Small Intestine Structure and Function Flashcards
What is the length of the duodenum, jejunum and ileum?
Duodenum - 25cm long
Jejunum - 2.5 m long
ileum - 3.5 m long
What is the function of the duodenum, jejunum and the ileum?
Duodenum - gastric acid neutralisation, digestion, iron absorption
Jejunum - Nutrient absorption 95%
Ileum - •NaCl/H2O absorption - chyme dehydration
Why is the jejunum usually flat?
Because it is usually empty
How much water does the ileum absorb each day?
Around 9 litres, 2200ml is lost in faeces each day
What is the name given to the circular folds that are present in the small intestine?
Plicae circularis
What is the function of the plicae?
Increase surface area but also create a swirling motion when food is being mixed
Where do the stem cells arise from?
The crypt of Lieberkuhn
What structure in the villi is responsible for removing the products of fat digestion?
The lacteal
What are the three mechanisms for increasing surface area in the small intestine?
What do the villus cells absorb?
- NaCl
- Monosaccharides
- Amino acids
- Peptides
- Fats
- Vitamins
- Minerals
- Water
What do crypt cells secrete?
Cl and water
How is glucose absorbed?
What is the purpose of the leaky potssium channel?
Prevents the cell from depolarizing - keeps inward drive for sodium
Why is H2O secretion of the intestines described as passive?
Epithelial cells lining the crypt of Lieberkuhn actively secrete chloride into the intestinal lumen - consequent movement of water
Why is H2O secretion important for normal digestive process?
A - Maintains lumenal contents in liquid state
B - Promotes mixing of nutrients with digestive enzymes
C - Aids nutrient presentation to absorbing surface
D - Dilutes and washes away potentially injurious substances
What is the transporter required to bring chlorine into the epithelial cell of the crypt of Lieberkuhn?
Sodium potassium chloride co transporter - transports potassium chloride and sodium (2 chloride and one of each (K and Na)
What is the name of the Chloride transporter on the lumenal side of the epithelial cell?
CFTR - Cystic Fibrosis Transmembrane Conductance Regulator - activated by the indirect action of adenylate cyclase
What are the two distinct types of intestinal motility?
Segmentation and peristalsis
Describe the process of segmentation
- Most common during meal
- Contraction and relaxation of short intestinal segments
- Contraction (few seconds) moves chyme (up & down) into adjacent areas of relaxation
- Relaxed areas then contract and push chyme back
- Provides thorough mixing of contents with digestive enzymes
- Brings chyme into contact with absorbing surface
Slightly faster contraction at the proximal portion of the small intestine results in a net migration of chyme down the small intestine
What causes segmentation?
Initiated by deoplarisation generated by pacemaker cells in the longitudinal muscle layer.
Intestinal basic rhythm produces oscillations in membrane potential resulting in action potentials and contraction.
Frequency of segmentation is determined by BER
What changes the strength in contraction?
The action potential frequency
When does peristalsis happen?
Following absorption of nutrients: segmentation stops and peristalsis starts
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
- Arrival of food in stomach - cessation of MMC and initiation of segmentation
What is the effect of the MMC?
Moves undigested material into the large intestine
Limits the bacterial colonisation of small intestine
Undigested food includes the likes of cellulose – without moving the substance down – it will become manifested by bacteria – we encourage bacteria in the large intestine but not the small intestine.
What is motilin?
A hormone involved in initiation of MMC
Motilin levels in the blood rise when nutrient levels in the lumen fall below a certain level
Explain how a bolus moves down the intestine
When the intestinal smooth muscle becomes distended:
Muscle on the oral side contracts
Muscle on the anal side relaxes
Bolus moves to the area of relaxation (towards colon)
What mediates peristalsis?
Neuorones in the myenteric plexus
What is the gastroileal reflex?
Contraction of the ileocaecal sphincter when chyme enters the large colon, as a result of distension of the large bowel
What is the purpose of the gastroileal reflex?
Prevents backflux into the ileum
