Alimentary systems 5 - Large Intestine Flashcards
What does the large intestine consist of?
- Colon
- Cecum
- Appendix
- Rectum
- Anal canal
What is the cecum?
- A blind pouch just distal to the ileocecal valve, larger in herbivores.
- Appendix is attached to the cecum (thin, finger-like extension)
What are the principal functions of the colon?
- Reabsorption of electrolytes and water
- Elimination of undigested food and waste
What is the length and diameter of the colon?
1.5m long, 6cm diameter
List the parts of the colon
- 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
Describe the blood supply of the colon
- 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
List the defining features of the large intestine
- Appendices epiploicae (fatty tags)
- Taeniae coli (3 thick bands of muscle)
- Haustra (pouched appearance)
- Peyers patches
Describe briefly reabsorption in the colon
- 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
Describe the structure of the rectum
- 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.
Describe the mucosal organisation
- 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
What stimulates goblet cell secretion?
Acetylcholine
Describe the muscle layers of the large intestine
- 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.
Describe the motility of the large intestine
- 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
What is a mass movement in the large intestine?
- 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
Describe the nervous control of the large intestine
- 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
What stimulates the enteric nervous system?
Presence of food in the stomach
What hormone controls the large intestine?
Aldosterone causes sodium reabsorption
Name the disease where there is no enteric nervous system
Hirschsprungs
Where are the myenteric plexus ganglia?
Concentrated below taenia coli
Describe the process of defication
- Rectum is filled with faeces following mass movement in the colon
- Stores stool until convenient
- Controlled by sacral spinal chord
- A reflex following sudden distention of the walls of the rectum (myenteric plexus initiates peristaltic waves and inhibits internal anal sphincter)
What is the social part of the rectum?
The part that can distinguish between solid, liquid and gas
Describe the composition of feces
- 2/3 water
- Cellulose, bacteria, cell debris, bile pigments, salts
List the roles of intestinal flora
- Synthesise vitamins (eg. vitamin K)
- Prevent colonization of harmful pathogens
- Stimulate production of cross reactive antibodies to prevent entry into the blood
- Stimulate development of tissues (cecum and lymphatic tissue)
- Causes fibre breakdown
- Regulates gut hormone release by fatty acid production
What are the types of normal gut flora?
- Bacteriocides (gram-negative, anaeobic, non-spore forming)
- Bifidobacteria (gram-positive, non-spore forming, lactic acid bacteria) prevent colonisation of pathogens
