Physiology - Large Intestine Structure and Function Flashcards
What is/ was the function of the veniform appendix?
Contains lymph nodes
Immune function
What are the 4 parts of the colon?
Ascending, Transverse, Descending, Sigmoid
How do the longitudinal and circular muscles differ in the colon?
The circular muscle layer is complete but the longditudinal muscle is incomplete
What are teniae coli?
How do they form haustra?
Bands of longitudinal muscle which line the entire length of the colon.
Contraction of these create pouches called haustra.
This is due to the incomplete arrangement of longitudinal muscle
What type of epithelium is found in the mucosa of the large intestine?
Simple columnar epithelium
Are crypts present in the colon?
Yes.
These large straight crypts are lined by many goblet cells which secrete mucous.
This lubricates the colon for the movement of faeces
Describe the structure of the rectum
The rectum is a straight muscular tube lying between the sigmoid colon and the rectum.
It is lined with simple columnar epithelium.
Its muscularis externa is much thicker than other areas of the alimentary canal
Describe the structure of the anal canal
The anal canal is about 2-3cm in length lying between the distal rectum and the anus.
Its muscularis externa is thicker than that of the rectum (internal sphinter)
It also has an external sphincter which is skeletal muscle.
The anal canal shows a change in its mucosal epithelium from simple columnar to stratified squamous.
(Change from inside the body to outside)
What is the function of the colon?
No important nutrient absorption role in humans
Actively transports sodium from the lumen to blood->
Osmotic absorption of water->
Dehydration of chyme ->
Solid faecal pellets
What similarities are there between water absorption in the colon and water absorption in that gall bladder?
Its the exact same mechanism.
Sodium potasium pump creates a low concentration of sodium inside the cell. (Potassium recycled)
Sodium transported into cell from lumen.
Water moves pancellularly
What is colonic microflora?
What does it produce?
Long term residence of faeces in the colon causes bacterial colonisation. This is the colonic microflora.
The bacteria are about 10^14 (weighing about 1kg)
They ferment the undigested carbohydrate that we can’t (cellulose)
This produces:
-Short chain fatty acids (energy source in ruminants)
-Vitamin K (blood clotting)
-Gas (flatus)
What is the clinical significance of vitamin K production by colonic bacteria?
Babies are born with a sterile colon. They therefore don’t get any vitamin K until they establish their microflora.
Vitamin K injection given to newborns to help their bleeding risk
What chemicals are contained in flatus?
Nitrogen, CO2, hydrogen methane, hyodrogen sulphide
Describe how we get the urge to defaecate
Normally, the anus is closed by the internal (autonomic control) and the external (voluntary control) anal sphincters.
Following a meal you get a wave of intense contraction (Mass Movement Contraction) through the colon to the rectum.
Mass movement of faecal material into the rectum causes distension of the rectal walls.
Mechanoreceptors detect this and signal the defeacation reflex causing an urge to defaecate.
What is the defaecation reflex?
Under parasympathetic control via pelvic splanchnic nerves.
A. Contraction of Rectum
B. Relaxation of internal and contraction of external sphincters (the poo is docking)
C. Increased peristaltic activity in the colon
Increased pressure on external anal sphincter
When it relaxes under voluntary control there is the expulsion of faeces
Are toxins absorbed from faecal material following long periods of retention?
No
Do the frequency of bowel movements vary between individuals?
Yes
What are the symptoms associated with constipation?
Headaches
Nausea
Loss of appetite
Abdominal distension
What is diarrhoea?
Too frequent passage of faeces which are too liquid
Major killer of children under 5 years of agein developing countries.
1500 cases of child diarrhoea
2.5 million deaths/year
What are the causes of diarrhoea?
Pathogenic bacteria Protoxoans Virus' Toxins Food Stress
Give an example of 2 enterotoxigenic bacteria.
Vibrio Cholerae
Escherichia Coli
What are enterotoxigenic bacteria and what is their pathology?
Produce protein enterotoxins which maximally turn on intestinal chloride secretion from crypt cells.
This causes huge H2O secretion
They act by elevating intracellular second messengers: cAMP, cGMP, Calcium
H2O secretion is massive compared to villus absorptive capacity. This results in profuse watery diarrhoea
(25 litres per day for cholera)
How do you treat secretory diarrhoea?
Enterotoxins dont damage villus cells
Give a sodium/glucose solution to drive water absorption and cause rehydration.
Secretion will still occur to wash infection away
This is called oral rehydration therapy and saves 2 million children lives per year
