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4. the large intestine
- 1. THE LARGE INTESTINE PHYSIOLOGY:GASTROINTESTINAL
- 2. ABSORPTION IN THE LARGEINTESTINE
- 3. ABSORPTION IN THELARGE INTESTINE OVERVIEW ‣ The large intestine, or colon, has several roles including water absorption and immunity ‣ The chyme that enters the colon is already very concentrated as most of the water has already been absorbed ‣ The large intestine is specialized to work in the environment this produces ‣ Learning Goal ‣ To review the functions of the intestine and how they are achieved
- 4. ABSORPTION IN THELARGE INTESTINE SPECIALIZED FUNCTIONS ‣ The large intestine is lined by mucosa with crypts of Lieberkühn containing glands and mucus-producing goblet cells ‣ These protect the intestinal wall from the plethora of anaerobic bacteria in the colon and from the pressure exerted on the walls by the concentrated chyme (soon to become feces) ‣ The walls also contain gastrointestinal lymphoid tissue (GALT) that contributes to the body’s immune defences ‣ The colon helps to absorb a small volume of water from the lumen (400ml/day) ‣ As the chyme is very concentrated by the time it reaches here, the colon must work against a larger osmotic pressure gradient than in the rest of the GIT (it must move water against the gradient for osmosis) ‣ The colon also helps to transport ions
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- 6. ABSORPTION IN THELARGE INTESTINE WATER AND ION ABSORPTION ‣ In the large intestine, there is a net absorption of sodium and chloride ions and secretion of bicarbonate ‣ Sodium – this ion may be absorbed by various methods: ‣ Sodium-hydrogen antiporter on the luminal membrane ‣ Epithelial sodium channels ‣ Enhanced by absorption of short-chain fatty acids in the colon via specialized symporters ‣ Chloride - the movement of sodium into the plasma produces an electrochemical gradient to allow absorption of chloride ‣ Bicarbonate - chloride ions are exchanged for bicarbonate ions causing secretion
- 7. ABSORPTION IN THELARGE INTESTINE WATER AND ION ABSORPTION ‣ Water – the absorption of these electrolytes creates an osmotic gradient to allow further absorption of water ‣ Potassium – absorption of water along the length of the bowel concentrates potassium in the lumen ‣ This provides an electrochemical gradient for the movement of potassium into the plasma ‣ In the colon potassium may be absorbed or secreted depending on the remaining concentration in the lumen and the electrochemical gradient created by the active absorption of sodium ‣ Secretion usually occurs when the luminal concentration of potassium ions is below 25mM ‣ Vitamins and fats – short-chain fatty acids, crucial B vitamins (such as B6 and B12) and vitamin K (required for blood clotting) are produced by the digestion of chyme by the commensal microbial flora of the colon
- 8. ABSORPTION IN THELARGE INTESTINE REGULATION OF ABSORPTION ‣ Absorption in the gastrointestinal tract is regulated by neuroendocrine mechanisms ‣ In the colon endocrine mechanisms used include: ‣ Aldosterone – increases the net absorption of water and electrolytes by stimulating the basolateral sodium-potassium ATP-ase ‣ This increases the electrochemical gradient and driving force for sodium absorption ‣ It also increases transcription of epithelial sodium channels ‣ Glucocorticoids and somatostatin – act to increase water and electrolyte absorption by increasing the action of the basolateral sodium-potassium ATP-ase ‣ The intestines are innervated by the enteric nervous system whose: ‣ Parasympathetic innervation promotes net secretion from the intestines ‣ Sympathetic innervation promotes net absorption from the intestines
- 9. ABSORPTION IN THELARGE INTESTINE REVIEW QUESTIONS ‣ How is Vit K produced and what is it necessary for?
- 10. ABSORPTION IN THELARGE INTESTINE REVIEW QUESTIONS ‣ How is Vit K produced and what is it necessary for? ‣ Vitamins and fats – short-chain fatty acids, crucial B vitamins (such as B6 and B12) and vitamin K (required for blood clotting) are produced by the digestion of chyme by the commensal microbial flora of the colon
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- 12. MOTILITY OF LARGEINTESTINE OVERVIEW ‣ The large intestine is the final section of the gastrointestinal system before the rectum ‣ In this section of the GI tract water is reabsorbed and any remaining waste material is stored as feces to be removed ‣ Learning Goal ‣ To consider how waste material is moved through the large intestine and clinical conditions that are relevant to its function
- 13. MOTILITY OF LARGEINTESTINE HAUSTRAL SHUTTLING ‣ The large intestine is naturally separated into segments known as haustra ‣ Along the course of the walls are groups of cells called pacemaker cells ‣ These send signals to the smooth muscle cells on the walls of the large intestine causing them to contract at regular intervals ‣ The contraction causes the food to be churned in the intestine exposing the gut contents to a larger surface area of epithelium maximizing absorption ‣ Each group of cells control a certain number of haustra ‣ The pacemaker cells closer to the ileum emit signals slightly faster than those towards the end of the length of bowel ‣ This gradient allows a gentle progression of bowel contents towards the rectum
- 14. MOTILITY OF LARGEINTESTINE MASS MOVEMENT ‣ Whilst haustral shuttling occurs continuously mass movement only occurs once or twice per day ‣ This involves a sudden, uniform peristaltic contraction of smooth muscle of the gut which originates at the transverse colon and rapidly moves formed feces into the rectum, which is normally empty ‣ The result of this is feeling the urge to defecate ‣ The contraction may be stimulated by eating ‣ When this occurs it is called the gastro-colic reflex
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- 17. https://teachmephysiology.com/gastrointestinal-system/large-intestine/large-intestinal-motility/ FBC - fullblood count / U&E - urea and electrolytes / LFTs - liver function tests / CRP - C-reactive protein / ESR - erythrocyte sedimentation rate
- 18. LARGE INTESTINAL MOTILITY CLINICALSIDE NOTE: IBS VS IBD ‣ But despite having similar acronyms and symptoms, these two conditions are very different ‣ IBS is a disorder of the gastrointestinal (GI) tract ‣ IBD is inflammation or destruction of the bowel wall, which can lead to sores and narrowing of the intestines ‣ It's possible to have both IBD and IBS
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- 25. LARGE INTESTINAL MOTILITY REVIEWQUESTIONS ‣ What does the blood test fecal calprotectin help differentiate between? ‣ Ulcerative colitis and Crohn's ‣ Ulcerative colitis and IBS ‣ Crohn's and IBS ‣ IBD and IBS
- 26. LARGE INTESTINAL MOTILITY REVIEWQUESTIONS ‣ What does the blood test fecal calprotectin help differentiate between? ‣ Ulcerative colitis and Crohn's ‣ Ulcerative colitis and IBS ‣ Crohn's and IBS ‣ IBD and IBS ‣ Faecal calprotectin is a good indicator between IBD and IBS, but it cannot differentiate between different types of inflammation seen in Crohn's and UC
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- 28. DEFECATION OVERVIEW ‣ The largeintestine is the final section of the gastrointestinal system before the rectum ‣ In this section of the GI tract water is reabsorbed and any remaining waste material is stored as feces to be removed ‣ Learning Goal ‣ To consider the physiological process of defecation and clinical conditions that may occur during dysfunction
- 29. DEFECATION RECTUM ‣ The rectumis responsible for temporary storage of feces before defecation ‣ As it becomes filled the rectal walls expand and stretch receptors stimulate the desire to defecate ‣ The urge to defecate arises from contraction of rectal muscles, relaxation of the internal anal sphincter and an initial contraction of the external anal sphincter ‣ If the urge is not acted upon further water is absorbed and the feces is stored until the next mass movement occurs
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- 31. DEFECATION DEFECATION ‣ There aretwo main anal sphincters: internal and external ‣ The internal anal sphincter is controlled by parasympathetic fibres which relax involuntarily ‣ The external anal sphincter is skeletal muscle that is controlled by somatic nerve supply ‣ Inferior anal branch of the Pudendal nerve (S2,3,4) ‣ Allows conscious control of defecation
- 32. DEFECATION DEFECATION ‣ When therectum is distended the rectosphincteric reflex is initiated and relaxes the internal sphincter ‣ If defecation is not desired, voluntary contraction of the external sphincter can delay it ‣ If defecation is appropriate, then a series of reflexes takes place that leads to: ‣ Relaxation of the external sphincter ‣ Contraction of abdominal wall muscles ‣ Relaxation of pelvic wall muscles
- 33. DEFECATION DEFECATION ‣ Peristaltic wavesthen facilitate the movement of feces through the anal canal ‣ Defecation can also be assisted by taking a deep breath and attempting to expel the air against a closed glottis, this is known as the Valsalva maneuver ‣ However, if a delay in defection is needed then voluntary contraction of the external sphincter is usually sufficient to override the reflexes that anal distension initiates
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- 37. DEFECATION REVIEW QUESTIONS ‣ ConsciousControl of Defecation is controlled by which nerve? ‣ Pudendal nerve (S2, S3, S4) ‣ Ilioinguinal ‣ Genitofemoral nerve ‣ Sacral nerve
- 38. DEFECATION REVIEW QUESTIONS ‣ ConsciousControl of Defecation is controlled by which nerve? ‣ Pudendal nerve (S2, S3, S4) ‣ Ilioinguinal ‣ Genitofemoral nerve ‣ Sacral nerve
- 39. DEFECATION REVIEW QUESTIONS ‣ Whatcan cause fecal incontinence? ‣ Childbirth ‣ Caffeine ‣ Alcohol abuse ‣ Opioids
- 40. DEFECATION REVIEW QUESTIONS ‣ Whatcan cause fecal incontinence? ‣ Childbirth ‣ Caffeine ‣ Alcohol abuse ‣ Opioids
- 41. References These slide reflecta summary of the contents of TeachMePhysiology.com and are to be used for educational purposes only in compliance with the terms of use policy. Specific portions referenced in this summary are as follows: ‣ https://teachmephysiology.com/gastrointestinal-system/large-intestine/ ‣ https://teachmephysiology.com/gastrointestinal-system/large-intestine/ defecation/ ‣ https://teachmephysiology.com/gastrointestinal-system/large-intestine/large- intestinal-motility/ Additional sources are referenced on the slide containing that specific content.