Large Intestine Structure and Function Flashcards
What valve is between the ileum and the caecum?
Ileocaecal valve
What is the length of the colon?
Ranges from 1.5 to 1.8m
What are the 4 parts of the large intestine?
Ascending colon
Transverse colon
Descending colon
Sigmoid colon
Explain how the circular and longitudinal muscles of the colon are different from the small intestine?
Circular muscle is complete but the longitudinal is not
What are the three bands present along the entire length of the colon?
Teniae coli
What are tenaei coli?
Three seperate longitudinal ribbons of smooth muscle running through the entire colon
What are the pouches present throughout the colon called?
Haustra
What produces haustra?
Contractions of the teniae coli
What is the classification of the mucosa in the colon?
Simple columnar epithelium that is flat
What is contained in the crypts within the colon?
Goblet cells
Why are goblet cells present in the colon?
Secrete mucous that is used as lubrication for faeces to move
What is the rectum?
Straight, muscular tube between the end of sigmoid colon and anal canal
What is the classification of the mucosa of the rectum?
Simple columnar epithelium
How does the muscularis externa of the rectum compare to other regions of the alimentary canal?
It is thicker
What is the anal canal?
2-3cm between distal rectum and anus
How does the muscularis externa of the anal canal compare to the rectum?
Is thicker forming the internal anal sphincter
What is the internal anal sphincter formed from?
Muscularis externa (smooth muscle)
What is the external anal sphincter formed from?
Skeletal muscle
How does the epithelium change in the anal canal?
Changes from simple columnar to stratified squamous
Does the colon have an important nutrient absorpion role?
Not in humans
What is the function of the colon?
Actively transport sodium from lumen into the blood, causing osmotic absorption of water
This causes dehydration of chyme causing solid faecal pellets
What cause faeces to be solid?
Water is absorbed, causing dehydration of faeces, due to the colon transporting sodium from the lumen to the blood creating an osmotic difference
What causes bacterial colonisation of the colon?
Large residence time
How much bacteria is present in the colonc microflora?
1014 (about 1kg)
What happens to undigested carbohydrates in colon?
Bacterial fermentation
What are consequences of bacterial colonisation of the colon?
Bacterial fermentation of undigested carbohydrates, short chain fatty acids and vitamin K
Production of gas
Why does defaecation not occur passively?
Anus is closed by internal anal sphincter (smooth muscle under autonomic control) and external anal sphincter (skeletal muscle under voluntary control)
Explain the mechanism that occurs following a meal that leads to defaecation?
1) Wave of intense contraction (mass movement contraction) from the colon into the rectum
2) Distension of rectal wall produced by mass movement of faecal material into rectum
3) Activated mechanoreceptors activate the defaecation reflex
4) Causes urge to defaecate
What controls the defaecation reflex?
Parasympathetic nervous system (via pelvic splanchnic nerves) with no sympathetic influence
How can defaecation be voluntary delayed?
By descending neural pathways
Explain the mechanism of the defaecation reflex under parasympathetic control?
1) Contraction of rectum
2) Relaxation of internal and contraction of external anal sphincters
3) Increased peristaltic activity in colon (increased pressure on external anal sphincter causing expulsion of faeces)
What is constipation?
Condition where there is difficulty emptying the bowels
Does constipation lead to absorption of toxins?
No, there is no absorption of toxins from faecal material following long periods of retention
How does frequency of bowel movements vary from person to person?
Vary considerably
What are some symptoms associated with constipation?
Headaches
Nausea
Loss of appetite
Abdominal distension
Why does constipation cause symptoms?
It causes distension of the rectum
What is diarrhoea?
Too frequent passage of faeces which are too liquid
What are some causes of diarrhoea?
Pathogenic bacteria
Protozoans
Viruses
Toxins
Food
How much deaths occur per year due to diarrhoea?
0.5 million deaths/year, mostly children under 5
What is E-coli an example of?
Enterotoigenic bacteria that normally lives in the intestines
What are examples of enterotoxigenic bacteria?
Vibrio cholerae
Escherichia coli
What does enterotoxigenic bacteria produce that causes diarrhoea?
Enterotoxins that maximally turn on intestinal chloride secretion from crypt cells, increasing H2O secretion
How does enterotoxigenic bacteria maximally turn on intestinal chloride secretion from crypt cells?
Enterotoxins that they produce elevate the levels of intracellular second messengers like:
cAMP
cGMP
Calcium
Why does excess H2O secretion in the colon lead to diarrhoea?
It swamps absorptive capacity of villus cells leading to profuse watery diarrhoea
How much water per day is lost in diarrhoea due to cholerae?
25L
Do enterotoxins damage villus cells?
No
What is the treatment of secretory diarrhoea?
Sodium/glucose solution which drives H2O absorption leading to rehydration
What is the administration of sodium/glucose solutions to people suffering from secretory diarrhoea called?
Oral rehydration therapy (ORT)
What does ORT stand for?
Oral rehydration therapy
