GI Topic 4 - Lower GI Tract, Diarrhoeal Diseases Flashcards
What causes inflammatory diarrhoea?
Widespread destruction of absorptive epithelium - insufficient water absorption
What is the gastrocolic reflex?
Increased colonic motility after a meal
Describe water movement in the intestines
- Water always moves to correct osmotic imbalance
- Small intestine
- Sodium is actively absorbed by co-transport with glucose/amino acids, water follows sodium and is absorbed
- Cl-/HCO3- antiporter pumps Cl- into the lumen, Na+ follows to correct the electrochemical gradient, water follows and is secreted
- Large intestine
- Osmotic gradient produced by Na+ absorption - water follows
Which antibiotics affect the gut microflora?
4 C’s - clindamycin, coamoxiclav, cephalosporins, ciprofloaxin
Describe the structure of the caecum
Blind-ended pouch, attached to the ileum at the ileocaecal valve
Describe the motility of the large intestines
- Short duration and long duration contractions
- Mixing movements - circular and longitudinal muscle contraction (haustrations)
- Ensures all contents are exposed to the intestinal wall - absorption of water/electrolytes
- Propulsive movements - slow, from prolonged contractions
- Mass movements - 10-30 minutes every 12 hours
- Contraction of 20cm+ of colon - propel faecal material en masse through colon
- Result from distention of stomach and duodenum/irritation of colon
- Stimulated by extrinsic autonomic nerves
What are the characteristics of inflammatory diarrhoea?
Low volume, bloody
Why is the microflora of the large intestine so extensive compared with other parts of the GI tract?
- Less movement
- Less acidic pH (buffered by bicarbonate)
Where is bacterial activity highest in the colon?
Most activity in the proximal colon - distal more for storage
How are diarrhoeal diseases treated/managed?
- Prevention - vaccination (e.g. rotavirus, measles), improve sanitation
- Rehydration
- Antibiotics (if bacterial)
Describe the structure of the external anal sphincter
- Lower 2/3 (overlaps with internal sphincter)
- Voluntary
- Joins with puborectalis muscle of the pelvic floor superiorly
Give examples of organisms which cause inflammatory enteric infections
Shigella, C. diff
Give examples of organisms which cause non-inflammatory enteric infections
Vibrio cholerae, staph. aureus
List the pathogenic mechanism types of enteric bacterial toxins
- Neurotoxin e.g. clostridium botulinum, staph. aureus, bacillus cerus
- Secretory (most common) e.g. Vibrio cholera, E. Coli, salmonella, shigella dysernteriae
- Cytotoxin e.g. shigella, C. Diff, H. Pylori
What is the function of the anal canal?
Defecation and maintaining faecal continence
What are the consequences of hypo/hyperkalaemia?
Arrhythmias
What are the benefits of short chain fatty acids produces by gut microflora?
- Increase cell proliferation in gut
- Promote water absorption, prevent osmotic diarrhoea
- Inhibit growth of pathogenic bacteria
- Energy sources - acetic acid for fat synthesis, propionic acid for gluconeogenesis
- Butyric - fuel for colonic cells, programmed cell death of cancer cells, increased Na+ and Cl- absorption
Why is the anorectal flexure important?
Contributes to faecal continence
Describe the innervation of the colon
- Mid-gut - SM plexus
- Hind-gut - IM plexus
- Parasympathetic - pelvic splanchnic nerves
- Sympathetic - lumbar splanchnic nerves
Describe the action of colloids, when are they used?
- High molecular weight, used to increase intravascular volume e.g. after major haemorrhage when the volume of the vascular bed decreases, for resuscitation
- Initially nearly 100% remains intravascular
Describe the venous drainage of the caecum
Ileocolic vein, drains into the superior mesenteric vein
What causes anorectal constipation?
- Hirschprung’s disease - failure of migration of neural crest cells to distal colon - aganglionic and contracted
- Obstructive defecation - paradoxical contraction of puborectalis + external sphincter during defection
- Rectocoele
- Anal fissure - associated pain on defecation
List the flexures of the rectum
- Sacral (anterior) and anorectal (posterior)
- 3 lateral flexures - superior, intermediate and inferior
List the ions which are most important for movement of water in the intestines
- Na+
- Cl-
- K+
- HCO3-
Describe the arterial supply of the anal canal
- Above the pectinate line - superior rectal artery (branch of the inferior mesenteric artery) and middle rectal artery
- Below the pectinate line - inferior rectal artery (branch of the internal pudendal artery), middle rectal artery
Which parts of the colon are intra/retroperitoneal?
- Ascending and descending parts are retroperitoneal
- Transverse and sigmoid parts are intraperitoneal
Describe the composition and types of crystalloids
- Water and electrolytes - saline, dextrose, Ringer-lactate, Hartmann’s
- Dextrose - contains glucose which is metabolised to water, used if Na+ is high, 10% remains IV
- Saline - if sodium is low, 25% remains IV
Describe the junction of the rectum and the anal canal
- Anorectal ring joins the rectum and the anal canal
- Muscular ring, made of internal and external anal sphincters and puborectalis muscle
Describe the relationship between enteric infection and Guillain-Barre Syndrome
- Guillain-Barre neuropathic syndrome can occur after campylobacter infection
- Neuropathic symptoms occur weeks to months after infection - weakness, neurological symptoms (demyelinating polyneuropathy)
- Especially occurs in young men
What is the function of the caecum?
Used to be the site of cellulose digestion, now just storage of chyme
List the parts of the lower GI tract
- Vermiform appendix
- Caecum
- Ascending colon
- Transverse colon
- Descending colon
- Sigmoid colon
- Rectum
- Anal canal
Describe the innervation of the rectum
- Sympathetic - lumbar splanchnic nerves, superior and infection hypogastric plexuses
- Parasympathetic - S2-4 pelvic splanchnic nerves and inferior hypogastric plexuses
What is the role of Na+/K+ ATPase in fluid and electrolyte absorption?
Maintains electrochemical gradient
On basolateral surface of cells lining the intestines
List the drugs which decrease colonic motility
- Opiates (via Muscarinic receptors)
- Anti-cholinergics
- Loperamide (Muscarinic receptor agonist, decreases myenteric activity, slow transit - more water absorbed) - diarrhoea management
How is potassium absorbed in the intestines?
H+/K+ ATPase - K+ absorbed, H+ secreted
How is defecation controlled?
- Local enteric nervous control in the rectal wall stimulated by distention of the rectum - afferent signals to the myenteric plexus, peristaltic waves in the descending/sigmoid colon and rectum, forces faeces towards anus
- Internal sphincter relaxed by inhibitory signals from myenteric plexus
- If external relaxed voluntarily - defecation occurs
- Parasympathetic defecation reflex - sacral spinal segments, pevic nerves - relax internal sphincter and increase peristaltic waves
List the causes of osmotic diarrhoea
- Laxatives
- Antacids
- Acarbose (alpha glucosidase inhibitor)
- Orlistat (lipase inhibitor)
- Digestive/pancreatic enzyme insufficiency
- Inflammatory disease
- Short bowel syndrome (or bowel resection)
- Loss of enterocytes
- Bacterial overgrowth
- Lymphatic obstruction
Describe the venous drainage of the appendix
Appendicular vein, drains to ileocolic vein (drains to SMV)
Aside from SCFA, what substances do colonic flora produce?
- Vitamin B12 - can’t be absorbed
- Thiamine
- Gases - CO2, H2, methane
How is the rectum structually different to the colon?
Rectum has no taenia coli, haustrations or epiploic appendices
Describe the structure of the marginal artery
- Formed from anastomoses of branches of the inferior mesenteric and superior mesenteric arteries
- Gives rise to long, straight arterial branches - vasa recta
How is sodium absorbed in the intestines?
- Co-transported with glucose and amino acids
- Antiported - sodium/hydrogen exchange
- Partially absorbed with Cl- - dragged by negative charge
Where does the small intestine join the colon?
At the ileocaecal valve
Describe the structure of the internal anal sphincter
- Upper 2/3 of the anal canal
- Involuntary smooth muscle
What causes motility disorders of the colon?
Diabetes mellitus, post-surgical
Describe the potassium content of the body
Plasma - 3.5-5.0 mmol/L
Total body - 3600 mmol
Daily intake - 30-100 mmol
What is pseuodomembraneous collitis? How is it treated?
- Caused by various infective agents e.g. C. diff
- Results in distention of colon which can progress to toxic megacolon if untreated
- Treated with vancomycin
How can rehydration be acheived?
- Oral rehydration solutions
- IV solutions - colloids or crystalloids
Describe the fluid replacement regimes
- 500 ml - 2 hourly (emergency)
- 500ml - 4 hourly
- 500ml - 6 hourly (standard)
- 500ml - 8 hourly (slow rehydration)
Describe the arterial supply of the colon
- Ascending
- Ileocolic and right colic arteries (branches of superior mesenteric artery)
- Ileocolic branches from anterior and posterior caecal branches
- Transverse
- Right colic and middle colic arteries (branches of superior mesenteric artery)
- Left colic artery (branch of inferior mesenteric artery)
- Descending
- Left colic artery (branch of inferior mesenteric artery)
- Sigmoid
- Sigmnoid arteries (branch of inferior mesenteric artery)
Marginal artery (of Drummond) - supplies colon collaterally, in case of occlusion/stenosis
Define diarrhoea
3+ loose, watery stools per day
Describe the daily intake and excretion of water in the body
- Intake = 2,000mL
- Excretion =
- Urine - 1200mL
- GI tract - 100mL
- Insensible loss (breathing and sweating) = 700-800mL
- Turnover in the GI tract = 9000mL
How is bicarbonate secreted/absorbed in the intestines?
- Absorbed in the duodenum and jejunum - makes up for secretion in pancreatic juices/bile
- Sodium ions absorbed in exchange for H+
- H+ + HCO3- combines to form carbonic acid (H2CO3) which dissociates to form H2O and CO2
- H2O absorbed/stays in gut, CO2 absorbed into blood and expired
- Secreted in the ileum and large intestines
- Antiporter - bicarbonate secreted, chloride absorbed
What are the uses of Vancomycin?
- Used as alternative in penicillin allergy
- Used to treat C. Diff infection (oral)
How long does it take for the contents of the colon to reach the rectum?
8-15 hours
What is the histological difference between the colon and the rectum?
Shorter crypts of Leiberkuhn
Define secretory diarrhoea
Endotoxins stimulate secretion, usually of Cl- ions via the CFTR transporter
Describe the histological layers of the large intestines
- Mucosa - simple columnar epithelium, lamina propria with lacteals and network of capillaries + lymphatic nodules, muscularis mucosae
- Submucosa
- Muscularis propria - circular and longitudinal smooth muscle, arranged as 3 taenia coli
- Serosa - visceral peritoneum
List the flexures of the colon
- R colic (hepatic) flexure
- L colic (splenic) flexure
- Sigmoid flexure
How is water absorbed in the intestines?
- Through tight junctions between apical cells (paracellular)
- Through cells (transcellular)
How is Hirschpring’s disease treated?
Surgical resection of affected part
Describe the innervation of the appendix and caecum
- Sympathetic - ileocolic branch of the superior mesenteric plexus
- Parasympathetic - vagus nerve
Give examples of organisms which cause penetrating enteric infections
Salmonella typhi
What GI risks are involved in antibiotic use
- Particularly broad spectrum antibiotics - inhibit growth and metabolism of normal colonic flora
- Increased risk of diarrhoea/infection e.g. C Diff.
Describe the epithelium of the anal canal
- Upper 2/3 - simple columnar
- Lower 1/3 - stratified squamous
- Above pectinate - non-keratinised
- Below pectinate - keratinised
Describe the location of the anal canal
- In anal triangle of perineum between the left and right ischioanal fossae
- 4cm long, terminates at anus
How are short chain fatty acids utilised clinically?
SCFA enema - treatment of ulcerative collitis
What is the function/structure of the CFTR transporter?
- G protein coupled receptor, cAMP/PKA activate it by phosphorylation
- Moves Cl- out of cells into the intestinal lumen
- Na+ and therefore water follow
Describe the types of enteric infections
- Non-inflammatory - proximal small bowel, watery diarrhoea, no leukocytes, mild/no lactoferrin
- Inflammatory - colon, dysentery, faecal leukocytes, high lactoferrin
- Penetrating - distal small bowel, enteric fever, faecal leukocytes
Describe the inferior portion of the rectum
Ampulla - temporary storage of faeces, passes through the pelvic floor, continuous with anal canal
How can colonic transit be measured?
- Senal X-Ray with radio-opaque markers
- No marker seen - normal
- Marker seen in rectosigmoid area - pelvic outlet obstruction
- Marker scattered throughout colon - slow transit constipation
Describe the venous drainage of the anal canal
- Above the pectinate line - superior rectal vein, drains to the inferior mesenteric vein
- Below the pectinate line - inferior rectal vein, drains to the internal pudendal vein
Describe the location of the rectum
- Begins at rectosigmoid junction - at level of S3
- Retroperitoneal
- 15cm long
Describe the structure and function of the vermiform appendix
- Highly variable position
- Lots of lymphoid tissue
- No vital function
List the types of diarrhoea
- Osmotic
- Secretory
- Inflammatory
List causes of inflammatory diarrhoea
- Inflammatory bowel disease - Crohn’s, ulcerative collitis
- Infectious disease - shigella, salmonella
- Irritable colon
What causes anorectal incontinence?
- Excessive rectal distention - acute/chronic diarrhoeal disease, chronic constipation
- Anal sphincter weakness - damage to muscle/pudendal nerve
Describe the arterial supply of the appendix
Appendicular artery from the ileocolic artery
Travels in the mesoappendix - field of mesentery
Define osmotic diarrhoea
- Due to fluid accummulation in the GI tract
- Fluid accummulates - rapid propulsion - reduced absorption of fluid - more fluid accummulation
- Due to non-absorbable solutes/failure to digest or absorb nutrients
What are the paracolic gutters?
- Lateral to the ascending and descending colon, between the colon and posterior abdominal wall
- Depression for materials to pass through
Describe the venous drainage of the colon
- Ascending - ileocolic and right colic veins, drain to SMV
- Transverse - middle colic vein, drains to SMV
- Descending - left colic vein, drains to IMV
- Sigmoid - sigmoid veins, drain to IMV
Describe the venous drainage of the rectum
- Superior, middle and inferior rectal veins
- Superior drains to the IMV
- Middle drains to the internal iliac vein
- Inferior drains to the internal pudendal vein
How does the structure of the anal canal allow for maintenace of continence?
Internal and external anal sphincters collapse to prevent passage of faeces, except during defecation
Describe the anatomical location of the caecum/appendix
R iliac fossa, intraperitoneal
Describe the innervation of the anal canal
- Above the pectinate line - inferior hypogastric plexus (detects stretch)
- Below the pectinate line - inferior anal nerves (branches of the pudendal nerve)
- Detects pain, temperature, touch and pressure
How does spinal cord injury affect bowel function?
- T12 or above - bowel opens spontaneously but without control (reflex arc intact)
- Sacral nerve roots - flaccid bowel, no reflex arc, incontinence
What must be taken into consideration when rehydrating?
- Need to correct potential electrolyte shifts
- Calculate rehydration rate/volume of fluids using daily need, anticipated loss and previous deficits
- Check electrolye shifts, blood gases (acidosis due to bicarbonate loss), glucose, albumin, urea and creatinine, plasma osmolality
- Speed of fluid replacement dependent on age, renal function and cardiovascular status
How are SCFA absorbed?
Antiporter - HCO3- secreted
What is the significance of E. Coli 0157?
- Damages vascular endothelium - renal failure, haemolytic uraemia syndrome
- Antibiotics contraindicated - results in release more toxins
- Causes cytotoxic bloody diarrhoea
Describe the maximum safe K+ supplementation
- Maximum concentration - 40 mmol/L
- Maximum rate - 10 mmol/L
Describe the volume of fluid secreted and absorbed by the GI tract
- Secretions
- Salvia = 1.5L
- Gastric juice = 2.5L
- Bile = 0.5L
- Pancreatic juice = 1.5mL
- Instestinal secretions = 1L
- Absorbed
- Proximal colon = 150-2000mL
Describe the distinct histological features of the colon
- Crypts of Leiberkuhn - straight tubular glands down to the muscularis mucosae
- Goblet cells - mucous for lubrication
- Absorptive enterocytes - extract water and electrolytes
Why are the colonic flora beneficial?
Convert 60g of carbohydrates (fibre, starch, oligosaccharides) per day to short chain fatty acids (acetic, proprionic, butyric acids) which are rapidly absorbed
What is the prodominant type of micoflora which colonises the large intestine?
Mostly strict anaerobes e.g. Clostridium spp.
Describe the process of defecation
- Faeces moves into rectum - desire for defecation
- Contraction of rectum, relaxation of anal sphincter
- Involuntary defecation prevented by internal and external anal sphincters (external always constricted unless conscious signals inhibit)
List the functions of the large intestine
- Absorb water and electrolyes - form solid faeces (first 1/2)
- Storage of faecal matter until expelled (second 1/2)
List the drugs which increase colonic motility
- Stimulant laxatives (short action, big effect)
- Prucalopride (serotonin receptor agonist - for treating chronic constipation)
- Linaclotide (guanylase C receptor agonist, increases Cl- and HCO3- secretion, increased fluid secretion, speeds transit) - IBS treatment
Describe the location of the sigmoid colon
- L lower quadrant
- From L iliac fossa to the level of S3
- S shape
- Attached to posterior pelvic wall by sigmoid mesocolon (mesentery)
List the signs of dehydration
Dry mucosa, reduced skin turgor, purple fingernails
How are oral rehydration solutions designed to give the most efficient rehydration?
- Contain glucose - needed to absorb sodium effectively
- Reduced osmolarity - better absorbed
List the causes of secretory diarrhoea
- Rotavirus
- E. Coli
- Shigella
- Campylobacter
- Salmonella
- Vibrio cholera
- Cryptosporidium protozoa
- Hormone-like peptides - carcinoid/Zollinger-Ellison’s syndrome
- Drugs e.g. antidepressants
- Metals, organic toxins + plant products
Describe the arterial supply of the rectum
- Superior rectal artery (terminal branch of the inferior mesenteric artery)
- Middle rectal artery (branch of internal iliac artery)
- Inferior rectal artery (branch of internal pudendal artery)
Describe the arterial supply of the caecum
- Ileocolic artery - branch of the superior mesenteric artery
- Splits into anterior and posterior caecal arteries
List the distinctive characteristics of the colon
- Epiploic appendices - pouches filled with fat surface
- Taenia coli = 3 strips of longitudinal muscle - mesocolic, free + omental coli, end at the rectosigmoid junction
- Haustra - where taenia coli contract, gives sacculations
- Wider diameter than small intestine
How do endotoxins cause activation of CFTR? What effect does this have?
- Endotoxins activate cAMP/PKA, CFTR is phosphorylated and therefore activated
- Causes increased movement of Cl- and therefore water secretion into the intestinal lumen - secretory diarrhoea
Describe the IV rehydration solutions with extreme osmolalities
- Sodium bicarbonate (8.4%) - hyperosmotic, used only in extreme resuscitation
- Saline (0.45%) - hypoosmotic, causes oedema
