The Large Intestine and its Function

Large Intestine (Colon)

The large intestine is the final segment of the digestive tract.
It receives waste material and water from the small intestine after nutrient absorption.
It primarily functions to absorb water and electrolytes, compact waste, and host a diverse microbial population.

The ileocecal valve marks the junction between the ileum (the end of the small intestine) and the cecum (the beginning of the large intestine).
It is located on the right side of the abdomen.
It is a flap-like valve that opens when chyme accumulates behind it, allowing the chyme to enter the large intestine.

The cecum is the pouch-like region of the large intestine situated below the ileocecal valve.
It is a vestigial structure, more prominent in herbivorous animals for storage and fermentation.
The appendix is a further reduced extension of the cecum.
Appendicitis refers to the inflammation and potential infection of the appendix, requiring prompt medical intervention to prevent rupture and subsequent peritonitis.

Ascending Colon: The initial segment, ascending upwards on the right side of the abdomen.
Transverse Colon: Runs horizontally across the abdomen.
Descending Colon: Descends downwards on the left side of the abdomen.
Sigmoid Colon: An S-shaped curve connecting the descending colon to the rectum.
Rectum: The terminal part of the large intestine with distinct anatomical features.

The large intestine has infoldings of the mucosa.
Unlike the small intestine, it lacks villi.
The infolds are typically linear and unbranched.
The mucosa consists of columnar absorptive cells and goblet cells in roughly equal proportions.
Goblet cells secrete large amounts of mucus into the intestinal glands or crypts.
Columnar cells possess a low brush border of microvilli on their apical surface.

The muscularis externa has two layers:
- A normal circular layer of smooth muscle.
- A longitudinal layer modified into three or four strips called taeniae coli.
Taeniae coli exhibit constant muscle tone, causing the colon to form pouches.
The pouches, or sacculations, formed by the taeniae coli are called haustra.
Contractions of the circular and longitudinal muscles compact waste and propel it towards the rectum.

The large intestine reabsorbs remaining water (10-15%) and electrolytes from the chyme.
Most water reabsorption occurs in the small intestine, but the large intestine's role is still crucial.
Diarrheal diseases, often caused by infections like cholera, result from inadequate water reabsorption, leading to dehydration and potential death, especially in infants.
Cholera bacteria bind to intestinal cells, causing excessive chloride ion secretion into the intestinal lumen.
Sodium ions and water follow the chloride ions, leading to electrolyte and water loss.

The large intestine compacts and stores waste until elimination is appropriate.
The gastrocolic reflex, triggered by food entering the stomach, stimulates mass peristalsis in the large intestine to create space for incoming waste.
Peristaltic movements move waste through the colon into the rectum.
The distension of the rectum stimulates reflexes and parasympathetic nervous system output, leading to relaxation of the internal anal sphincter and contraction of the intestinal wall.
These actions result in defecation.
Potty training involves learning to override the natural relaxation of the external anal sphincter, which is controlled by skeletal muscle.

The large intestine hosts a diverse population of beneficial bacteria, or "friendly flora."
Modern lifestyles (antibiotics, processed foods) have reduced the diversity of our microflora compared to more primitive populations.

Friendly bacteria prevent the establishment of pathogenic microbes by occupying surface area.
They produce antimicrobial factors that inhibit the growth of non-friendly bacteria.

Bacteria break down undigested food, producing byproducts that can be beneficial.
Vitamin K, essential for blood clotting, is produced by bacteria in the large intestine.
Newborns receive vitamin K supplements because their bacterial flora is not yet fully established.
The bacteria also produce several vitamins.
Deficiencies in the intestinal microflora during intrauterine life and early development can lead to lasting issues.
Microflora affects the columnar cells lining the digestive tract, maximizing nutrient uptake, metabolic activities, vascular growth, and protection against infection.
They also play a role in the development of the enteric nervous system.
Animals raised in germ-free environments have weaker digestive tracts, poor digestion and absorption, and are more prone to irritable bowel conditions.

Genetic factors.
Environmental exposure to bacteria.
Diet, including breastfeeding.
Children raised in less sterile environments with pets tend to have stronger immune systems and less sensitive guts.

Intestinal gas is a byproduct of bacterial fermentation of undigested food.
Some bacterial byproducts are vitamins and hormones, while others are gases.
Specific foods, such as beans, are known to increase gas production due to bacterial fermentation.
A study showed that diets high in beans led to increased gas production.