What is the large intestine made of?
Colon (ascending, transverse, descending, sigmoid), caecum, appendix, rectum and anal canal
What is the caecum?
A blind pouch not usually used in humans - first part of large intestine. Distal to the iloececal (sphincter seperating small and large intestine) valve and larger in herbivores
What is the appendix?
A thin, finger like extension of the caecum, not useful in humans but may be important for microbiome
How long and wide is the colon?
1.5m long, 6cm diameter
What is the main function of the colon?
To reabsorb electrolytes and water, and eliminate undigested food and waste
Describe the location and pathway of the ascending colon
The ascending colon is on the right side of the abdomen. It runs from the cecum to the hepatic flexure (the turn of the colon by the liver).
Describe the pathway of the transverse colon and its connection to the stomach
The transverse colon runs from the hepatic flexure to the splenic flexure (the turn of the colon by the spleen). Hangs off the stomach, attached by a wide band of tissue called the greater omentum.
Describe the pathway of the descending colon
The descending colon runs from the splenic flexure to the sigmoid colon.
Describe the pathway and appearance of the sigmoid colon
Sigmoid (s-shaped) colon runs from descending colon to the rectum.
Blood supply to the colon (2)
- Proximal transverse colon supplied by middle colic artery (branch of superior mesenteric artery)
- Distal third of transverse colon supplied by inferior mesenteric artery.
(Difference is because of embryological division between mid and hindgut - the area between the two is sensitive to ischaemia)
Features of the large intestine
- The peritoneum carries fatty tags (appendices epiploicae)
Suggestive to have protective features against intra abdominal infections - The muscle coat has 3 thick longitudinal bands (taeniea coli) which creates pouchinges (haustra). This is needed for motility.
- Has nodule of lymphoid tissue: solitary nodule
Where is most of the water absorbed, SI or LI?
Small
How is water reabsorbed in the LI?
How much?
Na+ and Cl- absorbed by exchange mechanisms and ion channels.
Water follows by osmosis.
K+ moves passively into lumen.
Large intestine can reabsorb approx 4.5 litres water (usually 1.5 litres).
The structure of the rectun
- Dilated distal portion of the alimentary canal
- Histology similar to the colon, but distinguished by transverse rectal folds in its submucosa (needed ti hold stool) and the absence of taenia coli in its muscularis externa
- Terminal portion is anal canal.
- Surrounded by internal (circular muscle) and external (striated muscle) anal sphincters.
Mucosal structure - compared to SI
Has lots of enterocytes and goblet cells, crypts and stem cells in crypts like SI
However, LI has no villi so smooth surface, the enterocytes are shorter and have irregular microvilil and re mainly involved with salt uptake
Goblet cells in the LI - distribution compared to in SI
What stimulates goblet cell secretion?
More goblet cells than the small intestine with number higher in the crypts (they dominate crypts) and more distally in the colon towards rectum - contents of LI become drier so more mucus needed to ease movement
ACh released by enteric nervous system and PNS
Paneth cells - their roles and distribution in LI
Contain large acidophilic granules containing: lysozymes (protect stem cells), glycoproteins and zinc (for enzymes). They also engulf bacteria and protozoa. May have a role in regulating intestinal flora. They are not found in the LI because we want bacteria!
Enteroendocrine cells in the LI
They are rarer than in the SI
Microvilli in the LI
- Microvilli form the brush border
- There are several thousand microvilli per cell
- The surface of the microvilli are covered with glycocalyx
What is glycocalyx?
- It is a rich carbohydrate layer on apical membrane
- It is involved in protection from the digestional lumen, yet allows for absorption.
- It traps a layer of water & mucous known as the “unstirred layer” which regulates rate of absorption from intestinal lumen
- Unlike glycocalyx in SI, it doesn’t contain enzymes
Muscle layers of the LI
- Like small intestine, muscularis externa consists of an inner circular and outer longitudinal layer
- The circular muscles are segmentally thickened
- The longitudinal layers are concentrated in 3 bands - taenia coli
- Between the taenia the longitudinal layer is thin
- Bundles of muscle from the teniae coli penetrate the circular layer at irregular intervals
- Shorter than circular muscle layer, ovoid segments called haustra- can contract individually
- Apart from rectum and anal canal- substantial and continuous
- Movements of large intestine more complicated than small intestine
The motility of the LI
- Colonic contractions lead to a kneading process
- Slow propulsion promotes absorption of electrolytes and water
- In the proximal colon, anti-propulsive patterns dominate to retain the chyme
- In transverse and descending colon, haustral contractions cause back and fourth mixing
- There are then short propulsions every 30 minutes that increase in frequency following a meal
- Mass movement occur 1-3 times daily and can propel contents 1/3-3/4 length of large intestine in a few seconds – these movements are promoted by presence of indigestible fibre or food in the stomach
What are mass movements in the LI?
They occur 1-3 times daily and can propel contents 1/3-3/4 length of large intestine in a few seconds – these movements are promoted by presence of indigestible fibre or food in the stomach
Nervous control of the LI
- PARASYMPATHETIC – ascending colon, most of transverse colon by vagal nerve. More distal by pelvic nerves.
- SYMPATHETIC – lower thoracic and upper lumbar spinal cord
- External anal sphincter controlled by somatic motor fibres in the pudenal nerves.
- ENTERIC – complicated but important
- Myenteric plexus ganglia concentrate below the taeniae coli
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Oesophagus and stomach42
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Liver44
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Small intestine45
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Pancreas43
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Introduction31
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Liver Failure and Jaundice48
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The large intestine43
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Ions, vitamins and minerals46
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Embryology6
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Infection and immunology of the gut50
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Nutrition17
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Cancer of gut23
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Abdominal pain and pancreatitis23
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Appetite regulation18
