RCSI Functions of the Colon 2024 PDF

RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Functions of the Colon Class Year 2 Course GIHEP Title Functions of the Colon Lecturer Simon Furney...

RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Functions of the Colon Class Year 2 Course GIHEP Title Functions of the Colon Lecturer Simon Furney Presented by Ebrahim Rajab Date 12 Septmber 2024 LEARNING OUTCOMES Describe the mechanisms involved in colonic motility, haustrations and mass movements and describe the defaecation reflex Describe the fluid volumes secreted and absorbed Explain the mechanism of electrolyte and water reabsorption in the large intestine Describe secretions of the colon and explain the mechanism of chloride secretion and the effects of cholera toxin Anatomy of the large intestine COLON MOTILITY SEGMENTATION – Contractions (called haustrations) are ~2.5 cm long and almost occlude the lumen – They occur at 2/min in the caecum and increase progressively to 6/min in the sigmoid colon – At adjacent sites, contractions usually occur independently – They slowly move contents back and forth, mixing and exposing them to the mucosa for absorption of water and electrolytes PERISTALSIS – Weak and slow MASS MOVEMENT – Propulsion forward usually occurs in a characteristic sequence of events called mass movement; during this movement, segmentation ceases with loss of haustrations – 1-3 times/day, typically after meals – Duration of approximately 15 min but highly variable – Triggered by gastrocolic and duodenocolic reflexes – Propels faeces into the rectum – Colon transit is slow (18-24 hours approximately) by comparison with the small intestine Fig. 6.3 Two mass movements. (A) Appearance of the colon before the entry of barium sulfate. (B) As the barium enters from the ileum, it is acted on by haustral contractions. (C) As more barium enters, a portion is swept into and through an area of the colon that has lost its haustral markings. (D) The barium is acted on by the returning haustral contractions. (E) A second mass movement propels the barium into and through areas of the transverse and descending colon. (F) Haustrations again return. This type of contraction accomplishes most of the  DEFAECATION REFLEX Consists of an intrinsic reflex and an extrinsic reflex INTRINSIC REFLEX Rectal distension causes peristalsis in the descending and sigmoid colon and in the rectum and relaxation of the internal anal sphincter Controlled by intramural (myenteric) plexuses Therefore, with lesions of the extrinsic nerves or spinal cord, defaecation is still possible  DEFAECATION REFLEX Extrinsic reflex Distension activates stretch receptors running in the sacral parasympathetics. This reflexly activates parasympt. efferents causing reflex peristalsis in the colon and rectum and relaxation of the internal anal sphincter Sympathetics do the opposite  DEFAECATION (CONTINUED) Conscious control is through the pudendal nerves to the external anal sphincter (striated muscle) During defaecation, there is voluntary relaxation of the sphincter and a Valsalva manoeuvre Rectal stretch receptors convey the sense of fullness to the brain Conscious control is absent in infants, cord and nerve injuries and intellectual disability LEARNING OUTCOME Describe secretions of the colon and explain the mechanism of chloride secretion and the effects of cholera toxin – Fluid Volumes – Chloride secretion by enterocytes in small intestine and colon – Cholera toxin causes salt and water secretion – Salt absorption in small intestine (ileum) – Sodium absorption & HCO3- secretion by the colon – K+ and HCO3- luminal concentrations  NEXT WE CONSIDER ABSORPTION OF H2O, NA+, CL-, HCO3- FLUID VOLUME (L/DAY) INGESTED 2 SALIVA 1 GASTRIC JUICE 3 BILE 1 PANCREATIC JUICE 1 INTESTINAL JUICE 2 TOTAL 10 ABSORBED (SMALL INT.) 8.5 ABSORBED (LARGE INT.) 1.4 EXCRETED IN FAECES 0.1  Chloride secretion by enterocytes in enterocyte small intestine and colon The crypt of Lieberkühn is a gland found in the epithelial lining of the small intestine and colon. The crypts and intestinal villi are covered by epithelium that contains two types of cells: goblet cells that secrete mucus and enterocytes that secrete water and electrolytes (intestinal fluid secretion; 1-2L/day – driven by Cl- secretion) In the crypts of Lieberkuhn, chloride is secreted by the cystic fibrosis transmembrane regulator (CFTR), into the gut lumen Sodium ions follow through channels between enterocytes into gut lumen  Cholera Toxin causes salt and water secretion Cholera toxin from the bacterium Vibrio cholerae activates the CFTR causing excessive NaCl and water secretion, producing as much as 20 L/day of watery stool.  SALT ABSORPTION IN SMALL INTESTINE (ILEUM) + H + HCO3- H2CO3 CO2+ SUGAR H2 O OR A.A Na+ CL- Gut Lumen Na+ H+ CL- HCO3- HCO3- H2CO3 Na+ K + CO2 + H2O Na+ enters the cells with amino acids and sugars There is a Na+/K+ pump in the basolateral membrane This is a major route for salt absorption and is unaffected by bacterial toxins HCO3- ABSORPTION IN SMALL INTESTINE (ILEUM) Start Gut Lumen here HCO3- is absorbed indirectly (as in the kidney). For each HCO3- destroyed in the lumen by secreted H+, a HCO3- formed in the cell enters the blood by facilitated diffusion at the basolateral membrane. This is a major way of absorbing the HCO3- secreted by the bile and pancreas Cl- is absorbed through tight junctions due to the positive charge of the intercellular space caused by the Na+/K+ pump H2O is absorbed through the junctions due to accumulation of HCO3-, Na+ and Cl- SODIUM ABSORPTION & HCO3- SECRETION BY THE COLON -30mV Na+ Na+ CL- Gut Lumen H+ HCO3- K+ Na+ H2CO3 K+ CO2 + H2O Similar exchanges to the small intestine but there are no amino acid or sugar carriers Na and Cl absorption, by Na+/H+ and HCO3-/Cl-exchangers, but these are inhibited by bacterial toxins Distal colon does not have the Na+/H+ exchanger so there is net HCO3- secretion  BACTERIAL TOXINS They stimulate salt and water secretion – cystic fibrosis transmembrane regulator (small intestine & colon) They inhibit salt and water absorption – Na+/H+ and HCO3-/Cl-exchangers (colon) This can cause diarrhoea They have no effect on absorption caused by sodium/sugar and sodium/amino acid co-transporters* (small intestine/ileum) Therefore, diarrhoea can be treated with oral solutions containing sodium and glucose  Gut Lumen Potential difference across wall of colon causes K+ secretion HCO3- is secreted up to a luminal concentration of 45 mMoles/Litre due to the HCO3- /Cl- exchanger The Na+/K+ pump creates a luminal negative charge of -30 mV which causes K+ secretion up to a luminal concentration of 25 mMoles/Litre CAN YOU EXPLAIN WHY DIARRHOEA CAUSES HYPOKALAEMIA AND METABOLIC ACIDOSIS ?  LEARNING OUTCOMES Describe the mechanisms involved in colonic motility, haustrations and mass movements and describe the defaecation reflex Describe the fluid volumes secreted and absorbed Explain the mechanism of electrolyte and water reabsorption in the large intestine Describe secretions of the colon and explain the mechanism of chloride secretion and the effects of cholera toxin