8th February 2024

Small and large intestines

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. 

Large intestine

The large intestine is the site of water reabsorption, digestion of chyme by bacterial intestines and formation and storage of faeces. It also acts as the site of vitamin K absorption, as the enteric bacteria produce this. 

Its motility is stimulated by parasympathetic stimulation, CCK, drugs (cholinergic, laxatives) and the gastrocolic reflex (food in the stomach increases motility and mass movement in the colon). Anticholinergics and sympathetic stimulation inhibit it. 

References

Tortora, G. J., & Derrickson, B. (2017). Tortora’s principles of anatomy and physiology. John Wiley and Sons.

Kumar, P. and Clark, M., (2012). Kumar & Clark’s clinical medicine. 8th ed.

Fairweather, J., Findlay, M., & Isles, C. (2020). Clinical companion in nephrology. Springer International Publishing.