How long is the small intestine and its parts?
Total ~ 6metres
Duodenum - 25cm
Jejunum - 2.5m
Ileum - 3.5m
What are the functions of the 3 sections of the small intestine?
- Gastric Acid neutralization
- Iron Absorption
- 95% of nutrient Absorption
- Reabsorbs Salt & so water
- Also bile salts which are then recycled back through the liver and used again
How much water is reabsorbed int he gut?
Around 9 litres a day passes through but only 200ml reaches the faeces
What are plicae circulares?
Fold in the small intestinal walls that forms a cork screw thread.
Liquid contents of the gut follow this thread helping in mixing and absorption.
What is the surface area of the gut?
around 200m2 thanks to the plicae, villi and microvilli
What are crypts of Lieberkuhn?
Glands in the intestinal epithelial lining extending don through the mucosa
What is the function of mucosal crypts?
All mucosal cells start life at the base of these crypts as stem cells.
They undergo a process of differentiation and move up the crypt, changing as they go.
- These crypts secrete Cl- ions which takes water with it.
- They also secrete a lot of mucous from goblet cells
- Paneth cells near the base are involved in the innate immune system
- Plus several other functions/cell types`
What is absorbed by intestinal villi?
NaCl Monosaccharides Amino acids & Peptides Fats vitamins & Minerals Water
What process of transport is responsible for most absorption in the gut?
Na Coupled Secondary Active Transport
Molecule enters epithelium coupled to Sodium using a conc. gradient produced by NaKATPase pumping sodium out the other side.
What is the purpose of secreting water out the crypts?
- Maintains liquid state of luminal contents
- Promotes mixing of the nutrients and digestive enzymes
- Aids nutrient presentation to the absorbing surface
- Dilutes and washes away any potential dangerous substances.
Water leaves the crypts by osmotic diffusion through the tight junctions, however Cl ions must be pumped out to drive this diffusion, how is this done?
Cl, Na and K move into the epithelial cell from the blood through a NaKCl co-transporter.
- Sodium is pumped back out by NaKATPase and K passes out through its own channel
- This turns the inside of the cell very -ve which causes the Cl ions to be pushed out the other side by the charge through CFTR channel
What is CFTR and what regulates it?
Cystic Fibrosis Transmembrane Regulator.
- Adenylate Cyclase converts ATP -> cAMP
- cAMP activate Protein Kinase A
- Protein Kinase A stimulates CFTR
What are the two types of intestinal motility?
Short intestinal segments contract
- This forces chyme up and down into adjacent relaxed segments
- They then contract and push it back
- Slightly more is pushed down than up so there’s a net slow movement towards the colon
What is the purpose of segmentation vs peristalsis?
Segmentation mixes the chyme and digestive enzymes + brings it into contact with the absorbing surface.
Peristalsis moves undigested material along and limits bacterial colonization.
How is a segmentation contraction generated?
Rate of segmentation is determined by the BER
Strength of contraction is determined by rate of APs which in turn is determined by parasympathetic (Vagal) and hormonal signals.
It can be inhibited by the sympathetic stimulation throuh thoracic splanchnic nerves
What are the thoracic splanchnic nerves and what do they supply?
They supply the gut viscera with sympathetic innervation
How does BER vary?
It is unaffected by the ANS.
However it decreases from duodenum to rectum.
When does the small intestine switch from segmentation to peristalsis and vice versa?
Arrival of food in the stomach turns off peristalsis and triggers segmentation.
When the nutrients are absorbed peristalsis begins again
What hormone intiates peristalsis?
How does peristalsis occur?
A Migrating Motility Complex (MMC) is a pattern of peristaltic activity which travels down the small intesinte from gastric antrum -> Terminal ileum.
One begins as another ends.
Triggered by motilin and inhibited by food entry to the stomach
What is the law of the intestine?
If smooth muscle is distended. The muscle on the oral end contracts and anal side relaxes moving the bolus towards the colon.
Mediated by neurons of the myenteric plexus. (i.e. totally devoid of external stimulus)
What is the gastroileal reflex?
When the stomach starts emptying, segmentation begins, not just int he duodenum but all the way down in the ileum..
- -> Ileocaecal valve opens
- -> Chyme enters large intestine
- -> Colon distends
- -> Reflex contraction of ileocaecal sphincter
Prevents reflux into small intestine.
What is BER?
Basic Electrical Rhythm