Makindo Medical Notes

Sections of the Large Intestine

• Caecum: The first part of the large intestine, located in the right lower quadrant. It connects to the ileum via the ileocecal valve, which regulates the flow of material from the small intestine.
• Appendix: A small, finger-like projection from the cecum, the appendix is often considered vestigial, but recent evidence suggests it may play a role in immune function.
• Ascending Colon: Extends upward from the cecum on the right side of the abdomen, leading to the transverse colon.
• Transverse Colon: Runs across the upper abdomen from right to left, connecting the ascending and descending colons.
• Descending Colon: Travels downward along the left side of the abdomen, leading to the sigmoid colon.
• Sigmoid Colon: The S-shaped portion of the large intestine that connects the descending colon to the rectum. It stores fecal matter until it is ready to be excreted.
• Rectum: The final section of the large intestine, which serves as a temporary storage site for feces before defecation.
• Anus: The terminal end of the GI tract, where feces are expelled from the body. It is surrounded by the internal and external anal sphincters, which control defecation.

Physiology of the Large Intestine

The large intestine plays a crucial role in the final stages of digestion, particularly in the absorption of water and electrolytes, as well as the formation and elimination of feces.

Functions of the Large Intestine

• Absorption of Water and Electrolytes: The large intestine absorbs water and electrolytes (mainly sodium and chloride) from the indigestible food residue, converting liquid chyme into more solid feces. Around 90% of the water entering the large intestine is reabsorbed.
• Bacterial Fermentation: The large intestine houses a vast community of gut bacteria (microbiota), which ferment undigested carbohydrates (fiber) and produce gases (e.g., methane, carbon dioxide) and short-chain fatty acids (SCFAs), such as acetate, butyrate, and propionate. These SCFAs are absorbed and used as an energy source for colonocytes and systemic metabolism.
• Formation and Storage of Feces: The indigestible material is compacted into feces, which are temporarily stored in the rectum until defecation.
• Mucus Secretion: Goblet cells in the mucosa of the large intestine secrete mucus to facilitate the passage of feces and protect the intestinal wall from mechanical damage.
• Immune Function: The large intestine plays a role in immune defense, as the gut-associated lymphoid tissue (GALT) provides a barrier against harmful pathogens, while the gut microbiota also helps prevent colonization by harmful bacteria.

Defecation

Defecation is the process of eliminating feces from the body and involves the rectum and anus.

• Rectal Distension: When feces enter the rectum, the walls stretch, triggering sensory nerves and initiating the urge to defecate.
• Internal and External Anal Sphincters: The internal anal sphincter (involuntary control) relaxes when the rectum is distended. The external anal sphincter (voluntary control) allows the conscious release of feces.
• Peristalsis and Relaxation: Peristaltic waves in the rectum and relaxation of the anal sphincters work together to expel the feces from the body.

Blood Supply to the Large Intestine

• Arterial Supply: The large intestine receives blood from the superior and inferior mesenteric arteries.
  • The superior mesenteric artery supplies the cecum, ascending colon, and most of the transverse colon.
  • The inferior mesenteric artery supplies the distal part of the transverse colon, descending colon, sigmoid colon, and rectum.
• Venous Drainage: Blood from the large intestine drains into the portal venous system via the superior and inferior mesenteric veins, which eventually drain into the hepatic portal vein and flow to the liver for filtration and processing.

Nerve Supply

• Sympathetic Innervation: Reduces motility and secretion in the large intestine, while promoting the contraction of the internal anal sphincter.
• Parasympathetic Innervation: Increases motility and secretion in the large intestine, aiding digestion and defecation. The vagus nerve innervates the proximal colon, while the pelvic splanchnic nerves innervate the distal colon, rectum, and anus.

Clinical Relevance

• Constipation: A common issue that results from reduced motility of the large intestine, leading to excessive water absorption and hard, dry feces.
• Diarrhea: Increased motility or secretion in the large intestine can prevent adequate water absorption, leading to watery stools.
• Colon Cancer: A common malignancy arising from the epithelial cells of the large intestine, often affecting the sigmoid colon and rectum.
• Inflammatory Bowel Disease (IBD): Includes conditions such as Crohn's disease and ulcerative colitis, where chronic inflammation affects the large intestine, leading to symptoms such as diarrhoea, abdominal pain, and bleeding.
• Diverticulitis: Inflammation of diverticula (small pouches) in the colon, typically seen in the sigmoid colon, causing pain, fever, and altered bowel habits.