GI physiology 4 (small + large intestine) Flashcards
Why are muscle contractions important?
They mix the stomach content, ensure the luminal content is in contact with epithelial cells and transport chyme
What are the two types of contraction in the small intestine?
Segmentation and Peristalsis
What is the function of segmentation?
To mix content in the intestine
What is the migrating motility complex?
Propulsive movements that ‘sweep’ the intestine clean between meals- it removes bacteria and sloughed off epithelial cells
How is the contraction in the small intestine controlled?
via pacemaker cells
What are pacemaker cells?
Specialised neurones that can generate slow waves that then contract the smooth muscle
What else is released that effects the membrane potential of the smooth muscle?
peptides and other neurotransmitters
Where are most carbohydrates and proteins broken down?
The small intestine
What are some adaptations of the small intestine?
Folds in the inner surface (mucosa), villi with microvilli (all to increase surface area)
What is the structure of enterocytes?
They contain an apical membrane that is in contact with the intestinal lumen, Tight junctions that connect adjacent enterocytes and a basolateral membrane that acts as extracellular space
How does absorption work in the small intestine?
Nutrients are transferred from the lumen, across the apical membrane (if they are organic)
Transcellular absorption
absorption through the cell through the enterocytes (organic nutrients)
Paracellular absorption
Inorganic ions and water diffusing through tight junctions that connect the enterocytes
What are the two phases of digestion?
Luminal and membranous, they are both catalysed by enzymes
Where and how are carbohydrates digested?
By amylase, in the proximal part of the stomach
Where does the final degradation of proteins into amino acids occur?
In the membrane of the stomach
How are fats broken down in the small intestine?
Fats are emulsified by bile produced from the pancreas, FFA’s and monoglycerides then diffuse into the epithelial cell where they become chylomicrons and enter the lymph vessel
Levels of Absorption of calcium in the small intestine
It is essential that the level of calcium within cells remains low so calcium is bound to a protein during its transport from the apical membrane to the basolateral
What is the function of calcitrol?
Increases the number of channels in the apical membrane and increases synthesis of the transporter proteins so more calcium is taken up
Absorption of sodium
All sodium is re-absorbed, co-transported with glucose
Absorption of potassium
Mostly absorbed via diffusion through the apical membrane in the gut
Where does microbial digestion occur?
In the large intestine, it is most of benefit to simple stomached herbivores (e.g horses/ rabbits)
What occurs to Carbohydrates in the large intestine?
they are degraded to VFA’s by microorganisms, these can then be absorbed by the epithelium
What happens with the excess acid after VFA production in horses? (too many VFA’S produced)
large amounts of HCO3- produced in the pancreas and ileum are transferred to the LI to neutralise the acids
What are the 4 types of contraction in the large intestine
Segmentation, Peristalsis, Antiperistalsis and Mass Movements
What is the most common form of contraction in the large intestine?
Segmentation, it forms ‘Haustra’ over several minutes, it is a movement of content slowly back and forth and occurs in horses and pigs
What are the two contractions found in the caecum?
segmentation and mass movements
What is the purpose of segmentation in the caecum?
maintains high microbial activity for degradation of
food and absorption of water and ions
What is the purpose of mass movements in the caecum?
replace segmentation at intervals, pushing
content forward into the colon
What is the most important function of contractions in the colon?
ensure the luminal content is well mixed and
exposed to epithelial surfaces for maximum absorption of water, ions and
VFA
What is the function of the pelvic flexure?
- critical for transport of
content - functions similarly to a
sphincter - larger particles cannot
pass through, so
retained until they are
broken down further
How is faeces produced?
Mass movements,
How does the proximal colon receive faeces?
In a fluid-like consistency it is then mixed in the ascending and transverse colon
What happens when faeces enters the rectum?
there is reflexive relaxation of the internal anal sphincter known as the rectosphincteric reflex which causes fedecation
Malabsorptive diarrhoea
occurs when absorption is inadequate to recover a sufficient portion of
secreted water
- Can be caused by viral or bacterial infection
Hypersecretory diarrhoea
occurs when rate of intestinal secretion increases and overwhelms absorptive
capacity
- E. coli and salmonella will cause this
How does Hypersecretory diarrhoea occur?
Produces enterotoxins
- These bind to epithelium and stimulate cAMP activity, causing
opening of chloride channels
- This causes water and electrolytes to be secreted from the crypt epithelium
What is absorbed across the tight junctions?
water and ions
What is absorbed across the extracellular space?
water and ions
