Defecation, constipation Flashcards
List the functions of faeces
- Expulsion of waste products of digestion
- Home to the microbiome
- Source of essential vitamins
- Train and maintain immune function
- Supply essential short-chain fatty acids
List the pathogens of faeces
- Bacteria: Vibrio cholerae (cholera), Clostridium difficile (pseudomembranous colitis), Salmonella typhii (typhoid fever), Shigella, Escherichia coli, Campylobacter (gastroenteritis)
- Viruses: HAV (hepatitis), HEV (hepatitis), Poliovirus (poliomyelitis), Enteroviruses, Norovirus, Rotavirus, Adenovirus (gastroenteritis)
- Protozoans: Entameba histolytica (amoebic dysentery), Giardia (giardiasis), Cryptosporidium (cryptosporidiosis), Toxoplasma gondii (toxoplasmosis)
- Helminths: Tapeworms, helminths (wide range of diseases)
Describe the composition of faceces
- Approximately 3/4 is water
- Of the remainder, ~1/3 is food residue (8%); the rest is produced “locally” (bacteria, epithelia)
- Quantity and quality dependent on various factors like food intake, water content, and drugs
Describe the role of the colon
- Faecal production: Bacterial flora produce vitamin K and B12 and produce gas (flatus)
- Re-absorption of water (500 - 1500 ml/day) and electrolytes
- Not essential for life (colectomy)
- Creation of an ileostomy:
- Normal output is 1500-1800 ml/d
- Adapts to 500-800 ml/d over months
- “High output” if >1500 ml/d after time
- Creation of an ileostomy:
Describe the architecture, anatomy, musculature of colon
Colon Architecture
- Crypts of Leiberkuhn: Site of the intestinal stem cells
- Lumen: Where the bacteria have a niche
- Thick and thin mucous layers house different bacteria
### Anatomy of the Colon
- Adult human colon: 130 cm long from TI to RS junction
- Caecum diameter: 6–8 cm
- Sigmoid diameter: 25 mm in the sigmoid colon
- Mesocolon: Transverse colon, sigmoid colon (most mobile parts)
- Retroperitoneal parts: Ascending, descending, and upper 1/3 rectum (least mobile parts of the colon)
- Blood supply is segmental
Musculature
- Taenia: 3 longitudinal bands along the colon. Contract to form haustra
- Circular muscle: Contracts as a group in large sections to push chyme along
Describe the ENS
-
Myenteric plexus: A mix of ANS (both sympathetic and parasympathetic ganglia) and neurones from the CNS
- Sits between the long and circular muscle layers and is responsible for motility
- Uses all the same neurotransmitters as the CNS including 5-hydroxytryptamine, dopamine, γ-aminobutyric acid, histamine, adrenalin, nor-adrenalin, several opiate receptors, acetylcholine; the “Brain of the gut”
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Submucous plexus: Has only parasympathetic neurones
- Sensory for the mucosa and innervate the mucosa and the circular muscle
- Enteric Ns stimulated by stretch,local chemical stimulation, or neuronal stimulation i.e. nerves and reflexes
- Hirshsprung’s disease: No ganglia; bowel obstruction
Describe the innervation of the colon
- Proximal colon: Innervated by the thoracic and lumbar spinal cord via the (lumbar) splanchnic nerve
- Distal colon: Dual spinal innervation via both the (lumbar) splanchnic nerve and (sacral) pelvic nerves
- Rectum: Innervation from the lumbosacral spinal cord, via the sacral pelvic nerves
Broadly describe colonic motility
- Propulsion of Chyme: Towards sigmoid, ultimately to anus
-
Three Types of Motility:
- Haustral: Small mixing movements locally
- Peristaltic movements: In both directions, especially retrograde RPC; net movement from caecum to anus
- Mass movements, usually while sleeping, to sigmoid colon and rectum GMC
- Parasympathetic innervation sent through vagus nerve branches to proximal colon; sympathetic fibres from splanchnic nerves
- Initiated by food intake:
- Gastrin (hormone)
- Gastrocolic reflex (ANS) - stomach distention
- Duodenocolic reflex (ANS) - food into duodenum
- also local stretch, and irritants
Describe RPC, GMC, and spontaneous GMC
Rhythmic Phase Contractions
- Characteristics:
- 2 to 4 cycles/min; low amplitude
- Short or long
- Rhythmic, or arrhythmic
- “Bursts” of non-propagated pressure activity
- Functions:
- Mixing and contact with mucosa for water absorption
- Moving bacterial populations to prevent colonization of specific regions
Giant Motile Contractions
- Stimulation: Food intake
- Mass movements start in the transverse colon
- High amplitudes
- In adults, 5–6 times a day
- Contraction lasts a few minutes
- Sigmoid fills within a few minutes, leading to an urge to defecate
Spontaneous GMCs
- Associated with Urges, and Defaecation:
- First three GMCs start near the splenic flexure and terminate in the sigmoid colon
- Second and third GMCs cause urges to defecate
- Fourth GMC propagates to the end of the colon, and causes defaecation
Describe the hierarchy of control ssytems
- Most control systems are involuntary
- Several control systems amplify and synchronize responses to initiate voiding
- Gut can function without voluntary CNS input
- Parasympathetic fibres are most important in the resting state, while sympathetic fibres stimulate motility; balance of these ANS effects
- somatic nerves provide local control over sphincters and muscles, but also brain and thinking, inspiring us to go ^[Valsalva is voluntary]
- Comatose Patients: Retain gut function
Describe the anatomy of the rectum and anus
-
Rectum:
- Commences at the mesocolic–mesorectal transition: ‘sigmoid take off’
- 3 transverse folds “valves of Houston” or “Kohlrausch folds”
- Horizontally placed mucous membranes to support weight of faecal matter
- High compliance (distensibility) rectal ‘reservoir’
- very muscularised to expel or to
-
Anal Canal:
- Dentate line to anal verge
- Longer in men (3.6 cm) than in women (2.9 cm)
- ## highly vascularised and innervated### Rectum and Anus
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Puborectalis: Forms anorectal angle at 65 – 108º
- Adds up to 10–20% of anal resting tone
-
Internal Anal Sphincter: Continuation of the circular muscle layer of the rectum
- Forms a smooth muscle ring
- Accounts for 85% of resting pressure
-
External Anal Sphincter: Composed of skeletal muscle under spinal and cortical control
- Responsible for voluntary control of continence
Describe the anatomical elements of continence
Anatomical Elements of Continence
- First Line of Defense: Controlled by Enteric NS
- Circular smooth muscle
- Second Line of Defense: Controlled by ANS
- Internal anal sphincter (smooth muscle): 70 - 85% of sphincter pressure. Always contracted, unless not. Positively innervated, to maintain continence
- Venous spongy body (haemorrhoids): 10 - 20% of sphincter pressure
- Third Line of Defense: Controlled by Central NS (spinal reflexes)
- External anal sphincter (striated muscle)
- Puborectal muscles (levator ani; striated muscle)
Anatomical Elements of Continence - pelvic floor
- At Rest:Contracted puborectalis forms anorectal angle at 65–108º
- contributes 20% of resting tone, of muscles of pelvic floor
- resting tone of muscles = pelvic function - essential for bowel and bladder continence
- The pelvic floor muscles (levator ani: pubococcygeus, iliococcygeus, and puborectalis) are primary supports, along with the pubourethral ligament, uterosacral ligament, and cardinal ligament
- striated, therefore trainable
- Lower rectus abdominus muscles and adductor muscles provide additional support
Anatomical Elements of Continence
- Internal Anal Sphincter:
- Forms a smooth muscle ring in a spiral
- Not under voluntary control
- Contraction: Shortening and narrowing
- Relaxation: Lengthening
- Resting tone: Neural or myogenic, contributing 85% of resting pressure
- Phasic contractions generate tone
- slow twitch, fatigue resistant smooth muscle
- unaffected by respiration or anaesthesia
- S2-4 supply
- External Anal Sphincter: Composed of skeletal muscle under spinal and cortical control
- Small contribution to anal canal resting pressure
- Responsible for generating maximal squeeze pressure
- Voluntary control of continence
- Supported by transverse perineal and bulbospongiosus muscles
- pudendal nerve supply
Describe the physiology of anal continence
- ENS: Constricts internal sphincter and relaxes sigmoid
-
Anal Sphincters:
- Both internal and external sphincters are tonically active to keep the anus closed
Describe the phases of defecation
-
Phases:
- Continence - basal phase
- Rectal filling - pre-expulsive
- Urge - pre-propulsive
- Deferral, a conscious decision
- Void
- Return to continence
Initiation of Defaecation: Filling
- Stretch stimulates sensory nerves to spinal cord, activating parasympathetic fibres, releasing ACh, leading to contraction of rectum and sigmoid
- Receptors and Reflexes:
- Touch and/or pressure receptors signal via pelvic plexus a feeling of urge at about 20 ml threshold
- Elicits either recto-anal inhibitory reflex (ENS & ANS: “GO”) or recto-anal contractile reflex (CNS: “STOP”)
- Fill rectum
- mass movements i.e. gastrocolic reflex, or small movements in sigmoid i.e. distention or enema, or emptying of sigmoid leading to increase in abdominal pressure
- Rectum is compliant, accommodating increases in volume with little change in pressure (adaptive relaxation)
The Feeling to Go
- Sampling Reflex: Intermittent, transient relaxation of the internal anal sphincter allows descent of contents into the upper anal canal (occurs 7 times/hour)
- Specialized cells in the upper anus sample rectal contents (“chemoreceptors”) - can distinguish between gas, liquid, solid
- Leads to a drop in upper anal canal pressure, with rectal pressure > mid anal pressure
Ano-Rectal Angle
- Positioning:
- “Assuming the position” (squatting) releases the ano-rectal angle - almost vertical, easier flow through
- Commode position is superior to lying down flat
- Elevation of the knees above the hips releases the anorectal angle, allowing stool to pass lower into the rectum
Maintenance of Emptying: Yes, GO!
- Mechanisms:
- Concentric contraction of sigmoid (ENS) and relaxation of internal sphincter (ENS and ANS)
- Behavioral response:
- Relaxation of external sphincter (CNS), amplification/synchronization of response
- Straining: Increased abdominal pressure, closure of glottis, Valsalva manoeuvre
- Increased rectal pressure: Relaxation of the anal sphincter and increased abdominal pressure, resulting in lower anal pressure than rectal pressure
- Timed with a wave of GMC: Segmental evacuation (Right colon 20%, Left colon 32%, Rectum 66%)
Closure Reflex
- Process:
- Begins under semi-voluntary control (sense of complete rectal emptying, stopping pushing)
- Removes inhibition to internal sphincter: Contraction of internal sphincter
- Followed by involuntary contraction of the external anal sphincter and pelvic floor, closing the anal canal and reversing the pressure gradient towards the rectum
- contraction of the puborectalis, returning the angle to its basal state
- Smooth muscles in sigmoid relax (ENS, re-establishing reservoir function)
- Cortically modulated, impaired in patients with spinal injury
Describe what we dont understand about defecation
- GMC and further propulsion cause the urge to defecate in 62% of people; this association is often lost with constipation ^[increasing constipation suspends feeling to go]
- Balloon distension of the rectum is associated with pain, not the urge to defecate
- Low anterior resections with ano-colonic anastomosis (excision of the rectum and sigmoid colon, e.g., for cancer) preserve the urge to defecate
- BUT…Low spinal anaesthesia (L5–S1) abolishes rectal sensation and causes loss of continence
