Large Intestine

Question 0

What are some of the factors of the colon which discourage infection?

Answer 0

Mucus layer

Anaerobic environment

Large amount of water

Question 1

What are some of the causes of appendicitis? What is the pattern of pain in classic appendicitis? What can mimic appendicitis?

Answer 1

Obstruction of appendix e.g. faecolith, lymphoid hyperplasia, bezoar

note: chickenpox inflaming lymph nodes in appendix can cause purulent appendicitis

Starts as vague peri-umbilical pain (inflammation of visceral peritoneum) and shifts to right iliac fossa (inflammation of parietal peritoneum)

Mesenteric adenitis can cause right iliac fossa pain which can be mistaken for appendicitis

Question 2

How is water absorbed in the large intestine?

Answer 2

Na+ diffuses into cell and actively pumped into the blood

Cl- follows and generates an osmotic gradient

Water follows into blood

Question 3

How is calcium absorbed in the large intestine?

Answer 3

Diffuses into cell and actively pumped into blood

Vitamin D required for both processes

PTH stimulates both processes

Question 4

How is iron absorbed in the large intestine?

Answer 4

Fe2+ binds to transferrin (secreted by mucosal cells) in the lumen

Endocytosed

Fe2+ released and exported into blood

Re-binds to transferrin in the blood

note: gastric acid required to solubilise iron complexes & gastroferrin secreted by stomach also helps solubilise (prevents resequesteration in the intestines when the pH increases)

Question 5

How are water-soluble vitamins absorbed in the large intestine?

Answer 5

Vitamin C & most vitamin Bs are absorbed passively

Vitamin B12 (involved in RBC production) is absorbed with the co-factor intrinsic factor (produced by stomach mucosa)

Vit. B12 absorbed in the terminal ileum ONLY

Therefore, stomach damage/terminal ileum removal —> B12 deficiency —> pernicious anaemia

Question 6

How does the large intestine mix intestinal contents?

Answer 6

HAUSTRAL SHUTTLING (not peristalsis!)

Contraction of smooth muscle gently moves contents back and forth

• slow absorption of remaining water & salts
• formation of faeces
• enters sigmoid colon

note: longitudinal muscles = taeniae coli

Question 7

How does the smooth muscle change between the small and large intestine?

Answer 7

Longitudinal muscle reduced to taenia coli in large intestine

Colon divided into segments called haustra

Question 8

What is mass movement?

Answer 8

Peristaltic propulsion (via longitudinal muscle) in the transverse & descending colon

Forces faeces into rectum, creating the urge to defecate (pressure receptors)

Internal anal sphincter (PS) relaxes during mass movement
External anal sphincter (voluntary striated - sacral reflex modified by higher centres) stays contracted until voluntarily relaxed

Infrequent (1-2/day)

Triggered by eating = gastro-colic reflex

Question 9

What is the difference between the internal and external anal sphincters?

Answer 9

INTERNAL EXTERNAL
- Smooth muscle - Voluntary striated muscle
- Parasympathetic - Stays contracted until voluntarily relaxed
- Relaxes during - Increased intra-abdominal pressure
mass movement expulses faeces
- Sacral reflex modified by higher centres
(overidden by +++rectal pressure)

Question 10

What are diverticula? Where do they develop?

Answer 10

Small mucosal herniations/outpouchings protruding through intestinal layers

Usually occur through small openings by nutrient vessels/vasa recta

Most frequently occurs in sigmoid colon (highest intra-luminal pressure)

Question 11

What is the difference in pathology above and below the pectinate line in the rectum?

Answer 11

ABOVE:
- columnar epithelium —> adenocarcinoma
(non-painful pathology)

BELOW:
- stratified squamous epithelium —> squamous cell carcinoma
(painful pathology)

Question 12

What are haemorrhoids? What symptoms can these cause?

Answer 12

Vascular structure in anal canal which help with stool control

note: only pathological when inflamed/swollen and causing symptoms

Internal haemorrhoids (above pectinate line):

• painful rectal bleeding during/following bowel movement (haematochezia)
• mucous discharge
• itchiness
• faecal incontinence

External haemorrhoids (below pectinate line):

• significant pain & swelling
• itchiness
• bright red bleeding
• prolapsed haemorrhoidal tissue

Question 13

What are intestinal crypts? What cells are present?

Answer 13

Tubular structure protruding from inner lining of the intestine containing stem cells which replace epithelial cells lost via anoikis (programmed cell death induced by detachment from extracellular matrix)

+ paneth cells = secrete anti-microbial compounds into the lumen in response to bacteria/bacterial antigens