Large Intestine Structure and Function Flashcards
How long is the colon?
1.5-1.8m
What are the 4 parts of the colon?
- Ascending
- Transverse
- Descending
- Sigmoid
How do the longitudinal and circular muscle layers differ?
Circular muscle layers is complete by longitudinal muscle layer is incomplete
What are the 3 bands of the colon called?
Teniae coli
What does contraction of the teniae coli result in?
Pouches (haustra) resulting in a puckered appearance
What is the mucosa of the large intestine composed of?
Simple columnar epithelium
What provides lubrication for movement of faeces?
Large straight crypts lined with large numbers of goblet cells
What is the rectum?
Straight, muscular tube between end of sigmoid colon and anal canal
What is the mucosa of the rectum?
Simple columnar epithelium
How does the muscularis externa of the rectum compare to that of other regions of the alimentary canal?
Thicker
What is the anal canal?
2-3cm between distal rectum and anus
How does the muscularis of the anal canal compare to that of the rectum?
Thicker, contains internal anal sphincter
What is the external anal sphincter?
Skeletal muscle
How does the epithelium of the anal canal transition?
Simple columnar to stratified squamous
What nutrient absorption role does the colon play?
No important role
What is the result of the active transport of sodium from the lumen into the blood?
- Osmotic absorption of water
- Dehydration of chyme
- Production of solid faecal pellets
What does the long residence time in the colon lead to?
Bacterial colonisation
How much colonic microflora is there?
10^14 bacteria (~1kg)
What does the bacterial fermentation of undigested carbohydrate lead to?
- Short chain fatty acids
- Vitamin K
- Gas
What are the components of flatulence?
- Nitrogen
- CO2
- Hydrogen
- Methane
- Hydrogen sulphide
Why is the anus normally closed?
Closed by internal anal sphincter and external anal sphincter
What is the control of the internal anal sphincter?
Smooth muscle under autonomic control
What is the control of the external anal sphincter?
Skeletal muscle under voluntary control
Describe the production of the urge to defecate.
- Following meal wave of intense contraction (MMC) from colon through to rectum
- Distension of rectal wall produced by mass movement of faecal matter into rectum
- Stimulates mechanoreceptors
- Defaecation reflex
- Urge
What is the control of the defecation reflex?
Under parasympathetic control via pelvic splanchnic nerves
What influence does the sympathetic system have on the defecation reflex?
No influence
What are the components defecation reflex?
- Contraction of rectum
- Relaxation of internal and contraction of external anal sphincters
- Increased peristaltic activity in colon
What is the result of the defecation reflex?
- Increased pressure on external anal sphincter
- Relaxes under voluntary control
- Expulsion of faeces
What is voluntary delay of defecation due to?
Descending pathways
Describe the frequency of bowel movements?
Varies considerably from person to person
What is the absorption of toxins from faecal material following long periods of retention?
No absorption
What are the symptoms associated with constipation?
- Headaches
- Nausea
- Loss of appetite
- Abdominal distension
What is constipation due to?
Distension of rectum
What is diarrhoea?
Too frequent passage of faeces which are too liquidy
What are the causes of diarrhoea?
- Pathogenic bacteria
- Protozoans
- Viruses
- Toxins
- Food
How many deaths a year is diarrhoea responsible for?
2.5 million of the 1,500 million cases a year
Give 2 examples of enterotoxigenic bacteria.
- Vibrio cholerae
- Escherichia coli
What doe enterotoxigenic bacteria produce?
Produce protein enterotoxins which maximally turn on intestinal chloride secretion from crypt cells which leads to increased H2O secretion
How do enterotoxigenic bacteria act?
Elevate intracellular second messengers:
- cAMP
- cGMP
- Calcium
How does H2O secretion lead to profuse watery diarrhoea?
H2O secretion swamps absorptive capacity of villus cells
What damage do enterotoxins do to villus cells?
No damage
What is the treatment for secretory diarrhoea?
- Give sodium/glucose solution
- Oral rehydration therapy
What does sodium/glucose solution do?
Drives H2O absorption leading to rehydration
