Alimentary systems 5 - Large Intestine Flashcards Preview

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Flashcards in Alimentary systems 5 - Large Intestine Deck (24)
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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
16
Q

What stimulates the enteric nervous system?

A

Presence of food in the stomach

17
Q

What hormone controls the large intestine?

A

Aldosterone causes sodium reabsorption

18
Q

Name the disease where there is no enteric nervous system

A

Hirschsprungs

19
Q

Where are the myenteric plexus ganglia?

A

Concentrated below taenia coli

20
Q

Describe the process of defication

A
  • 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)
21
Q

What is the social part of the rectum?

A

The part that can distinguish between solid, liquid and gas

22
Q

Describe the composition of feces

A
  • 2/3 water

- Cellulose, bacteria, cell debris, bile pigments, salts

23
Q

List the roles of intestinal flora

A
  • 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
24
Q

What are the types of normal gut flora?

A
  • Bacteriocides (gram-negative, anaeobic, non-spore forming)

- Bifidobacteria (gram-positive, non-spore forming, lactic acid bacteria) prevent colonisation of pathogens