Large Bowel Flashcards
What is the anatomy of the large bowel?
The large bowel consists of the colon, caecum, rectum and anal canal
The caecum is a blind pouch just distal to the ileocaecal valve (it is larger in herbivores)
The appendix is a thin, finger like extension of the caecum and is not physiologically relevant in humans
What is the function of the large bowel?
Reabsorption of electrolytes and water
And the elimination of undigested food and waste
It is 1.5m long and 6cm in diameter
What are the parts of the colon?
Ascending colon - on the right side of the abdomen, runs from the caecum to the hepatic flexure (the turn of the colon by the liver)
Transverse colon - runs from the hepatic flexure to the splenic Flexure (the turn by the spleen). Hangs of the stomach, attached by a wide band of tissue called the greater omentum (posterior side, mesocolon)
Descending colon - runs from the splenic Flexure to the sigmoid colon
sigmoid colon - runs from descending colon to the rectum
What is the blood supply of the colon?
The proximal transverse colon is supplied with blood by the middle colic artery (a branch of the superior mesenteric artery)
The distal third of the transverse colon is perfused by the inferior mesenteric artery
This reflects the embryo logical division between the midgut and the hind gut
The region between the two is sensitive to ischemia
What is the further anatomy of the colon?
The peritoneum carries fatty tags (appendices epiploicae) and the muscle coat has 3 thick longitudinal bands (taeniae coli). The gut wall is pouched in appearance (haustra)
Structural or functional purpose of appendices epiploica are unknown (may be protection against intra abdominal infection)
Taenia coli necessary for large intestine motility
Nodules of lymphoid tissue are common in the walls of the distal small intestines (peyers patches) and large intestine (solitary nodules)
What is the haustra?
As the taenia coli are shorter than the small intestine
This causes formation of pouched ovoid segments
What is the reabsorption function of the colon?
Main function
Absorbs electrolytes and water
More in proximal colon
Sodium and chloride absorbed by exchange mechanisms and ion channels
Water followes by osmosis
Potassium moves passively into lumen
Large intestine can reabsorbed approximately 4.5 L of water (usually 1.5). above the threshold is diarrhoea
What is the rectum?
The dialated distal portion of the alimentary canal
Histologically similar to the colon but distinguished by transverse rectal folds in its submucosa and the absence of taenia coli in its muscularis externa
What is the anal canal?
Terminal portion is anal canal. Surrounded by internal (circular muscle) and external (striated muscle) anal sphincters
What is the mucosal structure of the large bowel?
4 main layers inside to out
-mucosa.
Usually contains multiple glands
-submucosa
Also contains glands
- muscularis
Contains the myenteric plexuses
- serosa
Vessels and nerve endings
Like small intestine, enterocytes (dominating cell type) and goblet cells are abundant. Also lots of crypts which also contain stem cells
The mucosa appears smooth as it has no villi (so a smaller SA). The enterocytes have small itregular villi and are primarily concerned with the reabsorption of salts. Crypts dominated by goblet cells
No paneth cells and rarely enteroendocrine cells
Glycocalyx does not contain digestive enzymes
What are the goblet cells of the large bowel like?
Higher number than in small bowel
More prevalent in crypts, and number increases dostally towards rectum
Apical ends are packed with mucous filled secretion granules awaiting release
Mucous- facilitates the passage of the increasingly solid colonic contents and covers bacteria and particulate matter
Acetylcholine - (parasympathetic and enteric nervous system) stimulates goblet cell secretion
What are the muscle layers in the large bowel?
Like the small bowel: inner circular layer and outer longitudinal layer
Circular muscle is segmentally thickened
Longitudinal layer concentrated in three bands - taenia coli
Between these thelayer is thin
Bundles of muscle from the Taenia coli penetrate the circular layer at a regular intervals
The longitudinal muscle is shorter than the circular layers.
Ovoid segments (haustra) can contract individually
Haustra are sunstantial and continuous along the large bowel, apart from on the rectum and anal canal
What is the motility of the large bowel like?
Colonic contractions - kneading process - minimally propulsive - 5-10 cm/hour at most
Promotes absorption of electrolytes and water
In the proximal colon “anti propulsive” patterns dominate to retain chyme
In the transverse and descending colon, localised segmental contractions of circular muscle could haustral contractions cause back-and-forth mixing
Short propulsive movements every 30 mins
Increase in frequency following a meal
What is mass movement of the large bowel?
1-3 times daily, resembles peristaltic wave
Can propel contents 1/3 - 3/4 of length of large intestine in a few second
Food that contains vibes promotes rapid transport through colon
How is the large bowel controlled?
Parasympathetic: ascending colon and most of transverse colon innervated by vagus nerve. More distal innervated by pelvic nerves
Sympathetic - lower thoracic and upper lumbar spinal cord
External anal sphincter controlled by somatic motor fibres in the Pudendel nerves
Afferent sensory neurones detect pressure
The enteric nervous system is also important (hirchsprungs dosease - no enteric intramural ganglia)
Myenteric plexus ganglia concentrated below taenia coli
Presence of food in stomach can stimulate mass movement
Also hormonal and paracrine
Control (aldosterone promotes sodium and water absorption)
