Small Intestine Structure And Function Flashcards
What is the small intestine composed of
Duodenum —> Jejunum —> Ileum
Total length of small intestine
~6m
(25cm duodenum, 2.5m jejunum, 3.5m ileum)
Role of the duodenum
Neutralises gastric acid from stomach (digestive enzymes released here from pancreas via sphincter of oddi), digestion, iron absorption
Role of jejunum
Nutrient absorption (95% of the absorption happens here)
Role of ileum
Absorb salt and water (dehydration of the chyme from the stomach)
Gives large intestine better chance to dispose of waste material in solid manner
How does the shape of the mucosa of the small intestine aid with mixing and churning of digestive enzymes
Folds in mucosa work like threads of a screw, as material passes down it follows these folds and is mixed
What is on the surface of the folds of the mucosa in the small intestine
Villi
What is at the base of the villi, how are these involved in the lifespan of villi
At base there is a blind ended sac called a crypt
In these crypts are stem cells which mature and move up into the endothelial cells
When reach the top they die and shed of into lumen (lifespan 5days)
Why is the GI system so badly effected by chemotherapy
Surface of GI system constantly renewing itself, chemo targets rapidly dividing cells so affects GI system badly
How does the structure of the villi aid with nutrient absorption
Massive surface area so good contact with foodstuffs
At core of each villus is blood supply (arterial and venous drainage) - lots of capillaries so nutrients don’t have to travel far
Middle of core is lacteal (lymphatic capillary) - lipids can’t get through capillaries so enter through lacteal, ultimately transported to thoracic duct and emptied into bloodstream at subclavian vein
What do the villus cells absorb (8)
NaCl
Monosaccharides
Amino acids
Peptides
Fats
Vitamins
Minerals
Water
What do the crypts secrete
Cl & water
(Water is secreted osmotically as a consequence of active secretion of Cl ~1500ml water/day)
NORMALLY ALL WATER ABSORBED BY CRYPTS IS REABSORBED BY VILLI
Why is water secretion important for digestive processes (4)
Maintains Lumenal contents in liquid state
Promotes mixing of nutrients with digestive enzymes
Aids nutrient presentation to absorbing surface
Dilutes and washes away potentially injurious substances
What are the two types of intestinal movement (motility)
Segmentation
Peristalsis
When does segmentation occur
During a meal
What is the mechanism of segmentation
Contraction moves chyme into adjacent area of relaxation
Relaxed areas then contract pushing chyme back
Provides thorough mixing of contents with digestive enzymes and brings chyme into contact with absorbing surface (allowing for absorbition)
What initiates segmentation contractions (electrical)
Initiated by depolarisation generated by pacemaker cells in longitudinal muscle layer
The intestinal basic electrical rhythm (BER) produces oscillations in membrane potential —> threshold —> action potential —> contraction
What determines the strength of contraction and frequency in segmentation
Strength: action potential frequency
Frequency: BER
What happens BER as you move down intestine towards rectum
BER decreases
(segmentation produces slow migration of chyme towards large intestine)
What is the effect of the parasympathetic and sympathetic nervous systems on contraction in segmentation
Parasympathetic (vagus) = increased contraction
Sympathetic = decreased contraction
(Autonomic has no effect on BER)
What is the migrating motility complex
Pattern of peristaltic activity travelling down small intestine (starts in gastric antrum)
As one MMC ends (terminal ileum) another begins
What does MMC act to do (2)
Move undigested material into large intestine
Limit bacterial colonisation of small intestine
What stimulates the end of segmentation and start of peristalsis
Segmentation stops once nutrients absorbed stimulating start of peristalsis
What stimulates the end of peristalsis and initiation of segmentation
Arrival of food in stomach
