to stercobilin — a breakdown product of bilirubin.
• Bilirubin is converted to stercobilin by bacteria.

Odor of Feces: - Due to indole, skatole, and mercaptans — produced by bacterial action.

Intermediate Level

Parts of the Large Intestine

Functions of the Large Intestine

Absorption in the Large Intestine

Bacterial Flora

Motility of the Large Intestine

• Occurs 2–3 times per day.
• Often after meals (gastrocolic reflex).
• Moves feces from colon to rectum.
• Triggers the defecation reflex.

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Large intestine is about 1.5 meters long.
• 2. It consists of cecum, colon, rectum, and anal canal.
• 3. Colon has four parts: ascending, transverse, descending, sigmoid.
• 4. Appendix is a blind tube about 9 cm long.
• 5. Appendix contains lymph nodules.
• 6. No further digestion occurs in the large intestine.
• 7. Water and electrolytes are absorbed.
• 8. Vitamin K and Biotin are absorbed.
• 9. Bicarbonate is secreted to neutralize acids.
• 10. Mucosal barrier protects from infections.
• 11. Bacterial flora produce Vitamin K and Biotin.
• 12. Feces are formed and stored in the rectum.
• 13. Defecation is the elimination of feces through the anus.
• 14. Internal anal sphincter is involuntary.
• 15. External anal sphincter is voluntary.

Remember

• - Large intestine = 1.5 m long
• - Parts = cecum, colon, rectum, anal canal
• - Colon parts = ascending, transverse, descending, sigmoid
• - Appendix = 9 cm; lymph nodules
• - Ileocecal sphincter = guards junction
• - No digestion in large intestine
• - Absorbs = water, electrolytes, Vitamin K, Biotin
• - **Secretes** = bicarbonate
• - Bacterial flora = produce Vitamin K and Biotin
• - Feces = stored in rectum
• - Defecation = elimination through anus
• - Internal anal sphincter = involuntary
• - External anal sphincter = voluntary

Common Mistakes

• 1. Thinking digestion occurs in the large intestine — No further digestion occurs.
• 2. Believing the large intestine is longer than the small intestine — It is shorter (1.5 m vs. 6 m).
• 3. Confusing the appendix with the cecum — The appendix is attached to the cecum.
• 4. Thinking Vitamin K is absorbed from food — Vitamin K is produced by bacteria in the large intestine.
• 5. Forgetting the role of bicarbonate — Bicarbonate neutralizes acids in the large intestine.
• 6. Mixing up the parts of the colon — Ascending, transverse, descending, sigmoid.
• 7. Believing the internal anal sphincter is voluntary — It is involuntary.
• 8. Thinking the external anal sphincter is involuntary — It is voluntary.
• 9. Confusing feces with chyme — Chyme is in the stomach/small intestine; feces is in the large intestine.
• 10. Forgetting the role of bacteria — Bacteria produce Vitamin K and Biotin.

Quick Revision

The large intestine is about 1.5 meters long and consists of the cecum, colon (ascending, transverse, descending, sigmoid), rectum, and anal canal. The appendix is a blind tube about 9 cm long attached to the cecum and contains lymph nodules. No further digestion occurs in the large intestine. Its main functions are absorption of water and electrolytes, absorption of vitamins (Vitamin K and Biotin) produced by bacteria, secretion of bicarbonate to neutralize acids, and acting as a mucosal barrier against infections. Bacterial flora produce Vitamin K and Biotin. Feces are formed and stored in the rectum. Defecation is the elimination of feces through the anus. The internal anal sphincter is involuntary; the external anal sphincter is voluntary.

60-Second Revision

• Large intestine = 1.5 m; cecum, colon, rectum, anal canal
• Colon = ascending, transverse, descending, sigmoid
• Appendix = 9 cm; lymph nodules
• No digestion in large intestine
• Absorbs = water, electrolytes, Vitamin K, Biotin
• Secretes = bicarbonate
• Bacteria = produce Vitamin K and Biotin
• Feces = stored in rectum
• Defecation = elimination through anus
• Internal anal sphincter = involuntary
• External anal sphincter = voluntary
• Warning: No digestion in large intestine; only absorption and feces formation

NEXT TOPIC

• Chapter:
• 1. Digestive System of Man

• Topic:
• 1.8 DEFAECATION

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