Small Intestine

Prof. K. Sivapalan.

2013 Small Intestine 2

Small Intestinal Motility• Segmental movement [mixing 7/min in ileum]• Peristalsis [propagation average 1 cm/min].• Stimulated by stretch of the wall (dietary fibers).

– Obstruction causes over distention and violent peristalsis which causes colicky pain.

• Facilitated by,– Parasympathetic stimuli– Gastro-enteric reflex.

• Inhibited by,– Sympathetic stimuli.

• Irritation to mucosa can cause peristaltic rush and diarrhoea.

• Injury to peritoneum causes reflex inhibition and paralysis of the intestine- Paralitic ileus.

• Migrating Motor Complex

2013 Small Intestine 3

Intestinal Secretions• Brunner’s glands in duodenum

secrets thick mucus in response to acid to protect from acid.

• Crypts of Liebercühn – water and electrolytes similar to ECF,

2000 ml/day [?Much more]– Enzyme enterokinase.– Cell division and migration to top of villi-

life span of the epithelial cell is 5 days• Control of crypts of Liebercühn -

– autonomics- no effect– VIP- vasodilatation, may stimulate.

2013 Small Intestine 4

Intestinal Villi

• Surface area:– Intestine [cylinder]- 0.33 M2

– Valvulae c.- 1.00 M2

– Villi- 10.0 M2

• Microvilli- 200 M2 • Surface enzymes:

– Disaccharidases– Peptidases– Nucleic acidases

• Cells at the top face constant aberration and hypoxia

2013 Small Intestine 5

Fat Absorption • Fat is emulsified by bile

salts and lipase digests it.• Micelles formed in the

presence of bile salts.• Fatty substences diffuse

into the mucosal cell.• Chylomicrons formed and

released by exocytosis into ECF.

• Chylomicrons are taken by the lymph vessels.

2013 Small Intestine 6

Absorption of CHO and Protein.• Na+ pushed into ECF by

active transport• Na+ enters cell from

lumen.• The channel is a co-

transporter of glucose, maltose and amino acids, peptides.

• This is secondary active transport - Importance of salt and glucose in re-hydration.

• Rate of absorption and dietary fibers

2013 Small Intestine 7

Circulation in Villus

• Every villus has a central lacteal which transports chylomicrons.

• Blood with oxygen goes towards the tip and nutrients away from the tip.

• Arrangement of vessels sets up counter current flow preventing transport of oxygen to tip and nutrients out.

• The muscle in the villus contracts from time to time and empties it into the circulation.

2013 Small Intestine 8

ILLEO-CAECAL Valve