Flashcards in Physiology - Small Intestine Function Deck (21)
What are the functions of the duodenum, jejunum and ilium?
Duodenum = Gastric acid neutralisation, digestion, Iron absorption
Jejunum = Nutrient absorption (95%)
Ilium = NaCl/H20 absorption -> chyme dehydration
Give 4 ways in which the small intestine increases its absorptive area
1. Small intestine is about 6m long (ranges 4.5-9m) and is packed into a small area
2. The mucosa and submucosa inside is folded into Plicae Circularis or circular folds
3. Mucosa is further structured into villi which are lined by enterocytes
4. Each absorptive cell has many microvilli on its apical surface massively increasing the surface area
How do the plicae circularis improve digestion?
Create a greater surface area for absorption
Fold is like a spiral so food follows the small intestines "rifling", folding over itself so that every part of the chyme makes contact with the surface. Like a washing machine
Describe the life cycle of an epithelial cell in the small intestine
Cells begins life at the bottom of a crypt of Lieberkuhn.
As the cells proliferate the cell is pushed up out of the crypt and up the villi maturing as it goes.
When it reaches the top (about day 5) it is shed off into faeces.
Highly proliferative cells so will be targeted by cancer treatment
Which parts of the small intestine structure deal with absorption and which deal with secretion?
Give some examples of what villi secrete from the small intestine lumen
Monosaccharides, Amino acids, peptides, Fats
What to Crypts of Lieberkuhn secrete?
Cl and water
How should water move in the small intestine structure?
Secreted by crypts and absorbed by villi
Describe intestinal fluid secretion
Small intestine secretes about 1500ml/day
H2O secretion comes from epithelial cells lining crypts of Lieberkuhn.
H2O secreted passively (osmotically) as a consequence of active secretion of chloride into intestinal lumen.
H2O secretion important for normal digestive processes:
1. Maintains luminal contents in liquid state
2. Promotes mixing of nutrients with digestive enzymes
3. Aids nutrient presentation to absorbing surface
4. Dilutes and washes away potentially injurious substances
Describe how water is pumped into lumen
Transporter on basolateral surface moves Cl, Na and K into cell
Sodium pumped out with sodium potassium pump
Potassium allowed to exit cell through channel
Calcium builds up inside cell and is allowed to leave through CFTR.
Water follows path of chlorine
Describe the control of CFTR
How does cholera affect this process?
Adenylate cyclase activated
ATP -> cAMP
cAMP phosphorylates Protein Kinase A (PKA)
This activates CFTR
Cholera turns on CFTR constantly
What are the two types of intestinal motility movement?
Segmentation and peristalsis
What is segmentation?
Most common during meal
Contaction -> relaxation -> contraction of short intestinal segments
Contraction (few seconds) moves chyme (up and down) into adjacent areas of relaxation
Relaxed ares then contract moving chyme back
Provides thorough mixing of contents with digestive enzymes
Brings chyme into contact with absorbing surface
Describe the generation of segmentation contractions
Initiated by depolarisation generated by pacemaker cells in longitudinal muscle layer (like gastric motility)
Intestinal BER produces oscillations in membrane potential
AP frequency = strength of contraction
Frequency of contractions controlled by BER
What happens to the BER as you move down the intestine?
BER and therefore frequency of contraction decreases towards rectum.
This produces a slow migration of chyme towards large intestine with more being pushed down than up
How does neural control effect segmentation?
Parasympathetic NS (vagus) increases contraction
Sympathetic NS decreases contraction
No effect of autonomic NS on BER
What occurs once nutrient absorption is complete?
What is the Migrating Motility Complex (MMC)?
Pattern of peristaltic activity travelling down small intestine (starts in the gastric antrum)
As one MMC (terminal ilium) another begins
Arrival of food in the stomach -> cessation of MMC and initiation of segmentation
The MMC acts to:
-Move undigested material into small intestine
-Limit bacterial colonisation of small intestine
What hormone is involved in the initiation of MMC?
What is "The Law of the Intestine?"
If intestinal smooth muscle is distended (e.g. by bolus of chyme)
Muscle on oral side of bolus contracts
Muscle on anal side of bolus contracts
Bolus is moved into area of relaxation towards colon
This is mediated by neurones in the myenteric plexus