Physiology - Small Intestine Function

Question 1

What are the functions of the duodenum, jejunum and ilium?

Answer 1

Duodenum = Gastric acid neutralisation, digestion, Iron absorption

Jejunum = Nutrient absorption (95%)

Ilium = NaCl/H20 absorption -> chyme dehydration

Question 2

Give 4 ways in which the small intestine increases its absorptive area

Answer 2

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

Question 3

How do the plicae circularis improve digestion?

Answer 3

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

Question 4

Describe the life cycle of an epithelial cell in the small intestine

Answer 4

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

Question 5

Which parts of the small intestine structure deal with absorption and which deal with secretion?

Answer 5

Villi absorb

Crypts secrete

Question 6

Give some examples of what villi secrete from the small intestine lumen

Answer 6

NaCl, Water
Monosaccharides, Amino acids, peptides, Fats
Vitamins, Minerals

Question 7

What to Crypts of Lieberkuhn secrete?

Answer 7

Cl and water

Question 8

How should water move in the small intestine structure?

Answer 8

Secreted by crypts and absorbed by villi

Question 9

Describe intestinal fluid secretion

Answer 9

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

Question 10

Describe how water is pumped into lumen

Answer 10

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

Question 11

Describe the control of CFTR

How does cholera affect this process?

Answer 11

Adenylate cyclase activated
ATP -> cAMP
cAMP phosphorylates Protein Kinase A (PKA)
This activates CFTR

Cholera turns on CFTR constantly

Question 12

What are the two types of intestinal motility movement?

Answer 12

Segmentation and peristalsis

Question 13

What is segmentation?

Answer 13

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

Question 14

Describe the generation of segmentation contractions

Answer 14

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

Question 15

What happens to the BER as you move down the intestine?

Answer 15

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

Question 16

How does neural control effect segmentation?

Answer 16

Parasympathetic NS (vagus) increases contraction
Sympathetic NS decreases contraction
No effect of autonomic NS on BER

Question 17

What occurs once nutrient absorption is complete?

Answer 17

Segmentation stops

Peristalsis starts

Question 18

What is the Migrating Motility Complex (MMC)?

Answer 18

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

Question 19

What hormone is involved in the initiation of MMC?

Answer 19

Motilin

Question 20

What is “The Law of the Intestine?”

Answer 20

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

Question 21

Describe the Gastroilial Reflex

Answer 21

Gastric emptying = Increased segmentation activity in the ileum

• Opening of ileocaecal valve (sphincter)
• Entry of chyme into large intestine
• Distension of colon
• Reflex contraction of ileoceacal sphincter (prevents backflux into small intestine)