Large bowel Flashcards
What is the primary function of the large bowel?
To reabsorb nutrients, electrolytes, and water; synthesises specific vitamins in addition to formant eliminate faeces
What are the 5 main features of the large bowel?
Caecum Colon Appendix Rectum Anal canal
What is the initial region of the large bowel?
Caecum
Which valve connects the ileum and caecum?
Ileocecal valve
What is the vermiform appendix?
Tube that attaches to the caecum, contains lymphoid tissue considered vestigial
What are the three main colonic regions?
Ascending colon
Transverse colon
Descending colon
Where is the ascending colon in relation to the abdomen?
Right abdomen
Which flexure bends the ascending colon into the transverse colon?
Hepatic flexure
Which flexure bends the transverse colon into the descending colon?
Splenic flexure
Which structure attaches the transverse colon to the stomach?
The lesser omentum
What is the relation between the descending colon and the abdomen?
Descends down the left side of the posterior abdominal wall
Where does the descending colon end?
Runs inferiorly into the pelvis, becoming the W-shaped sigmoid colon -> Rectum
Which arteries happy the proximal transverse colon?
Middle colic artery - branch of the superior mesenteric artery
Which artery supplies the distal third of the transverse colon?
Inferior mesenteric artery
What does the arterial to the proximal and distal transverse colon supply reflect?
Reflects the embryological divisions between the midgut and hind-gut
Which regions of the transverse colonies sensitive to ischaemia?
Regions between the proximal and distal transverse colon (middle)
Is not supplied significantly by the inferior and superior mesenteric artery
What are Taenia coli?
These comprise of three bands of muscle that comprise the longitudinal muscle of the muscular layer of large intestine
What punches are formed upon the tonic contractions of the Tania coli?
Bunches the colon into succession of ovoid pouches referred as haustra
What is the function of Taenia coli?
Large intestinal motility through muscular contractions, shorter than small intestine
What are attached to Tania coli which provideprotecxting functions against intra-abdominal infections?
Fat filled sacs of visceral peritoneum called epileptic appendages
What are the lymphoid tissues within the walls of the distal small intestine called?
Peyer’s patches
What are the large intestine lymphoid tissues?
Solitary nodules
Which parts of the large bowel is absent of taenia coli?
Rectum and anal canal, have established layers of muscularis for defecation
Which epithelial lining lines the anal canal?
Stratified squamous epithelial mucosa of the anal canal connects to the skin on the external anus
What are anal columns?
Mucous membrane organised into longitudinal folds, encapsulating network of arteries and veins. Two superficial venous plexuses are located in the anal canal
What are the depressions between anal columns that secretes mucous that facilitates defecation?
Anal sinuses
Where is the majority of electrolytes and water absorbed within the large bowel?
Proximal colon
How are chloride and sodium ions reabsorbed iii the colon?
Exchange mechanism and ion channels, change sin osmolarity result in water absorption by osmosis.
How much water is reabsorbed by the large intestine?
4.5L
What folds are located within the submucosal layer of the rectum?
Transverse rectal folds, and the absence of taenia coli in its muscularis externally
Where does the anal canal open up into?
Opens into the anus
What are the two main sphincters in the anal canal?
The internal anal sphincter
External anal sphincter
What muscle type many forms the internal anal sphincter?
Circular muscle
Which anal sphincter is mainly involuntary in terms of contractions?
Internal anal sphincter
Which type of muscle forms the external anal sphincter?
Striated skeletal muscle
What type of control is the external anal sphincter subjected to?
Voluntary
What type of ability does the rectum have to distinguish between solid, gas and liquid?
Perceptual ability
Which space within the bowel wall harbour the submucosal glands?
Submucosal layer
Where is the myenteric plexus located?
Within the muscularis layer, intermediate between the internal circular muscles, and the external longitudinal muscles
What type of epithelial cells form the mucosa of the colon?
Simple columnar epithelium, predominantly consist of enterocytes
Why does the large bowel have a smooth mucosal appearance?
Absence of plicae circular and vili
Which type of mucosal cells represent are greater proportion within the large bowel?
Goblet cells
What concentrated substrate is secreted from goblet cells?
Mucin
What is the purpose of mucous within the large bowel?
Facilitates the passage of solid colonic contents
Protects the intestine from the present acids and gases produced by enteric bacteria
Which polar part of goblet cells contains mucous filled secretions granules?
Apical ends of goblet cells
What stimulates the secretion of mucin from goblet cells?
Acetylcholine released from post-ganglionic parasympathetic nerve terminals, as well as enteric fibres stimulates goblet cell secretions
Which part of the large intestine do goblet cells dominate?
Intestinal crypts
Describe the distribution of goblet cells distally towards the rectum?
Increases
What are the main functions performed by enterocytes within the large bowel?
Absorb water and electrolytes
Absorb vitamins produced by intestinal bacteria
Why is salt absorption into enterocytes important?
Decreases intracellular water potential, establishing a water potential gradient for the absorption of water into the cell via osmosis
What layer is superficial to the enterocytes and microvilli, assisting in the exchange of substrates and enhancing the absorption capacity of enterocytes?
Glycocalyx layer
Where are stem cells located within the large bowel?
Crypts
Paneth cells and enteroendocrine cells are located in the large bowel: True or false?
False
What is the function of the unstirred layer?
Regulates the rate of absorption from intestinal lumen
Traps a layer of water and mucous
What is the purpose of segmentation in mechanical digestion within the large bowel?
Mixing of chyme, promotes the absorption of water & electrolytes
Haustrum is distended with chyme, circular muscle contracts, causing the chyme to move into the adjacent hausturm
What is the function of antipropulsive patterns within the proximal colon?
Antipropulsive patterns dominant to retain chyme, increasing the time exposed for effective absorption
What effect does fibre exert on colonic contractions?
Fibre promotes power of colonic contraction optimising The activities of the colon
What is mass movements within the colon?
Waves being midway through the transverse colon, force contents towards the rectum (Propulsive movements increase in frequency post-prandially). Resembles peristaltic waves, gastrocolic reflex increases motility
Where do sympathetic preganglionic fibres arise from?
Arise from thoracolumbar (t1-l2) regions within the spinal cord.
Which motor fibres central the external anal sphincter?
Controlled by somatic motor fibres in the pudenal nerves
Which spinal nerves forth pudenal nerves?
S2-4 (sacral nerve)
Which nerve innervates the ascending colon and the majority of the transverse colon?
Vagus nerve
Which nerve innervates the distal regions of the colon?
Pelvic nerve
Where are myenteric plexus ganglia located in relation to the taenia coli?
Deep to the taenia coli, within the muscularis layer
What effect does Aldosterone have on sodium and water reabsorption?
Promotes sodium and water reabsorption
Translation of Na+ ion channels integrated on the apical membrane
What is Valsalva’s manoeuvre in terms of increase intra-abdominal pressure during defecation?
Faeces are eliminated through rectal muscle contractions, this process is assisted by voluntary procedure, Valsalva’s manoeuvre, increases intra-abdominal pressure by contracting diaphragm and abdominal wall muscles, closing the glottis.
How is the defecation reflex activated?
Mass propulsive movements force faecal matter from the sigmoid colon into the rectum, distention of the rectal wall, and provoking the defecation reflex causing the elimination of faeces from the rectum
How do pressure receptors inhibit the internal anal sphincter?
Pressure receptors send signals via myenteric plexus to initiate peristaltic waves in descending, sigmoid and rectum. Internal anal sphincter inhibited.
Weak intrinsic signal augmented by autonomic reflex.
Relaxation of the internal anal sphincter, and contraction of the external anal sphincter enables defecation to occur
What is the final stage of defecation?
Presence of faeces in the anal canal transmits electrical stimulation to the brain, providing information for voluntary opening of the external anal sphincter or closure. Delay in defecation would cause reflex reactions to circumvent, and causes rectal wall relaxation.
What fraction of faeces is comprised of water?
2/3 Water
What solids form faeces?
Cellulose, bacteria, cell debris, bile pigments, salts
What results in the odour of faeces?
Bacterial fermentation
What three molecules kill the majority of bacteria that enter the alimentary canal?
Lysozyme, defensives, and HCl
Which vitamins are produced by intestinal flora (3)?
Biotin
Pantothenic acid
Vitamin K
How does intestinal flora provide an immune function to the gut?
Prevents colonisation of pathogens by competing for attachment sites, as well as intraspecific competition for essential nutrients.
Antagonise external bacteria through toxic substance production which inhibit/kill non-indigenous species
What is the role performed by cross reactive antibodies within the gut?
Antibodies produces against normal components of intestinal flora (peptidoglycan) can cross react with specific pathogens, reducing infection/invasion?
Which immune cells pass into the mucosa upon cytokine release due to peptidoglycan component release from bacterial cell walls?
Macrophages
Which junctions are opened between epithelial cells by macrophages?
Tight junctions
Which antibodies are released within the gut?
IgA
