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What are the 5 steps of the defaecation reflex?

Distension of rectal wall causes: Contraction of rectum
Relaxation of internal S + initial contraction of external S
Increased peristalsis in sigmoid colon
Relaxation of external S
Faecal expulsion


Voluntary control of defaecation requires...

Reverse peristalsis of faces back into sigmoid colon
Valsalva manoeuvre to push faeces back into rectum to initiate defaecation reflex once more


Anal canal is guarded by 2 sphincters. What are they and which muscle type are they controlled by?

Internal anal sphincter - smooth muscle
External anal sphincter - skeletal muscle


At what positions are haemorrhoids classically found?

3, 7 and 11 o'clock on the anus


What are haemorrhoids?

Dilation of internal venous plexus, can prolapse


At what point do pelvic organs change their pattern of pain referral from suprapubic to perineal?

Pelvic pain line


Which veins form portosystemic anastamoses in rectum?

Superior rectal vein - portal
Middle and inferior rectal veins - systemic


Describe the blood supply to the rectum

IMA and branches of the internal iliac artery


Hoe many transverse rectal folds are there?

2 on left, 1 on right


Describe rectal pain referral

Above pelvic pain line, refers to L1/2, pubic region, covered in peritoneum
Below pelvic pain line, no peritoneum, refers to S2/4, perineal region


What 3 nerves are involved in defecation?

Sympathetic - contracts internal sphincter, stops poo
Pudendal (somatic) S2/4 - tonic contraction of external sphincter, stops poo
Parasympathetic (pelvic splanchnic nerves, S2/4) - relaxes internal sphincter, contraction of rectal wall, poo


What triggers the defaecation reflex?

Reflex response to distension of the rectal wall
Mediated by mechanoreceptors


Describe voluntary control of defaecation

Brain centres can override the reflex signals and so keep the external sphincter closed
Reverse peristalsis can pull contents of rectum back into the colon More water can be absorbed. Progressively harder stools
Voluntary defaecation involves the Valsalva manoeuvre


Describe the valsava manoeuvre

Full inspiration followed by forced expiration against a closed glottis causes the diaphragm to move downwards
Abdominal and thoracic muscles are contracted
Increased pressure in the abdomen forces faecal contents into the rectum
The defaecation reflex is initiated


What is the composition of faeces?

3/4 water
1/4 solid matter: dead bacteria, fat, inorganic matter, protein, undigested material, including bile pigment and sloughed epithelial cells


What are signs and symptoms of constipation?

Types 1 and 2 on the Bristol Stool Chart indicate constipation
Stool is hard
Stool is difficult and painful to pass
Infrequent urge to void
Haemorrhoids and anal fissures
In later stages, abdomen may become distended and diffusely tender and with cramps, occasionally with enhanced bowel sounds


What may constipation, anaemia and weight loss combined be a sign of?

Colon cancer


What could alternating constipation and diarrhoea in an otherwise healthy person be?



What are bulk laxatives?

Polysaccharide polymers that not broken down by the normal process of digestion
Retain water in the GI lumen, softening and increasing faecal bulk and promote increased motility
E.g. methylcellulose (“Citrucel”), plant gums such as bran, agar, guar or ispaghula husk (“Fybogel”)
Act: 1-3 days, Usage: Good first choice in constipation and IBS
Side effects: None, but some may decrease absorption


What are osmotic laxatives?

Poorly absorbed solutes: saline purgatives and lactulose
By osmosis, maintain an increased volume of fluid in GI tract. This accelerates small intestine transit and results in an abnormally large volume of fluid entering colon, Distension leads to purgation
Act: Potent, rapid action (1-2 hrs), watery evacuation, Usage: Bowel prep. prior to procedure, not suitable for routine alleviation of constipation, Side effects: Dehydration and electrolyte depletion
Should be avoided in small children and poor renal function


Describe how faecal softeners work

Surface-acting compounds that act similar to detergents and produce softer faeces. Act: slowly, i.e. 3-5 days to keep stools soft. Usage: constipation & fissures/piles. Side effects: None. Weak stimulant laxative


What is Bisacodyl?

Stimulant purgative
Usually given as a suppository
Stimulates rectal mucosa, resulting in mass movements and defaecation in 15-30 minutes
Usage: only short courses should be used, useful with opioids
Side effects: Danger of excessive laxative effect


What is Senna?

Stimulant purgative. Contains derivatives of anthracene combined with sugars to form glycoside
Passes unchanged into colon, where bacterial action releases free anthracene derivatives which are absorbed and have a direct on myenteric plexus
Act: 8hrs, taken at night, Usage: Very common, Side effects: Can appear in breast milk, griping abdominal pain, potential for abuse (anorexics)