Order of the large intestine:
Caecum (with vermiform appendix coming off) Ascending Colon Transverse Colon Descending Colon Sigmoid Colon Rectum Anal Canal Anus
What is the caecums function?
Its a blind ended sac acting like a big fermentation sac, much bigger in full herbivores who need bacteria to break down cellulose more
Where is the appendix and what does it do?
Most often retrocaecal but varies
It doesn’t have on but is associated with a lot of lymph nodes.
How long is the colon?
Whats different about the muscularis Externa of the large intestine?
The longitudinal muscle layer isn’t complete but instead comes in three strips that run the length of the colon called teniae coli
The colon has a quilted or puffa jacket appearance, what produces this?
The 3 teniae coli, when they contract the circular muscle layer is forced out the gaps and produces the haustra or sacculations.
Whats different about the mucosa of the large intestine to small?
It has a simple columnar epithelium just like the small intestine but with no villi
Whats different about the crypts in the large intestine to the small intestine?
They’re much longer in the colon and lined with goblet cells to lubricate movement of faeces
What is the rectum?
A straight muscular tube connecting the sigmoid colon and anal canal.
It has a thicker muscularis externa than most of the canal
What is the anal canal?
A short 2-3cm channel.
Its muscularis externa forms the internal anal sphincter (IAS).
Where is the external anal sphincter, and what is it made from?
In the anal canal, distal to the internal anal sphincter.
Its made of skeletal muscle and so is voluntarily controlled
How does epithelium change leaving the anal canal?
IT changes from simple columnar to stratified squamous like the skin.
What is the main function of the colon?
Actively absorbing sodium in order to take up water.
A huge amount of bacteria exists in the colon, (colonic microflora) why?
The long residence time of faeces allows extensive bacterial colonisation
What are the roles of colonic microflora?
- Breakdown undigested carbohydrates (cellulose) to short chain fatty acids which we use for energy
- Also produce vitamin K which we use for blood clotting
What gasses are there in the colon? (Flatus)
Nitrogen CO2 Hydrogen Methane Hydrogen Sulphide
What sphincters control defecation and how?
Internal Anal Sphincter - Smooth muscle - Autonomic (part of the anal canal)
External Anal Sphincter - Skeletal Muscle - Voluntary
What is Mass Movement Contraction?
Waves of intense peristaltic contraction from colon to rectum that occur after eating and result in:
- Rectal wall distends by mass movement of faeces
- Stimulates mechanoreceptors
- Initiates defaecation reflex (urge to shit)
Basically making room for fresh food
What controls the post-meal defecation reflex?
Parasympathetic control via pelvic splanchnic nerves
How does defaecation occur?
- Contraction of rectum
- Relaxation of Internal anal sphincter
- Increased peristaltic activity in colon -> increased pressure on External Anal Sphincter (EAS)
- Voluntary Relaxation of EAS
- Faeces Expulsed
As you know defaecation can be voluntarily delayed, why cant babies do this?
The descending pathways required for conscious control of defecation dont develop until your around 2. Prior to that potty training wont work because kids cant physically stop
What are the symptoms of constipation and why?
The build up of material causes rectum distension which can cause: = Headaches = Nausea = Loss of Appetite = Abdominal Distension
Unusually frequent and liquid faeces
What are some causes of diarrhoea?
- Pathogenic Bacteria e.g. C.Diff/Vibrio Cholerae
- Toxins (from bad food or bacteria)
- Certain foods cause it in certain people
How many people die from diarrhoea?
About 1/4 of <5yr deaths in the developing world are due to diarrhoea
Define an enterotoxin?
A toxin produced in or affecting the intestines
Name 2 enterotoxigenic bacteria?
How do enterotoxogenic bacteria cause diarrhoea?
- Produce protein enterotoxins
- Elevates intracellular secondary messengers (e.g. cAMP, calcium etc)
- Maximally activates intestinal Cl- secretion from crypt cells
- You passively secrete loads of water
- Secretion swamps absorptive capacity of villi
How do we treat diarrhoea?
Sodium/Glucose solution (Oral Rehydration Salts, ORS)
- Gut absorbs sodium & glucose
- Drives H20 absorption
- Keeps you hydrated
- Secretion is still ongoing and will wash away the infection eventually