Physiology and pharmacology of the large intestine Flashcards
How long is the Large intestine?
1.7m
What are the 4 parts of the large intestine?
Caecum and appendix
Colon
Rectum
Anal canal and anus
What are the four sections of the colon?
Ascending
Transverse
Descending
Sigmoid
What are the three smooth muscle layers in the caecum and colon?
Teniae Coli
Internal anal sphincter
External anal sphyincter
Do the caecum and appendix have a function in humans?
No
How many litres of fluid does the caecum normally receive from the ileum per day?
1-2L
How is entry from ileum to the caecum regulated?
The gastroileal reflex in response to gastrin and CCK through the ileocaecal valve
What is the appendix?
A blind ended tube with extensive lymphoid tissue connected to the distal caecum via the appendiceal orifice that may be obstructed by faecalith.
What are the 5 primary functions of the colon?
Absorption of Na+, Cl- and H20 Absorption of short fatty acid chains Secretion of K+, HC03 and mucus Reservoir of colonic contents Periodic elimination of faeces
Are there Villi in the mucosa of the colon?
No
What is present in the mucosa of the colon?
Colonic folds
Crypts
Microvilli
What do colonocytes mediate?
Electrolyte absorption by osmosis
What do crypt cells mediate?
Ion secretion
What do goblet cells secrete?
Copious mucus containing glycosaminoglycans
Trefoil proteins involved in host defense
What are Na+ absorption and K+ secretion mediated by?
Aldosterone
What might cause signifigant loss of K+ in the faeces?
Secretory diarrhoea
How much ileocaecal material is absorbed by the colon each day?
0.1L
What is Haustration?
Non-propulsive segmentation
What is Peristaltic propulsive movements?
Mass movement
What is Defaecation?
Periodic egestion
What are Haurstra?
Saccules caused by alternating contraction of the circular muscle
Where does haustration occur?
The proximal colon
What generates haustration?
Slow wave activity
Describe mass movement
simultaneous contraction of large sections (about 20 cm) of the circular muscle of the ascending and transverse colon (haustra disappear) - drives faeces into distal regions
When does mass movement occur and what triggers it?
occurs about one to three times daily
typically triggered by a meal (often breakfast) via the gastrocolic response involving
gastrin
extrinsic nerve plexuses
Describe the process of motility in the large intestine
Mass movement > Rectum fills with feacal matter > Activation of rectal stretch receptors and afferents to brain > Activation of parasympathetic efferents > Contraction of smooth muscle of sigmoid colon and rectum
How would a beneficial resident bacteria in the gut be described?
Commensal
What is the function of commensal bacteria?
increase intestinal immunity by competition with pathogenic microbes
promote motility and help maintain mucosal integrity
synthesise vitamin K2 and free fatty acids (from carbohydrate) that are absorbed
activate some drugs (e.g. used in treatment of IBD)
How do gases in the large intestine arise?
swallowing air
What is constipation and what causes it?
Presence of hard dried faeces within the colon (results from delay in defaecation and enhanced absorption of H2O)
What are the causes of constipation?
ignoring, or suppressing, the urge to defaecate
decreased colonic motility (e.g. improper diet, drugs, metabolic disorders, old age)
obstruction of faecal movement
paralytic ileus following abdominal surgery
impairment of motility/defaecation reflex (e.g. Hirschprung disease, involving absence of a section of the enteric nervous system)
What are the symptoms of constipation?
Abdominal discomfort
Headache
Loss of appetit
General malaise
what drugs are used to treat constipation?
Laxatives
What are Purgatives?
Agents that cause purging or cleansing of the bowel by promoting evacuation
When should laxatives and purgatives not be used?
When there is a physical obstruction of the bowel
What do laxatives do?
increase peristalsis and/or soften faeces causing, or assisting, evacuation
