Large intestine Flashcards
What does the large intestine consist of?
Colon Cecum Appendix Rectum Anal canal
What is the cecum? Where is it found?
It’s a blind pouch just distal to the ileocecal valve - larger in herbivores
What is the appendix?
A thin finger like extension of the cecum that is not physiologically relevant in humans
What are the functions of the colon?
- reabsorption of electrolytes and water
- elimination of undigested food and waste
What is the length and diameter of the colon?
1.5 m long
6cm diameter
Where is the ascending colon found?
Right side of the abdomen, runs from the cecum to the hepatic flexure (the turn of the colon by the liver)
Where is the transverse colon found?
Runs from the hepatic flexure to the splenic flexure (the turn of the colon by the spleen). Hangs off the stomach, attached by a wide band of tissue called the greater omentum (posterior side, mesocolon)
Where is the descending colon found?
From the splenic flexure to the sigmoid colon
Where is the sigmoid colon found?
From descending colon to the rectum - s shaped
Where is blood supplied from in the transverse colon?
Middle colic artery - branch of the superior mesenteric artery
What is blood from the distal third of the transverse colon perfused by?
Inferior mesenteric artery
What region of the colon is sensitive to ischemia?
The region between the distal third and the proximal transverse colon
What are the properties of the peritoneum?
- has fatty tags
- muscle coat has 3 thick longitudinal bands
- gut wall pouched in appearance
Where are the nodules of lymphoid tissue commonly found in the intestines?
- walls of distal small intestine (Peyer’s patches)
- large intestine (solitary nodules)
What is the suggested function of the appendices epiploica?
Protective function against intra-abdominal infections
What is necessary for large intestine motility?
Taenia coli
- three seperate longitudinal ribbons of smooth muscle that are shorter than small intestine
What are the formation of pouched ovoid segments called in taenia coli?
Haustra
How are Na+, Cl-, Water and K+ transported in colon?
Na+ and Cl- absorbed by exchange mechanisms and ion channels
Water follows by osmosis
K+ moves passively into lumen
What is the histology of the rectum?
Similar to colon but distinguished by transverse rectal folds in it’s submucosa and the absence of taenia coli in it’s muscularis externa
What is the terminal portion of the rectum called? What is it surrounded by?
Anal canal
Surrounded by internal circular muscle and external striated muscle anal sphincters
What cells are abundant in the large intestine?
Like the small intestine, enterocytes and goblet cells are abundant. Abundant crypts and stem cells are found in crypts
Where are mucosa found in the large intestine?
Mucosa appears smooth at gross level because no villi, smaller SA than small intestine
What kind of microvilli do enterocytes in the large intestine have?
Short, irregular microvilli and primarily concerned with resorption of salts
How is water absorbed in the large intestine? What is the result?
It passively follows electrocytes resulting in more solid gut contents
Where are goblet cells prevalent in the large intestine?
More in crypts than along the surface and increases distally towards the rectum
What does the mucus do in the large intestine?
Facillitates the passage of increasingly solid colonic contents and covers bacteria and particulate matter
What hormone stimulates goblet cell secretion?
Acetylcholine - parasympathetic and enteric nervous system
Where are paneth cells found in the large intestine?
Bases of crypts
What do paneth cells contain
Large acidophillic granules containing:
- antibacterial enzyme lysozyme which protects stem cells
- glycoproteins and zinc which are essential for some enzymes
they also engulf some bacteria and protozoa and may have role in regulating intestinal flora
What do the muscularis and externa layer consists of?
An inner circular and an outer longitudinal layer
What is special about the circular muscle layer of the large intestine?
Segmentally thickened
What is special about the longitudinal muscle?
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
What is special about the haustra?
Shorter than circular muscle layer, ovoid segments that can contract individually.
What are the colonic contractions like?
Kneading process that is minimally propulsive 5-10cm/hr at most
What does large intestine motility do?
Promotes absorption of electrolytes and water
What happens in proximal colon- large intestine motility ?
Antipropulsive patterns dominate to retain chyme
What happens in transverse and descending colon in large intestine motility?
Localised segmentation contractions of circular muscle called Haustral contractions cause back and forth mixing
When do movements in the large intestine increase?
Short propulsive movements happen every 30 minutes but increase in frequency following a meal
What does mass movement in large intestine resemble?
Peristaltic wave
What kind of food promotes rapid transport through colon
Food containing fibre (indigestible material)
What nerves control which parts of the large intestine parasympathetically?
Ascending colon and transverse colon - vagus nerve
More distal - pelvic nerve
What nerves and parts of spinal cord control which parts of the large intestine sympathetically?
Lower thoracic and upper lumbar spinal cord
How are the external anal sphincters controlled? What nerves?
Somatic motor fibres in pudendal nerves
What disease is the enteric nervous system important in and what happens in this disease?
Hirschsprung’s disease - no enteric intramural ganglia
Where are the myenteric plexus ganglia concentrated below?
Taenia coli
What does aldosterone promote in the large intestine?
Sodium and water absorption by synthesis of Na+ ion channel, Na+/K+ pump
How does defecation occur?
Rectum filled with faeces by mass movement into sigmoid colon and stores stool until convenient to void.
What is the defecation reflex controlled by?
Primarily by sacral spinal cord - both reflex and voluntary actions
What is defecation?
A reflex to sudden distension of walls of rectum
What causes the reflex of defecation?
Pressure receptors sending signals via myenteric plexus to initiate peristaltic waves in descending sigmoid colon and rectum. Internal anal sphincter inhibited.
What kind of control, autonomic or voluntary, does the external anal sphincter have? What about the intrinsic signal
Voluntary - external anal sphincter
Weak intrinsic signal augumented by autonomic reflex
What are the last few centimetres of the rectum known as?
The social part of the rectum
What ability does the social part of the rectum have? Why is this important?
Can distinguish between solid, liquid and gas. It is important in knowing what can be passed appropriately in what circumstance
What solids are passed in the faeces?
Cellulose, bacteria, cell debris, bile pigments and salts (K+)
What gives the colour and odour of faeces?
Bile pigments give colour
Bacterial fementation gives odour
How much poop does an adult do a day?
150g
How much of poop is water
2/3
What is different in bacterial flora in stomach and small intestine vs large intestine?
Stomach and small intestine have few bacteria - protected
Large intestine contains many essential to normal function
How much (in weight) live bacteria does an average adult human comprise of? What is the active biomass equivalent to?
1.5kg
major human organ
What are the roles of intestinal flora in large intestine? (7)
- make and excrete vitamins eg vit K
- prevent pathogen colonization by competing for attachment sites or essential nutrients
- antagonize other bacteria by the make stuff that inhibit or kill non indigenous species
- stimulate making of cross reactive antibodies that prevent infection or invasion
- stimulate development of certain tissues eg cecum and lymphatic tissues
- break down fibre
- make short chain FA
What problems can germ free animals have?
Clotting problems
How do cross reactive antibodies made in intestinal flora prevent infection or invasion?
Antibodies produced against components of the normal flora can cross react with certain related pathogens
How is fibre broke down in the intestines?
By colonic bacteria
What do short chain fatty acids do?
Regulate gut hormone release
Absorbed to be used as energy source or to influence functions eg food intake and insulin sensitivity directly
What is a type of normal gram negative bacteria in gut? What are the qualities of this bacteria?
Bacteriodes - gram negative, anaerobic, non spore forming bacteria
What do bacteriodes have implications on?
Initiation colitis and colon cancer
What is a type of normal gram postive bacteria in gut? Properties?
Bifidobacteria
Gram positive, non sporeforming, lactic acid bacteria, described as friendly bacteria
What do bifidobacteria do?
Prevent colonization by potential pathogens
What are gut bacteria recently discovered to have a link with? (5)
- drug metabolism
- insulin resistance
- bile acid metabolism
- lipid metabolism
- obesity
