Large intestine physiology Flashcards
What is the gastroileal reflex?
When there is vigorous gastric peristalsis of increased secretion, it stimulates peristalsis in the ileum
This causes the ileal contents to be pushed into the colon, in turn stimulating colonic peristalsis and an urge to defecate
What is the purpose of the ileocaecal sphincter and how does it achieve this?
Maintains a positive resting pressure - controls flow of ileal contents to colon
Relaxes in response to distension of the duodenum
Contracts in response to distension of the ascending colon
What is the nervous supply of the ileocaecal sphincter?
The vagus nerve, sympathetic nerves and enteric neurones
What are the functions of the large intestine?
Absorption of H2O, Na+ and Cl-, secretion of K+, HCO3- and mucus i.e. converts liquid content of ileocecal material into stool
Absorption of short chain fatty acids
Storage of colonic contents
Periodic elimination of faeces
What are haustra?
Saccules caused by contraction of the circular muscle in the large intestine
What purpose does haustration serve in the large intestine?
Non-propulsive segmentation
Similar to segmentation in function, but much lower frequency– contributes to long transit time (16 – 48 hours)
What is the gastrocolic response?
Segmentation and mass movement of faeces through the large intestine in response to a meal
Which receptors does the filling of the rectum with faeces activate?
Activates the rectal stretch receptors
This activates afferents to spinal cord and brain, signalling of the urge to defecate
What nervous signals contract the smooth muscle of the colon and rectum and relax the internal anal sphincter?
Parasympathetic nerves?
What do the parasympathetic nerves signal for in response to afferents sent to spinal cord and brain following activation of rectal stretch receptors?
Relaxation of internal anal sphincter
Contraction of muscles of colon and rectum
Where does Na+/H+ exchange occur and what is it driven by?
Occurs in the duodenum and jejunum and is stimulated by luminal HCO3-
Where does parallel Na+/H+ and Cl-/HCO3- exchange occur and when is it most important?
Occurs in the ileum and colon most important in the interdigestive period
Where does sodium exchange via epithelial Na+ channels (ENaC) occur and what is it driven by?
Occurs in the colon (distal particularly) and is regulated by aldosterone
What are some of the causes of diarrhoea?
Infectious agents – viruses, bacteria (e.g. traveller’s diarrhoea) Chronic disease Toxins Drugs Psychological factors
How does cholera produce such extreme and often fatal diarrhoea?
Cholera toxin enters enterocyte and stimulates adenylyl cyclase, producing lots of cGMP which stimulates chloride channels on the enterocyte
This causes hypersecretion of Cl-, with Na+ and water following
