Describe the gross anatomy of the large intestine (including caecum, appendix, colon and rectum):
Features of LI:
- right colic (hepatic) flexure
- left colic (splenic) flexure
- haustra = sacculations that contract to churn the food. (one haustra will gradually expand as it fills with chyme until it reaches maximum capacity and then contracts to move food into the next haustra)
- taenia coli = 3 longitudinal bands of muscle that contract to produce the haustra, thickening of muscularis externa
- appendices epiploicae/epiploic appendices (fatty tags covering the LI surface)
Parts of LI:
CAECUM
- lies in R iliac fossa
- the ileocaecal valve has 2 lips which protrude into the caecum and an increase in pressure in the caecum squeezes the two lips together to prevent the reflux of contents into the ileum
APPENDIX
- thin windy tube that opens off caecum
- ~8cm long (varies from 2-20)
COLON:
- retroperitoneal parts = ascending and descending colons
- transverse colon suspended from transverse mesocolon (a peritoneal fold) from lower pancreatic border
- ascending colon and proximal 2/3 of transverse = midgut derived, the remaining portion is hindgut
- arterial supply from branches of SMA and IMA: middle and right colic and left colic
- venous supply similar to arterial but from branches of SMV and IMV which drain into the hepatic portal system
- SMV branches are ileocolic and right colic veins
- IMV branches are left colic vein and sigmoid veins
RECTUM
- ~15cm long
- angle of 120 degrees at anorectal junction is maintained by puborectalis pt of levator ani muscles and is important to maintain rectal continence
Describe the anatomy of the anal canal:
- last 4cm of alimentary canal
- mucosal folds are present called COLUMNS
- anal canal contains veins which can form haemorrhoids if pressure increases
- internal anal sphincter surrounds upper 2/3 of canal, circular bands of involuntary smooth muscle
- external sphincter surrounds lower 2/3 of anal canal, voluntary muscle
- anorectal ring (a band of muscle) is at the junction of the rectum and anal canal, formed by fusion of internal and external sphincters and puborectalis muscle, palpable on DRE
Describe the histology of the LI:
- colon and rectum lined by simple columnar epithelium
- above pectinate line of anal canal = simple columnar epithelium
- below pectinate line of anal canal = non-keratinised stratified squamous epithelium
- pectinate line divides upper 2/3 anal canal from lower 1/3
- above pectinate line = superior rectal artery/vein, visceral innervation sensitive to stretch
- below pectinate line = inferior rectal artery/vein, somatic innervation sensitive to pain/touch/temperature/pressure
- lots of absorptive cells in colon
- many goblet cells to secrete mucous to lubricate the passage for the movement of colonic contents
- thick mucous layer prevents irritation of mucosa by faeces/fermentation products
- no circular folds or villi
- MICROVILLI ARE PRESENT (IN LARGE AND SMALL INTESTINES)
- mucosa thicker with deeper crypts and more goblet cells than the SI
Describe motility in the LI:
- slow movement from caecum -> transverse colon, allows time for absorption to occur and faeces to be produced
1) normal peristalsis = 3-12 contractions per minute
2) mass movements = few times a day, where large proportion of proximal colon is emptied as faeces is forced into the rectum producing the urge to defecate, triggered by distension of the stomach/duodenum
3) segmentation/haustral churning = main movement type in caecum and proximal colon, one haustra relaxes and fully fills and distends, once at certain distension the haustral walls contract and squeeze contents into the next haustra
How does the defecation reflex work?
1 - distension of rectal walls activates stretch receptors and signals are sent to spinal cord neurons
2 - spinal reflex initiated and parasympathetic motor fibres cause relaxation of internal anal sphincter and rectal wall contraction
3 - IF CONVENIENT to defecate, voluntary motor neurons that keep the external sphincter contracted are inhibited, so external sphincter relaxes and faeces can pass
4 - IF INCONVENIENT to defecate, rectal wall contractions end and voluntary motor neurons are NOT INHIBITED and defecation is delayed
5 - rise in abdominal pressure by closing the glottis an contracting the abdominal muscles aids defecation
- urge to defecate occurs when rectal pressures reach >15mmHg
- external sphincter involuntarily relaxes when rectal pressures >55mmHg (this occurs in babies, elderly and people with spinal cord injuries)
How is water absorbed in the GI tract and describe electrolyte transport?
- Na and Cl are pumped into intercellular spaces and water always follows
- absorbed amino acids and sugar increase the osmotic absorption of water
- 90% of water from colon contents is absorbed by the colon
- XS urea is secreted into the colon to get metabolised and broken down by bacteria
Describe the normal gut flora of the GI system:
- upper gut has few flora due to pancreatic enzymes and gastric activity
- large and complex bacterial ecosystem can develop in the colon due to the HCO3- ions which buffer the pH
- ~1000 different bacterial species in the colon and the majority (99%) are anaerobes
- when we are infants the gut is colonised by commensal bacteria developing the gut-associated immune system
- tolerance to this immune system is developed, preventing an immune response to colonic flora
What is the function (including benefits) of gut flora?
- enables colon to salvage energy and nutrients that escape absorption in the small intestine
- commensal bacteria keep pathogenic bacteria under control by competing with them for nutrients and space
- gut flora converts urobilinogen -> stercobilin
- bacteria degrade cholesterol and some drugs
- gut flora ferments indigestible carbohydrates into short chain fatty acids
Name some short chain fatty acids produced by bacteria in the gut and their benefits:
Acetic acid
- energy source
- substrate for fat synthesis in the liver
Propionic acid
- energy source
- reduces cholesterol synthesis
Butyric acid
- stimulates cell differentiation
- involved in programmed death of cancer cells
In general, SCFA’s inhibit pathogenic bacteria growth and increase cell proliferation throughout the whole gut
What dangerous effect can antibiotics have on the gut flora and commensal bacteria?
Broad spectrum antibiotics can inhibit the growth and metabolism of normal colonic flora
- this puts individuals at risk of diarrhoea, infections etc.
What are the different toxins that bacteria in the gut can produce and how do they exert their effect?
ENTEROTOXINS:
- produced by bacteria and adhere to the intestinal epithelium
- cause XS fluid to be secreted into bowel lumen = dehydration
- > activate cAMP
- > PKA phosphorylated
- > excretory Na channels activated
- > Cl- released from enterocytes and water follows
- some enterotoxins are preformed in food and cause vomiting
CYTOTOXINS:
- damage the intestinal mucosa
- may damage vascular endothelium also
What are the routes of transmission of enteric infection?
1) endogenous -> body’s own endogenous flora can cause infection
2) air borne -> spread from person to person by aerosol/droplet transmission
3) faecal-oral route -> by direct transfer of food and water with faeces
4) vector -> spread by an animal e.g. malaria
5) direct person to person transmission -> breast milk, blood transfusions, STI’s
What is an enteric infection?
- affects nutrient absorption and digestion and GI tract
- characterised by diarrhoea, abdominal discomfort, nausea, vomiting and anorexia
What measures should be put in place to prevent and control spread of infection?
- education of staff in healthcare environments and public
- use proper sanitation equipment
- isolation (automatic doors, air conditioning)
- appropriate disposal of healthcare waste
- use of PPE
- hand decontamination
- antibiotic stewardship
Briefly describe the anatomy of the spleen:
- Fist sized organ
- At the posterior of the abdomen at the level of ribs 9-11
- Is posterior to the mid-auxillary line
- In the greater sac of the peritoneal cavity
- Attached to the gastrosplenic ligament: part of the greater omentum attaching the greater curvature of the stomach to the hilum of the spleen
- Histology: has white pulp (lymphoid tissue) and red pulp (which filters the blood)
- Has special vessels called sinusoids which take up old RBC’s and the red pulp contains macrophages to destroy them
What are the large fluid movements in the GI tract and describe the potential for large losses in diarrhoea?
- massive water reabsorption occurs daily and only 100ml is excreted per day
- fluid INTAKE - 2000ml
- fluid TURNOVER - 9000ml
- fluid excretion - 100ml
Due to this huge fluid turnover, it means that there can be a massive fluid loss if something goes wrong with the system
What are the 4 important membrane transporters found in the GI tract cells and describe their movements?
- Na/K ATP pump drives the whole process by removing Na from the cell in exchange for K. This creates a concentration gradient which means Na can then passively move back into the cell. Various pumps transport different ions using Na.
- When Na is removed by the Na/K ATP pump, water follows and can move out through the permeable tight junctions between enterocytes
4 main membrane transporters:
1) Na ion channel
2) Na co-transporter/symporter (moves Na into cell with amino acids, peptides, bile salts and vitamins)
3) Na antiporter/exchange carrier (moves Na into cell and H+ out)
4) Another antiporter which moves Cl- into cell and HCO3- out
Na is taken in from the gut lumen side with the other co-transported molecules, and then when it is released by the ATP pump, Na and H2O are released into the bloodstream side (basolateral side)
What was the breakthrough discovery that made oral rehydration therapy possible?
In cholera epidemic it was discovered that NaCl cannot be absorbed alone and glucose must be present.
Glucose enhances Na absorption.
This is why you give solutions like saline and dextrose, or water with sugar and salt in it.
What is the importance of children’s diarrhoea worldwide and how is it treated and prevented?
Diarrhoea is a major cause of children’s death worldwide
88% of worldwide deaths from diarrhoea are due to unsafe water/poor sanitation/poor hygiene
How can infectious diarrhoea be prevented?
- vaccines against infections that cause diarrhoea
- early breast feeding and vitamin A supplementation
- handwashing with soap
- improved water quality
- community wide sanitation promotion
How can you treat infectious diarrhoea?
- fluid replacement (ORT)
- zinc supplements (decrease severity and duration of diarrhoea)
- use of appropriate fluids
- generally, increase fluid intake
Describe the body water compartments and what different osmolarities mean:
- 3 water compartments are
- intracellular fluid
- extracellular fluid
- intravascular fluid
High osmolarity = cellular dehydration
Low osmolarity = cellular over-hydration and oedema
How does osmotic pressure control fluid movement between body compartments?
- body always tries to equalise osmolarity
- too many osmotically active molecules in the blood = water leaves cells
- when plasma has low osmolarity (i.e. lots of water) water will move into cells and could result in oedema
What are the main categories of intravascular fluids used and give examples?
Why are IV fluids used?
1) COLLOIDS
- large molecular weight substances
- e.g. albumin, hydroxy-ethyl-starch (HES), haemacel
2) CRYSTALLOIDS
- water and electrolyte solution e.g. salt solution
- e.g. saline, dextrose, Hartmann’s ringer lactate
Are used to prevent hypovolaemic shock and maintain intravascular volume if a patients BP is low
-
Endocrine Week 157
-
Endocrine Week 278
-
Endocrine Week 359
-
Endocrine Week 438
-
Gastrointestinal Week 162
-
Gastrointestinal Week 298
-
Gastrointestinal Week 3101
-
Gastrointestinal Week 472
-
Reproduction Week 1100
-
Renal Week 291
-
Reproduction Week 278
-
Renal Week 182
-
HNN Week 1170
-
HNN Week 2149
-
HNN Week 397
-
HNN Week 4100
-
HNN Week 586
-
HNN Week 646
-
VS - Professionalism13
-
People and illness Week 181
-
People and illness Week 268
-
VS - Right thing to do27
