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Colon: Digestion, absorption and fermentation - Coggle Diagram
Colon: Digestion, absorption and fermentation
Pathway of food waste from point of entry into large intestine through its exit from the body as faeces
Defecation:
- When faeces fills the sigmoid colon, faeces flow over in the rectum and creates distention
- Stretch receptors send signals to the spinal cord
- Signals are subsequently sent to the internal anal sphincter for it to relax (it is felt as urge to defecate)
- The presence of faeces in the anal canal sends a signal to the brain, which gives you the choice of voluntarily opening the external anal sphincter (defecating) or keeping it temporarily closed
Interactions between mucus layer, gut microbes and health
Absorption of electrolytes, water and vitamins:
- Ion channels, carriers and pumps, located either on the luminal or basolateral membrane, allowing highly efficient transport of large amounts of salt and water
- Colonic absorption of ~1.5L of electrolyte-rich fluid per day
- Net transport is the result of well-balanced absorption and secretion
- Ion transported is disturbed during infectious diseases causing secretory diarrhoea (can cause life threatening dehydration by excessive loss of salts and water)
Secretion of compounds in to the lumen:
- Secretory epithelial cells contain Cl- and K+ channels in their luminal membranes allowing for secretion of KCL
- Bicarbonate (HCO3-) secreted into the lumen determines pH on the surface of the colonic mucosa as it is alkaline. It possibly works as a buffer.
- Mucus produced by goblet cells are also secreted into the lumen
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Colonic Digestion: The Enteric Nervous System (largest component of autonomic nervous system)
- It is equipped with intrinsic microcircuits that enable it to orchestrate GI function independently of CNS input
- Neural and endocrine control of digestion is closely coordinated
- Colonic motility is dependent on the ENA for propulsion of contents
Sensing of mechanical forces:
- Distension in the gut - ingested and secreted material
- gas produced by microbial fermentation
- Mechanosensory cells - mechanical force gets traduced by several proteins in the mechanoreceptor
-Responses are amplified and signalling molecules are released
- Mechanosensory circuits built into the GI wall allow for spatial and temporal integration of mechanical stimuli into a coordinated physiological response
GI Motility in the large intestine: Haustral contractions:
- Type of segmentation
- Triggered by stretch
- Occurs primarily in ascending and transverse colon
- Increase mixing, contact to mucosal surface, increase absorption
Peristalsis
- Coordinated, involuntary propulsion of food
Mass movements (peristalsis)
- Occur 3-5 times per day (during or after meals)
- Triggered bys stretch, luminal irritants and gastrocolic reflex
- Propulsion to rectum
- Occur primarily in transverse and descending colon