Small Intestine Structure & Function

Question 1

What is the structure and function of the small intestine?

Answer 1

Small intestine structure:

Duodenum - Gastric acid neutralisation, digestion, Iron absorption

Jejunum - Nutrient absorption (95%)

Ileum - Salt + water absorption (chyme dehydration)

The absorptive surface of the small intestine is enhanced by the presence villi & microvilli

Villi sit on top of Pilca circularis

Crypts of lieberkun found below villi

Brush border found along individual villi is made up of microvilli

Along brush border are goblet cells

Question 2

What is the basic electrical rhythm of segmentation contractions?

Answer 2

Contractions generated by pacemaker cells in longitudinal muscle layer

The basic electrical rhythm generating these contractions is controlled by the Basic Electrical Rhythm (BER)

The BER decreases as move down intestine

Segmentation gradually moves chyme towards large intestine

Vagal parasympathetic supply increases contractions

Sympathetic supply decreases contractions

ENS has no effect on BER

Question 3

What is the difference between segmentation and peristalsis?

Answer 3

Peristalsis - contractive waves that propels the bolus in one direction, using circular muscles

Segmentation - segments of the intestine contract and relax alternately. Circular muscles contract whilst longitudinal muscles relax and vice versa

In general, peristalsis projects food forward whereas segmentation ‘churns up’ food

Question 4

What are the features that act to increase absorptive surface area along the alimentary canal?

Answer 4

Villi & Microvilli (largest increase in surface area) absorb:

• Salt
• Monosaccharides
• Fats
• Vitamins
• Amino acids

Rugae folds

Circular plicae folds

Question 5

What is the neuronal control of intestinal motility?

Answer 5

Vagal parasympathetic stimulation - Increases motility

Sympathetic stimulation - Decreases motility

ANS stimulation - Increases motility

Myenteric plexus:

• ensures muscle on oral side of blus is contracted, and muscle on anal side of bolus is relaxed

GastroIleal reflex:

• Opening of Ileocaecal valve (sphincter)
• Entry of chyme into large intestine
• Distension of colon
• Reflux contraction of Ileocaecal sphincter (prevents backflux)