Small Intestine - Structure and Function Flashcards
What is the total length of the small intestine?
~6 metres
range: 4.5 - 9 metres
How long is the duodenum?
25cm
What are the functions of duodenum?
Gastric acid neutralisation
Digestion
Iron absorption
How long is the jejunum?
2.5m
How much nutrient absorption occurs in the jejunum?
95%
How long is the ileum?
3.5m
What occurs in the ileum?
NaCl/H2O absorption - chyme dehydration
How is the absorptive surface area enhanced?
By folds, villi and microvilli
What is the structure of the small intestine like?
Screw-like structure - contents follow along it, helps with mixing and increases surface area
What happens to villi in vegetarians?
Becomes flatter and leaf-like shape
Where do epithelial cells form?
In the crypts, start as stem cells and move up the crypt and mature, travel to the top of the villus and die once they reach the top (~5 days)
Why can gut cells get badly damaged by chemotherapy?
Chemotherapy targets fast dividing cells (tumours), gut cells are also fast dividing cells
What makes of the villus?
Simple columnar epithelium
Lacteal for absorbing fats
Capillary network
Goblet cells for secreting mucus
What does a villus cell absorb?
NaCl Monosaccharides Amino acids Peptides Fats Vitamins Minerals Water
What does a crypt cell secrete?
Cl
Water
How much water does the small intestine secrete daily?
~1500ml
Where does water secretion come from in small intestine?
Epithelial cells lining the crypts of Lieberkuhn
How is water secreted from the crypts?
Secreted passively due to active secretion of chloride into intestinal lumen
Why is water secretion important in digestion?
Maintains lumenal contents in liquid state
Promotes mixing of nutrients with digestive enzymes
Allows nutrients contact with absorbing surface
Dilutes and washes away potentially injurious substances
What happens to water secreted by the crypts?
Reabsorbed by villi - when osmotic gradient formed by the transport of sugar and sodium through
How does Cl get into the cells?
Na-K-Cl pump - Na and K are needed to pump in at the same time, Na pumped out by Na-K pump, K recycled by leaky K channel
Cl = -ve, inside of cell = -ve (Cl goes through cell out of channel) = osmotic gradient = WATER SECRETION
What are the 2 distinct types of movement in intestinal motility?
Segmentation
Peristalsis
What is segmentation?
Contration moves chyme into adjacent areas of relaxation
Relaxed areas contract = push chyme back
Result = thorough mixing of contents with digestive enzymes and brings chyme into contact with absorbing surface
How is segmentation contraction initiated?
Depolarisation by pacemaker cells in longitudinal muscle layer
BER produces membrane potential = threshold = AP = contraction
What determines the strength of contraction in segmentation?
AP frequency
What determines frequency of segmentation?
Basic electrical rhythm
What happens to BER as you move down intestine to the rectum?
BER decreases because segmentation produces slow migration of chyme towards large intestine = more chyme pushed down than up
What does the parasympathetic nervous system (vagus) do to segmentation contraction?
Increases force of contraction
What does the sympathetic nervous system do to segmentation contraction?
Decreases force of contraction
What effect does the autonomic nervous system have on basic electrical rhythm?
No effect - since frequency does not change, only force
When does peristalsis occur?
After absorption of nutrients, segmentation stops and peristalsis starts
What is the migrating motility complex?
Pattern of peristaltic activity travelling down the small intestine (starting in gastric antrum)
1 MMC ends (terminal ileum) another begins
What brings about the cessation of MMC and initiation of segmentation?
Arrival of food in the stomach
What does MMC do?
Moves undigested material to L.I
Limits bacterial colonisation in S.I
What is the hormone involved in initiation of MMC?
Motilin
What are the 3 main things that occur when intestinal smooth muscle is distended? + what are they mediated by?
Muscle on oral side = contracts
Muscle on anal side = relaxes
Bolus moved into area of relaxation towards colon
Mediated by neurones in myenteric plexus
What is the gastroileal reflex?
Gastric emptying leads to increased segmentation activity in the ileum
What happens during the gastroileal reflex?
Opening of ileocaecal valve
Entry of chyme into L.I
Distension of colon
Reflex contraction of ileocaecal sphincter to prevent backflux into S.I
