Buckle up! This 🧵 is a deep dive into the #physiology of defecation, the mechanics of pooping! 🧰 🚽 💩
nature.com/articles/s4157…
#medicine #MedEd #science #GITwitter #GI #gut #motility #constipation #IBS
Defecation requires
1. intact #GI tract
2. coordination of many systems: neural (enteric nervous system, modulated by the peripheral somatic, autonomic and the CNS); muscular (smooth and striated); hormonal (endocrine and paracrine); and cognitive (behavioural and psychosocial)
#Constipation is the3⃣rd most common #GI symptom
💰 Annual cost of fecal incontinence and constipation: $1,594 - $7,522
The #anatomy of defecation
Anatomy of defecation
Anatomy of defecation
Fun #anatomy facts
🦴the adult colon is ~130 cm
🦴the lumen of the anal canal is shaped like an ⏳
🦴 the anal canal is typically longer in men than women
🦴the internal anal sphincter is not under voluntary control, while the external anal sphincter is
⚠️Ehlers-Danlos syndrome and other connective tissue disorders can cause laxity of pelvic floor ligaments▶️ descending perineum syndrome (presents with #constipation). Multiple vaginal deliveries and gynecologic surgery also ⬆️ risk.
Image: link.springer.com/article/10.100…
The 4⃣phases of defecation
1⃣BASAL
💩the non-pooping phase
💩colon maintains homeostasis
💩rectum is empty
💩mean colon transit time is 24h (range 4-50), which is 70-80% of total gut transit ⏰
💩digesta moves ~1cm/h in a to&fro motion
💩during meal, pancolonic pressurization&relaxation of anal sphincter▶️sampling
💩after meal, pooling of gas in distal colon▶️flatal urge & expulsion of 💨
💩cyclic motor pattern (active during 😴) is the rectosigmoid brake, inhibiting transit (aka colonic gatekeeper)
What happens when you suppress a poop? The 💩 gets sucked back in!
2⃣PRE-EXPULSIVE
💩propagating&non-propagating activity begins to⬆️up to 1h prior to defecation
💩subconsious! (no urge to 💩)
💩coordinated motor patterns propel💩toward the rectum
💩Distension of the rectum beyond a threshold▶️ rectoanal inhibitory reflex (RAIR)▶️reflex relaxation of the IAS & contraction of the EAS
⚠️RAIR is absent in Hirschsprung disease
Image: link.springer.com/chapter/10.100…
How do we know if it's poop or a fart?
💩transient IAS relaxations occur ~7⃣x/h, ~40% may be consciously perceived
💩during these relaxations, intraluminal pressures w/in the proximal anal canal equalize w rectal pressures▶️sampling of luminal content by anal mucosa: 💨 or 💩?
💩rectal distension tells the 🧠 that it's ⏰ to poop!
💩🧠 involved: prefrontal cortex, anterior cingulate gyrus, insula, thalamus, somatosensory cortex
💩brain▶️spinal cord ▶️pudendal nerves▶️anal canal: ✅poop or 🚫poop
3⃣EXPULSIVE
💩⬆️ antegrade propagating contractions, each originating more proximally, and now associated with the urge to defecate
💩cyclic motor pattern is inhibited to allow for passage of stool
💩rectoanal pressure gradient is reversed via voluntary&involuntary processes: gradient exceeds frictional resistance of anal canal▶️deform solid feces to enable transit through anal canal
⚠️Inability to reverse gradient=dyssynergic defecation
Image: cghjournal.org/article/S1542-…
💩⬇️of anal pressure occurs via⬇️acuity of anorectal angle from 65–108° to 110–155° (⚠️squatting or a squatty potty helps facilitate this!)
💩⬆️rectal pressure via Valsalva maneuver
💩the anorectal luminal diameter also increases during defecation
💩during evacuation, rectosigmoid and total colonic volume⬇️by 44% and 19%
💩⬇️colonic gas volume
💩3⃣patterns of evacuation: a single, rapid expulsive motion (type 1); frequent, pulsatile expulsion of small volumes (type 2); and slow, sustained, steady expulsion (type 3)
4⃣END PHASE
💩closing reflex: contraction of anal sphincter and pelvic floor; relaxation of the conjoint longitudinal muscle of the anal canal to enable distension of the anal endovascular cushions; contraction of puborectalis to restore the anorectal angle; and perineal ascent
Normal pooping frequency? Between 3/d - 3/wk.
💩 💩 💩
Factors influencing defecation:
🧠stress, anxiety, abuse, stool withholding
🚽posture (squatty potty!)
💩consistency and volume
⏰transit time (slower time, harder 💩)
🦠microbiome (eg ⬆️methanogens)
😴circadian rhythm (motility inhibited during 😴 and stimulated upon waking)
🍲gastrocolic reflex
🥝dietary fiber (every 1 g increase in wheat fiber⬆️💩 volume by 3.7±0.09g/d)
🧓age⬇️motility
👩constipation more common in F>M
🤰parity▶️pelvic floor damage
💊opioids and other meds
🚴exercise
Disorders of defecation by phase
Closing remarks:
Other fun facts from the article!
💩 poop is predominantly water (median💧content 75%) plus a suspension of bacterial biomass, protein, carbohydrates, & lipids
💩median fecal wet mass of 128 g per day
🥇Best line in the article: "Propulsive HAPCs can be associated with morning waking and also with the morning call to defaecate"
"call to defecate" is my new favorite phrase
That's all, folks! I hope you enjoyed this deep dive into the mechanics of defecation!
#GITwitter #motility
@MTPapaD @LinChangMD @LindaNguyenMD @NGMJournal @NatRevGastroHep @KewinSiah @drkeithsiau @IBS_Maastricht @WalterChanMD @JeffConklinMD @AllonKahn @JohannahRuddy @IBSpatient @GiJournal @SultanMahmoodMD @IrisWangMD @NeenaSAbrahamMD @KristleLynchMD @JCPetrov @loulev_md
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