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?
No important nutrient absorption role in humans
What is the function of the colon?
Actively transports sodium from lumen into blood ⇒ osmotic absorption of water ⇒ dehydration of chyme ⇒ solid faecal pellets
What cause faeces to be solid?
sodium from lumen into blood ⇒ osmotic absorption of water ⇒ dehydration of chyme ⇒ solid faecal pellets
What causes bacterial colonisation of the colon?
Long residence time in colon
How much bacteria is present in the colonc microflora?
1014 bacteria (~1Kg)
What happens to undigested carbohydrates in colon?
Bacterial fermentation
What are consequences of bacterial colonisation of the colon?
Short chain fatty acids (energy source in ruminants)
Vitamin K (blood clotting)
Gas (flatus) - nitrogen, CO2, hydrogen, methane, hydrogen sulphide
Why does defaecation not occur passively?
Normally, anus closed by internal anal sphincter (smooth muscle under automatic control) and external anal sphincter (skeletal muscle under voluntary control)
Explain the mechanism that occurs following a meal that leads to defaecation?
Wave of intense contraction (Mass Movement Contraction) - colon → rectum
Distension of rectal wall produced by mass movement of faecal material into rectum → mechanoreceptors → defaecation reflex → urge to defaecate
What controls the defaecation reflex?
Distension of rectal wall produced by mass movement of faecal material into rectum → mechanoreceptors → defaecation reflex → urge to defaecate
How can defaecation be voluntary delayed?
Voluntary delay of defaecation - descending pathways
Explain the mechanism of the defaecation reflex under parasympathetic control?
Under parasympathetic control – via pelvic splanchnic nerves (no sympathetic influence)
Contraction of rectum
Relaxation of internal and contraction of external anal sphincters
Increased peristaltic activity in colon
↑ Pressure on external anal sphincter - relaxes under voluntary control ⇒ expulsion of faeces
What is constipation?
No absorption of toxins from faecal material following long periods of retention
Does constipation lead to absorption of toxins?
no
How does frequency of bowel movements vary from person to person?
Frequency of bowel movements vary considerably from individual to individual
What are some symptoms associated with constipation?
Headaches
Nausea
Loss of appetite
Abdominal distension
Why does constipation cause symptoms?
Due to distension of 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
first year medical exams…
How much deaths occur per year due to diarrhoea?
Major killer of children under 5 years of age in developing countries
What is E-coli an example of?
Enterotoxigenic Bacteria
What are examples of enterotoxigenic bacteria?
Vibrio cholerae, Escherichia coli
What does enterotoxigenic bacteria produce that causes diarrhoea?
Produce protein enterotoxins which maximally turn on intestinal chloride secretion from crypt cells
How does enterotoxigenic bacteria maximally turn on intestinal chloride secretion from crypt cells?
↑ H2O secretion
Act by elevating intracellular second messengers:
cAMP
cGMP
calcium
Why does excess H2O secretion in the colon lead to diarrhoea?
H2O secretion swamps absorptive capacity of villus cells → profuse watery diarrhoea (25 litres per day for cholera)
How much water per day is lost in diarrhoea due to cholerae?
25 litres
Do enterotoxins damage villus cells?
don’t damage villus cells
What is the treatment of secretory diarrhoea?
Give sodium/glucose solution
Drive H2O absorption ⇒ 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
