Large Intestine Structure + Function Flashcards
Haustra Def
Bulges in large intestine that enables segmentation
4 regions of large intestine
Caecum, colon (ascending, transversal, descending), rectum and anal canal. Small intestine attaches at caecum
Function of large intestine
Absorption of water, remaining nutrients and storage + elimination of stool
Type of cell in epithelium of large intestine
Simple columnar
4 layers of large intestine
mucosa, submucosa, musculasris, serosa
3 layers of mucosa
epithelium, lamina propria and muscularis mucosa
Main structural differnces between small and large intestine
no villi in LI, less Paneth cells in LI, less peyers patches in LI and enteroendocrine cells are less diverse in large intestine
Cells in colonic mucosa
Arise from STEM cells in crypt. Enterocytes, enteroendocytes, goblet cells and Paneth cells
Colonic Mucosa Function
Alkaline mucosal barrier, absorption, secretion and communication with microbiome
Lamina propria function
contains nerve and immune cells that regulate epithelial function
Enterochromaffin cells Outline
Baroreceptors of luminal content. Stores and releases serotonin. Serotonin release increases gastric motility and increases local fluid secretion
L cells Outline
Senses presence of specific nutrients in lumen (eg glucose). Relaese glucagon-like peptide 1 (GLP1). Stimulates pancreas insulin release and sends satiety signals to brain
Taeniae Coli Outline
Longitudinal muscle layer of colon is comprised of 3 discontinuous sheets of muscle. Longitudinal layer overall is sorter then circular layer causing bunched up (hustural appearance)
3 Types of Colonic Motility
Haustral, peristalsis and mass movements
Haustral Movements Outline
Slow segmenting (~25 mins). Mixes contents to allow greater epithelium exposure
Peristalsis Outline
Pushes bolus towards sigmoid colon and rectum. Done slowly to maximise absorption
Mass Movement
Triggered by gastrocolic reflex (3/4 times daily). Pushes material in colon towards rectum. Rectal distension triggers defecation reflex
What stimulates reflexes in GIT
Cholinergic and hormonal (CCK and gastrin) stimulation due to presence of food
Gastroileal Reflex Outline
Terminal ileum peristalsis and ileocecal valve relaxation
Gastrocolic Reflex Outline
Mass movement in colon, stimulating urge to defecate after a meal
Ileocecal Junction Outline
Place where bolus moves from ileum (SI) to caecum (LI) through ileocecal valve. Ileum contraction opens valve pushing bolus through. Caecum contraction closes valve preventing reflux
Content of bolus at ileocecal junction
water, indigestible food (eg fibre) and bile acids
Bacteria digesting fibre
Fibre can’t be digested by human enzymes. Bacterial short chain fatty acids ferment fibre into H2, O2 and methane by acting as a beta glycosidase (breaks beta glycosidic bonds)
Examples of fibre digesting short chain fatty acids (beta glycosidase)
acetatae, butyrate and propionate
Vitamins Mainly digested by bacterial enzymes
K (bllod coagulation + bone metabolism) , B7 (biotin, cell growth and fat metabolism) and B12 (cobalamin, erythropoiesis and healthy nerves)
What enables osmosis in colon
solute = NaCl and selectively permeable membrane = epithelium
Water absorption in colon
Na+ is pumped into cell from lumen by Na+ pumps. Excess Na+ is pumped out of cell into blood by Na+/K+ pumps (excess K+ in cell is pumped back into blood by K+ pumps). Cl- ions migrate from luminal to basolateral side through tight junctions. Na+ and Cl- join to form NaCl attracting water to basolateral side
Water secretion in colon
Na+/K+/2Cl- pump pushes all molecules from blood into cell. Na+ is pushed back into blood by Na+-K+ ATPase pumps (excess K+ in cell is pushed back into blood by K+ pumps). Cl- is pushed out of cell into lumen through CFTR pumps. Due to high conc in blood Na migrates through tight junction to lumen. Na and Cl form NaCl attracting water to luminal side
Rectum Outline
Final intestinal segment. Between sigmloid colon and anal canal. Temporarily stores faeces. Normally is empty but is filled by mass movement
Rectum Mucosa
Resembles colon mainly. Crypts are longer and lined mainly with goblet cells
Defecation reflex Involuntary
Peristalsis and mass movement push bolus into rectum stimulating stretch receptors. Baroreceptors stimulates spinal reflex. Spinal reflex = peristalsis stimulation, internal sphincter relaxation and external sphincter contraction
Defecation Reflex Involuntary
Not acted on: voluntary contraction and high pressure due to external sphincter returns faeces to sigmoid colon. Acted on: signal sent along pudendal nerve and external sphincter relaxes. Material sent through alimentary canal
Anal canal physiology
Stratified squamous epithelium. Protects against abrasion (no absorption needed)
Diahorrea Def
> 3 loose stools a day for 3 days. Acute = <14 days and chronic = >14 days
Constipation Def
<3 hard lumpy stools per week. Involves straining
3 causes of Diarrhea
Decreased absorption, increased secretion and increased motility
Diarrhoae Treatment
Usually self limiting, ensure to remain hydrated. Anti-motility agents (eg ioperamide), antimicrobials (eg rifaxamin), anti inflammatory and alternative medicines (eg charcoal)
Symptoms of severe constipation
Blood in stool, abdominal pain/swelling and haemorroids. Causes stress and decreases over all sense of well being
Constipation Causes
Diet (insufficient fibre and water), medication (diuretic and opiods), obstructions (strictures and tumours), neurological/hormone conditions (eg hypothyroidissm) and psychomatic (acute anxiety eg exams)
Types of Laxatives
Bulk forming (eg fibre), osmotic ( eg lactulose) and stimulant (eg senna)
Amitiza Function
Softens stool by mimicking secretion.
Colon Drug Absorption
Ascending colon is equally as permeable to lipophilic drugs as small intestine with several carrier proteins for hydrophilic. Local and systemic drugs target colon for absorption. pH rises throughout colon meaning alkaline drugs are best absorbed at proximal while acidic are absorbed best at ascending
Rectal Route Advantages
Large doses, safe/convienent for elderly + children, drug dilution minimised (little fluid in rectum), negligible digestive enzymes, 1st pass elimination is bypassed
