Digestion in the Small Intestine

In the small intestine, three secretions are mixed with the chyme — intestinal juice, bile juice and pancreatic juice — with the peristaltic movements of the muscularis layer ensuring they mix thoroughly. Bile and pancreatic juice enter through the hepato-pancreatic duct into the duodenum.

Bile juice is a dark green fluid containing bile pigments (bilirubin and biliverdin), bile salts (sodium glycocholate and sodium taurocholate), cholesterol and phospholipid. It contains no digestive enzyme of its own. Instead, bile salts neutralise the acidity of the chyme (making it alkaline), bring about the emulsification of fats (breaking large fat droplets into smaller ones), and activate lipases (fat-digesting enzymes). Bile pigments are what give faecal matter its colour.

Pancreatic juice, secreted by the acinar cells of the pancreas, contains pancreatic amylase, lipases, and the inactive enzymes trypsinogen and chymotrypsinogen, along with nucleases (enzymes that digest nucleic acids). Its actions are:

• Pancreatic amylase acts on starch and glycogen, converting them to disaccharides.
• Lipases hydrolyse fat molecules into fatty acids and monoglycerides (via an intermediate stage of fatty acids and diglycerides, with a further fatty acid removed to yield monoglycerides).
• Trypsinogen is converted to its active form, trypsin, by an enzyme called enterokinase present in intestinal juice. Trypsin then converts proteins, proteoses and peptones into polypeptides, and also converts chymotrypsinogen into active chymotrypsin.
• Chymotrypsin converts polypeptides into dipeptides.
• Nucleases break nucleic acids down to nucleotides (by nucleases), nucleotides to nucleosides plus phosphate (by nucleotidases), and nucleosides to sugar plus nitrogenous base (by nucleosidases).

The intestinal juice (succus entericus) is a mixture of mucus (from goblet cells) and enzymes secreted by the intestinal mucosa, including dipeptidases, lipases and disaccharidases. Its actions complete digestion:

• Maltase: maltose -> glucose
• Sucrase: sucrose -> glucose + fructose
• Lactase: lactose -> glucose + galactose
• Dipeptidase: dipeptides -> amino acids
• Lipase: emulsified fats -> fatty acids + monoglycerides …