Large Intestine Structure and Functions Notes

Functional Anatomy Of Large Intestine

The large intestine is also known as the colon. It extends from the ileocecal valve up to the anus. It consists of seven portions:

• Cecum with appendix
• Ascending colon
• Transverse colon
• Descending colon
• Sigmoid colon or pelvic colon
• Rectum
• Anal canal.

Read And Learn More: Medical Physiology Notes

Structure Of Wall Of Large Intestine

The wall of the large intestine is formed by four layers of structures, like any other part of the gut.

1. Serous layer: It is formed by the peritoneum.

2. Muscular layer: The smooth muscles of the large intestine are distributed in two layers, namely the outer longitudinal layer and the inner circular layer. The longitudinal muscle fibers of the large intestine are arranged in the form of three long bands called tenia coli. The length of the Tenia coli is less when compared to the length of the large intestine. Because of this, the large intestine is made into a series of pouches called haustra.

3. Submucus layer: It is not well-developed in the large intestine.

4. Mucus layer: The crypts of Leiberkuhn are present in the mucosa of the large intestine. But the villi, which are present in the mucous membrane of the small intestine, are absent in the large intestine. Only mucus-secreting glands are present in the mucosa of the large intestine.

Secretions Of the Large Intestine

The large intestinal juice is a watery fluid with a pH of 8.0.

• Composition Of Large Intestinal Juice: The large intestinal juice contains 99.5% of water and 0.5% of solids. Digestive enzymes are absent and the concentration of bicarbonate is high in large intestinal juice.
• Functions Of Large Intestinal Juice:
  • Neutralization of Acids: Strong acids formed by bacterial action in the large intestine are neutralized by the alkaline nature of large intestinal juice. The alkalinity of this juice is mainly due to the presence of a large quantity of bicarbonate.
  • Lubrication Activity: The mucin present in the secretion of the large intestine lubricates the mucosa of the large intestine and the bowel contents, so that, the movement of the bowel is facilitated. The mucin also protects the mucous membrane of the large intestine by preventing damage caused by mechanical injury or chemical substances.

Functions Of Large Intestine

1. Absorptive Function: Large intestine plays an important role in the absorption of various substances such as:

• Water
• Electrolytes
• Organic substances like glucose
• Alcohol
• Drugs like anesthetic agents, sedatives, and steroids.

2. Formation Of Feces: After the absorption of nutrients, water, and other substances, the unwanted substances in the large intestine form feces. This is excreted out.

3. Excretory Function: Large intestine excretes heavy metals like mercury, lead, bismuth, and arsenic through feces.

4. Secretory Function: Large intestine secretes mucin and inorganic substances like chlorides and bicarbonates.

5. Synthetic Function: The bacterial flora of the large intestine synthesizes folic acid, vitamin B₁₂, and vitamin K. By this function large intestine contributes in erythropoietin activity and blood clotting mechanism

Applied Physiology

1. Diarrhea: Diarrhea is the frequent and profuse discharge of intestinal contents in loose and fluid form. It occurs due to the increased movement of the intestine. It may be acute or chronic.

Diarrhea Causes:

• Normally, when digested food passes through the colon, a large portion of the fluid is absorbed and only a semisolid stool remains. In diarrhea, the fluid is not absorbed sufficiently, resulting in watery bowel discharge.
• Acute diarrhea may be caused by temporary problems like infection, and chronic diarrhea may be due to disorders of the intestinal mucosa.

The most common causes of diarrhea:

1. Dietary abuse: Diarrhea is caused by the intake of contaminated water or food, artificial sweeteners found in food, spicy food, etc.

2. Food intolerance: Indigestion is a common cause of acute diarrhea. Substances such as lactose, a sugar present in milk and milk products, may not be digested easily leading to diarrhea.

3. infections by

• Bacteria such as Escherichia coli, Salmonella, Shigella. etc.
• Viruses like rotavirus, hepatitis virus, etc.
• Parasites like Entamoeba histologic, Giardia lamblia, etc.

4. Reaction to medicines: Diarrhea is caused by medicines such as

• Antibiotics
• Antihypertensive drugs
• Antacids containing magnesium
• Laxatives.

5. Intestinal diseases: Chronic diarrhea occurs during inflammation of the intestine, irritable bowel syndrome, and abnormal motility of the intestine.

Diarrhea Features:

• Severe diarrhea results in loss of excess water and electrolytes, leading to dehydration and electrolyte imbalance. Chronic diarrhea results in hypokalemia and metabolic acidosis.
• Other features of diarrhea are abdominal pain, nausea, and bloating (a condition in which the subject feels the abdomen full and tight due to excess intestinal gas).

2. Constipation:

Failure of voiding feces, which produces discomfort, is known as constipation. It is due to the lack of movements necessary for defecation. Due to the absence of mass movement in the colon, feces remain in the large intestine for a long-time, resulting in the absorption of fluid. So the feces become hard and dry.

Constipation Causes:

1. Dietary causes: Lack of fiber or lack of liquids in the diet cause constipation.

2. Irregular bowel habit: Irregular bowel habit is the most common cause of constipation. It causes constipation by inhibiting the normal defecation reflexes.

3. Spasm of the sigmoid colon: The spasm in the sigmoid colon (spastic colon) prevents its motility, resulting in constipation.

4. Diseases: Constipation is common in many types of diseases.

5. Dysfunction of my enteric plexus in large intestine – Megacolon

• Megacolon is a condition characterized by distension and hypertrophy of the colon associated with constipation. It is caused by the absence or damage of ganglionic cells in my enteric plexus, which causes dysfunction of my enteric plexus.
• It leads to the accumulation of large quantities of feces in the colon. The colon is distended to a diameter of 4-5 inches. It also results in hypertrophy of the colon. Congenital development of megacolon is called Hirschsprung’s disease.

6. Drugs: The drugs like diuretics, pain relievers (narcotics), antihypertensive drugs (calcium channel blockers), antiparkinson drugs, antidepressants, and anticonvulsants cause constipation.

3. Appendicitis:

The inflammation of the appendix is known as appendicitis. The appendix is a small, finger-like pouch projecting from the cecum of ascending colon. It is situated on the lower right side of the abdomen.

The appendix does not have any function in human beings. But, it can create major problems when diseased. Appendicitis can develop at any age. However, it is very common between 10 and 30 years of age.

Appendicitis Causes: The cause of appendicitis is not known. It may occur by a viral infection of the Gl tract or if the connection between the appendix and the large intestine is blocked.

Appendicitis Features:

• The main symptom of appendicitis is the pain, which starts around the umbilicus and then spreads to the lower right side of the abdomen. The pain becomes severe within 6–12 hours
• Nausea
• Vomiting
• Constipation
• Diarrhea
• Difficulty in passing gas
• Low fever
• Abdominal swelling
• Loss of appetite.

If not treated immediately, the appendix may rupture and the inflammation will spread to the whole body leading to severe complications, sometimes even death. Therefore, the treatment of appendicitis is considered as an emergency. The usual standard treatment for appendicitis is surgery removal of the appendix called an appendectomy.

4. Uncerative Colitis:

• Ulcerative colitis is an inflammatory bowel disease (IBD) characterized by inflammation and ulcerative aberrations in the wall of the large intestine. It is also known as colitis or proctitis. The rectum and lower part of the colon are commonly affected. Sometimes entire colon is affected.
• Ulcerative colitis can occur at any age. More commonly, it affects people in the age group of 15-30 years. Rarely, it affects 50-70 years old people.

Ulcerative Colitis Cause: The exact cause of ulcerative colitis is not known. However, it is believed that the interaction between the immune system and viral or bacterial infection causes this disease.

Ulcerative Colitis Features:

1. Abdominal pain
2. Diarrhea with blood in the stools
3. Early fatigue
4. Loss of appetite and weight
5. Arthritis and osteoporosis
6. Eye inflammation
7. Liver diseases like hepatitis, cirrhosis, etc.
8. Skin rashes
9. Anemia.