The Large Intestine Flashcards
What does the large intestine consist of
Colon Cecum Appendix Rectum Anal canal
What is the caecum
Blind pouch distal to the ileocecal valve
What is the appendix
Thin, finger-like extension of the caecum (no physiological relevance)
What is the function of the colon
Reabsorption of electrolytes and water + elimination of undigested food and waste
What is the size of the colon
1.5 m
6cm diameter
Describe the colon structure
Caecum Ascending colon Hepatic flexure Transverse colon Splenic flexure Descending colon Sigmoid colon Rectum
How is the colon attached to the stomach
Wide band of tissue called the greater omentum
Describe the vasculature of the colon
Proximal transverse colon - middle colic artery (branch of the superior mesenteric artery)
Distal third of the transverse - inferior mesenteric artery
Describe the peritoneum
Carries fatty tags (appendices epiploicae) muscle coat (taeniae coli) - 3 longitudinal bands required for motility Peyer's patches in the small intestine, solitary nodules in the large intestine
What are haustra
Pouched ovoid segments that are caused by the shorter Taenia coli (than the circular muscle) in the large intestine
Can contract individually
Describe absorption in the large intestine
More in the proximal colon
Na+ and Cl- absorbed via exchange and ion channels
Water follows via osmosis
K+ moves passively into the lumen
How is the rectum distinguished from the colon
Histology similar to the colon but has transverse rectal folds in its submucosa
Absence of Taenia coli in the muscular externa
Describe the anal canal
Terminal portion of the rectum
Surrounded by internal/ circular muscle and external/striated muscle anal sphincters
Describe the mucosal structure of the large intestine
No villi (small SA)
Enterocytes and goblet cells are abundant
Enterocytes have short irregular microvilli for salt resorption (+glyocalyx without enzymes)
Abundant crypts
Stem cells + goblet cells in the crypts
Describe the goblet cells of the large intestine
More than in the SI
More prevalent in the crypts than on the surface
no. increases distally
Mucus produced facilitates passage of solid colonic contents + bateria
What stimulates goblet cells secretion
Acetylcholine (parasympathetic and enteric nervous system)
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 = 3 bands as Taenia coli
Bundles of taeniae coli penetrate the circular layer at irregular intervals
Describe the motility of the large intestine
Colonic contractions are 5-10cm/hr
Promotes absorption + mixing
Anti-propulsive patterns in the proximal colon to retain chyme
Transverse + descending colon = localised segmental contractions to cause back and forth mixing (haustral)
Short propulsive movements every 30mins
Describe mass movement in the large intestine
1-3 times daily
Resembles a peristaltic wave
propels contents 1/3 - 3/4 of the length of the LI
Fibre, cigarettes and coffee promote rapid transport
What controls the large intestine
Parasympathetic
ascending colon and transverse colon is innervated by the vagus nerve
Distal colon innervated by the pelvic nerves
Sympathetic
Lower thoracic and upper lumbar spinal cord
What controls the external anal sphincters
Somatic motor fibres in the pudendal nerves (voluntary movement)
What is Hirschsprung’s disease
No enteric intramural ganglia
Describe the enteric nervous system
Myenteric plexus ganglia concentrated below the Taenia coli
Presence of food stimulates mass movement
Describe defecation
The rectum is filled with faeces by mass movement in the sigmoid colon
Stores stool until convenient
Defecation is controlled by the sacral spinal cord (reflex and voluntary)
