A. Option 4. Secrete serotonin. Certain duodenal cells called enterochromaffin cells contain serotonin and the polypeptide ‘motilin’ that are thought to influence smooth muscle motility in the gut.
B. Option 2 Secrete alkaline mucus. Brunner’s glands in the duodenum secrete a thick alkaline mucus that protects the duodenal mucosa against acid when gastric juice enters the duodenum.
C. Option 1 Fat transport. Micelles are complexes of lipids and bile salts. The complexes are water-soluble and this aids their take up by the enterocytes lining the intestinal mucosa.
D. Option 5 Ingest bacteria. Kupffer cells are macrophages in the capillary sinusoids of the liver; when they are deficient due to liver disease there is increased risk of organisms passing from the gut to the systemic circulation via the portal circulation.
E. Option 3 Activates enzymes in pancreatic secretion. Trypsinogen is a proenzyme secreted by the pancreas that is activated in the lumen of the small intestine by the enzyme enterokinase secreted by the intestinal mucosa.