Alimentary systems 5 - Large Intestine Flashcards
1
Q
What does the large intestine consist of?
A
- Colon
- Cecum
- Appendix
- Rectum
- Anal canal
2
Q
What is the cecum?
A
- A blind pouch just distal to the ileocecal valve, larger in herbivores.
- Appendix is attached to the cecum (thin, finger-like extension)
3
Q
What are the principal functions of the colon?
A
- Reabsorption of electrolytes and water
- Elimination of undigested food and waste
4
Q
What is the length and diameter of the colon?
A
1.5m long, 6cm diameter
5
Q
List the parts of the colon
A
- Ascending colon (right of abdomen, cecum to hepatic flexure)
- Transverse colon runs from hepatic flexure to the splenic flexure (turn of colon by spleen)
- Descending colon runs from splenic flexure to the sigmoid colon
- Sigmoid runs from descending colon to rectum
6
Q
Describe the blood supply of the colon
A
- Proximal transverse colon supplied with blood by the middle colic artery (from superior mesenteric artery)
- Distal third of transverse colon supplied by the inferior mesenteric artery
7
Q
List the defining features of the large intestine
A
- Appendices epiploicae (fatty tags)
- Taeniae coli (3 thick bands of muscle)
- Haustra (pouched appearance)
- Peyers patches
8
Q
Describe briefly reabsorption in the colon
A
- Colon absorbs electrolytes and water
- (Mostly proximal colon)
- Sodium and chloride absorbed
- Water follows by osmosis
- Potassium moves passively into the lumen
- Can absorb 4.5l water but usually only absorbs 1.5l, over 4.5l and diarrhoea occurs
9
Q
Describe the structure of the rectum
A
- Dilated distal portion of the alimentary canal.
- Histology similar to the colon, but distinguished by transverse rectal folds in its submucosa and the absence of taenia coli.
- Terminal portion is anal canal. Surrounded by internal (circular muscle) and external (striated muscle) anal sphincters.
10
Q
Describe the mucosal organisation
A
- Mucosa has no villi
- Enterocytes have short irregular microvilli with glycocalyx (no digestine enzymes)
- Mainly concerned with reabsorption
- Crypts have goblet cells (more than SI) to allow passage of mucous
- Fewer enteroendocytes, no paneth cells
11
Q
What stimulates goblet cell secretion?
A
Acetylcholine
12
Q
Describe the muscle layers of the large intestine
A
- Muscularis externa consists of an inner circular and outer longitudinal layer.
- Circular muscles segmentally thickened.
- Longitudinal layer concentrated in three bands- taenia coli
- Between the taenia, longitudinal layer is thin
Bundles of muscle from the teniae coli penetrate the circular layer at irregular intervals.
13
Q
Describe the motility of the large intestine
A
- Kneading colonic contractions
- Antipropulsive patterns dominate to retain chyme
- Haustral contractions in the transverse and descending colon cause back and forth mixing
- Short propulsive movements every 30 mins/after a meal
14
Q
What is a mass movement in the large intestine?
A
- Resembles a peristaltic wave
- Propels contents 1/3-3/4 of the length of the LI in a few seconds
- Food containing fibre promotes this movement
15
Q
Describe the nervous control of the large intestine
A
- Parasympathetic (ascending/transverse by vagus nerve, more distal by pelvic nerves)
- Sympathetic (lower thoracic/upper lumbar)
- External anal sphincter contolled by somatic motor fibres in pudendal nerves
- Enteric nervous system
