Physiology - small + large intestine Flashcards
Length of small intestine - average & range
6m
Range 4.5 - 9m
Length of duodenum + 3 functions of duodenum
25cm
Neutralise gastric acid
Digestion
Fe absorption
Length of jejunum + function
2.5m
Nutrient absorption
Length of ileum + function
3.5m
NaCl/ H2O absorption –> chyme dehydration
3 structural features of small intestine
Folded to add surface area for absorption
Villi present on mucosal surface
Microvilli line the villi
What are found at the base of villi
Crypts
What secreting cells are found in the villi
Goblet cells
Function of villi (absorption of 8 things)
ABSORPTION OF NaCl Monosaccharides Amino acids Peptides Fats Vitamins Minerals - Fe, Zn Water
Function of crypt cells
SECRETION OF
Cl and water
What happens to the K+ that’s pumped into epithelial cells via Na/K ATPase
K is recycled via K recycler protein back out of cell because cell would become too positive inside which would remove the ELECTRICAL PART of the ELECTROCHEMICAL GRADIENT
What fluid does small intestine secrete
Intestinal
How much water do crypt cells secrete a day
1500ml
Why is secretion of water from crypt cells passive (osmotic)
Because crypt cells actively secrete Cl- into lumen so water follows that osmotic gradient
Why is H2O secretion important in SI (4)
- Maintains lumen contents in liquid state - easier for digestion
- Promotes mixing of nutrients with digestive enzymes
- Helps nutrients to get closer to absorbing surface
- Dilutes and washes away potentially harmful substances
What transporter transports Cl across the epithelial cells of crypts + how does it work
Na/K/Cl co-transporter
Moves 2 Cl- , 1 Na+ , 1 K+ into epithelial cell
The Na+ that enters gets pumped back out via Na/K pump
The K+ that enters also leaves the cell via K+ recycler protein (late K+ channels) to stop cell getting too positive
Once Cl- is pumped into epithelial cell via Na/K/Cl co-transporter, how does it get out of cell into intestinal lumen
Leaves through apical membrane via chloride channels - CFTR protein
How is the CFTR channel regulated (3)
An enzyme in the basolateral membrane called ADENYLATE CYCLASE (AC) converts ATP –> cAMP
cAMP then phosphorylates protein kinase A (PKA)
PKA then activates CFTR, opening it
3 features of SI that increases surface area
Circular folds (plicae)
Villi
Microvilli
2 types of intestinal motility
Segmentation
Peristalsis
Describe segmentation
Contraction + relaxation of short intestinal segments
Contraction moves chyme into adjacent areas of relaxation then relaxed areas contract and push chyme back to where it was so the content isn’t actually moving down the intestine
Purpose of segmentation
Allows mixing of contents in SI with digestive enzymes
What are segmentation contractions generated by + description of the contractions
Basic electrical rhythm
Segmentation contractions are constant due to pacemaker cells in the longitudinal muscle layer
What does the intestinal basic electrical rhythm do
Produces back and forth changes in membrane potential
When the membrane potential hits depolarisation threshold, AP fires –> causing segmentation contraction
What determines strength of segmentation contraction
AP frequency
Therefore frequency of segmentation contractions are determined by BER
Parasympathetic activity in vagus nerve and sympathetic activity in spinal nerves has what effect on segmentation
Parasympathetic - increases segmentation contractions
Sympathetic - decreases segmentation contraction
Describe peristalsis
Occurs after segmentation
Muscle on oral side of bolus contracts and muscle on anal side of bolus relaxed, pushing bolus down the SI
What complex does peristalsis involve + describe what it is
Migrating motility complex (MMC) is a wave of electrical activity triggering peristaltic waves
MMC starts in the gastric antrum and ends in terminal ileum
Function of MMC
Moves undigested material into LI
Limits bacterial colonisation of SI (inhibits migration of bacteria in colon into terminal ileum)
What hormone initiates a MMC
Motilin
What stops MMC and initiates segmentation
Arrival of food in stomach (gastric emptying)
Describe the gastroileal reflex (3)
Increased segmentation activity opens ileocaecal valve allowing entry of chyme into LI –> distends colon –> causing reflex contraction of ileocaecal valve to stop more chyme entering colon
Length of large intestine
1.5 - 1.8m
5 parts of large intestine
Caecum –> ascending colon –> transverse colon –> descending colon –> sigmoid colon
Which muscle layer of muscularis externa is incomplete in LI
Longitudinal
What is the longitudinal muscle layer called in the LI
Teniae coli
Contractions of the teniae coli create what structures
Haustra - pouches
Crypts in the large intestine are lined with lots of what cells + purpose of these cells
Goblet cells which secrete mucus onto mucosal surface, providing lubrication for faeces to move
What does the rectum do + type of epithelium in rectum
Connection between sigmoid colon and anal canal
Simple columnar
Where is the anal canal
Between distal rectum and anus
What’s special about the musularis externa of the rectum and anal canal
Muscularis of rectum is very thick compared to other regions of GI tract but muscularis of anal canal is EVEN THICKER THAN RECTUM
The muscularis externa of the anal canal forms what
Internal anal sphincter (smooth muscle)
What kind of muscle forms the external anal sphincter allowing us to control it voluntarily
Skeletal muscle
What does the epithelium change to in anal canal
Stratified squamous
Function of colon (3)
Reabsorbs sodium and therefore osmotic absorption of water into blood –> dehydrates chyme –> forming solid faeces
How much bacteria in colon
10^14
Importance of colonic bacteria
Allows fermentation of carbohydrates that can’t be digested by digestive enzymes, i.e. CELLULOSE
Products of bacterial fermentation of carbohydrates
Short chain fatty acids (energy source)
Vitamin K (needed for blood clotting)
Gas - due to nitrogen, CO2, hydrogen, methane products
What initiates defaecation reflex (3)
Distension of rectal wall due to mass movement of faecal material into rectum –> mechanoreceptors sense the pressure from the distension –> initiating defaecation reflex
What controls the defaecation reflex (neural)
Parasympathetic nervous system via pelvic splanchnic nerves S2-4
What actually happens in the defaecation reflex (4)
- Rectum contracts
- Internal anal sphincter relaxes
- External anal sphincter contracts
- Increased peristalsis in colon –> puts pressure on external sphincter –> relaxing it voluntarily –> expelling faeces
When does constipation occur
When there’s no absorption of toxins from faecal material following long periods of retention
What is diarrhoea
Too frequent passing of faeces which are too liquid
Causes of diarrhoea
Pathogenic bacteria - e.g. c.diff Protozoans Viruses Toxins Food
What specific type of bacteria cause diarrhoea
Enterotoxigenic bacteria
What do enterotoxigenic bacteria do
Produce enterotoxins which cause EXCESSIVE INTESTINAL CHLORIDE SECRETION from crypt cells –> EXCESSIVE H2O SECRETION INTO LUMEN
The enterotoxins produced by enterotoxigenic bacteria elevate levels of what
Intracellular second messengers - cAMP, cGMP, calcium
Treatment of secretory diarrhoea + how it works
Sodium/glucose solution - ORAL REHYDRATION THERAPY
Drives villi to reabsorb H2O –> rehydration
Colorectal area supplied by what artery
IMA
Superior rectal artery is a branch of what
Middle and inferior rectal arteries are branches of what
IMA
Internal iliac
Small and large intestine peristalsis is mediated by intrinsic and extrinsic neural control from what (2)
Intrinsic control - enteric nervous system
Extrinsic control - autonomic innervation
What is the enteric nervous system
Intrinsic nervous system, one of the main divisions of the autonomic nervous system (ANS) and consists of a mesh-like system of neurons that governs the function of the GI tract
enteric nervous system consists of what 2 plexuses
Myenteric plexus (Auerbach’s plexus)
Submucosal plexus (meissner’s plexus)
Location of myenteric plexus
between inner circular and outer longitudinal muscle layer
