For each of the structures and substances found in the alimentary tract A–E, select the most appropriate function from the list below

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.