8th February 2024

Small intestine

The small intestine has a wide range of functions from the duodenum, jejunum and ileum. The duodenum secretes various hormones, as highlighted in the sections above. It also continues the digestion processes from the stomach as it receives pancreatic and bile secretions. Paneth cells secrete watery secretions, while Brunner’s glands secrete an alkaline mucous. The enterocytes produce digestive enzymes. CCK, secretin and parasympathetic stimulation stimulate these, while they are inhibited by sympathetic stimulation. 

The majority of nutrient absorption occurs within the duodenum and upper jejunum. Specialised epithelium with brush borders is found in this area to help with absorption. The duodenum and upper jejunum absorb iron and calcium. Fatty acids, cholesterol and fat-soluble vitamins are passively absorbed, while carbohydrates (monosaccharides), proteins (amino acids), folic acid and water-soluble vitamins are absorbed through carrier proteins. 

As highlighted in previous sections, the terminal ileum also performs essential functions in absorbing vitamin B12 and bile salt reabsorption.