. 2021 Apr 14;4:308–319. doi: 10.1016/j.crfs.2021.04.004

Table 1.

Summary of the physical and chemical processes occurring in the human digestive system.

Section	Physical Process	Chemical Process	Conditions (adult)	References
Mouth	Mastication Food breakdown Mixing	Enzymatic hydrolysis Starch breakdown (α-amylase) Lipid breakdown (lingual lipase)	pH: 5-7 Transit time: 5s −2 min Saliva flow rate: 0.042–1.83 mL/min (unstimulated), 0.77–4.15 mL/min (stimulated) Biting force: 100–400 N	(Chen, 2009, Marze, 2017a, Marze, 2017b, Mosca and Chen, 2016, Rogers, 2011)

Esophagus	Peristalsis Bolus transport	–	Transit time: 8–10 s solid1-2 liquid	(Bender et al., 2005, Buettner et al., 2001, Rogers, 2011)

Stomach	Contractions/Peristalsis Food breakdown Mixing Gastric transport Gastric sieving	Enzymatic hydrolysis Protein breakdown (pepsin) Lipid breakdown (gastric lipase) Acid hydrolysis Food softening and dissolution (gastric acid)	pH: 1-3 Transit time: 15 min - 4 h Gastric juice secretion: 1–3 L/day Contraction frequency: 3 cycles/min	(Boland et al., 2014, Chen et al., 2011, Miftahof, 2017, Rogers, 2011, Sullivan, 2009)

Small Intestine	Peristalsis Chyme transport Segmentation Mixing Absorption Monosaccharides (glucose, galactose, fructose) Amino acids, peptides Monoacylglycerides, glycerol, free fatty acids Phosphates, nitrogenous bases, pentose sugars Minerals and vitamins	Enzymatic hydrolysis Starch breakdown (pancreatic amylase, dextrinase, sucrose, maltase, lactase, amyloglucosidase) Protein breakdown (trypsin, chymotrypsin, carboxypeptidase, elastase) Lipid breakdown (pancreatic lipases, phospholipase) Nucleic acid breakdown (nucleases, nucleosidases and phosphatases)	pH: 6–7.5 Transit time: 1–5 h Pancreatic juice secretion: ~1.5 L/day	(Rogers, 2011, Saladin, 2017, Smith and Morton, 2010, Sullivan, 2009)

Large Intestine	Peristalsis Chyme transport Segmentation Mixing Absorption Water, ions, minerals, vitamins, fats and organic molecules	Fermentation Production of short chain fatty acids and other by products	pH: 5-7 Transit time: 12–24 h Microbiota: ~10¹¹-10¹² (>1000 different species)	(Rogers, 2011, Saladin, 2017, Smith and Morton, 2010, Sullivan, 2009)