Digestion of Food

Digestion Begins in the Mouth

In the buccal cavity, the teeth and tongue, helped by saliva, chew (masticate) and thoroughly mix the food. Mucus in the saliva binds the chewed particles into a rounded mass called a bolus. Swallowing (deglutition) pushes the bolus into the pharynx and oesophagus, and waves of muscular contraction called peristalsis carry it further down to the gastro-oesophageal sphincter, which controls its entry into the stomach.

Saliva also carries out the first chemical digestion of the meal. Besides electrolytes (sodium, potassium, chloride, bicarbonate), it contains two enzymes:

• Salivary amylase -- hydrolyses about 30% of the starch present, at an optimum pH of around 6.8: starch -> maltose.
• Lysozyme -- an antibacterial agent that helps protect against infection.

Digestion in the Stomach

The stomach's mucosa contains gastric glands made of three cell types: mucus neck cells (secrete mucus), peptic/chief cells (secrete the inactive proenzyme pepsinogen), and parietal/oxyntic cells (secrete hydrochloric acid, HCl, and 'intrinsic factor', needed later for absorbing vitamin B12). Food stays in the stomach for about 4-5 hours while the muscular wall churns it, mixing it with acidic gastric juice into a semi-fluid mass called chyme.

On contact with HCl, inactive pepsinogen is converted into active pepsin, the stomach's chief protein-digesting enzyme: pepsinogen -> [HCl] -> pepsin. Pepsin breaks proteins down into proteoses and peptones (partly digested peptides). HCl keeps the stomach at about pH 1.8, the optimum for pepsin; the mucus and bicarbonate in gastric juice protect the stomach's own lining from this acid. Rennin, present in infants' gastric juice, helps digest milk proteins, and the gastric glands also secrete a small amount of lipase.

Digestion in the Small Intestine

In the small intestine, muscular movements mix the food (now chyme) with three secretions poured in here: bile, pancreatic juice and intestinal juice.

• Pancreatic juice carries several enzymes in an inactive form -- trypsinogen, chymotrypsinogen, procarboxypeptidases, amylases, lipases and nucleases. An enzyme called enterokinase, released by the intestinal mucosa, switches on trypsinogen into active trypsin, which then activates the other inactive pancreatic enzymes.
• Bile, released into the duodenum, contains bile pigments (bilirubin and biliverdin), bile salts, cholesterol and phospholipids -- but no enzymes at all. It emulsifies fats, breaking them into tiny droplets called micelles, and also activates lipases.
• Intestinal juice (succus entericus) comes from mucus-secreting goblet cells and the brush-border cells of the intestinal mucosa, and contains disaccharidases (such as maltase), dipeptidases, lipases and nucleosidases. Mucus, together with bicarbonate from the pancreas, keeps this region protected from acid and alkaline (about pH 7.8), the range these enzymes need; Brunner's glands in the sub-mucosa also add to this.

Putting it together, as food passes through the duodenum: proteins/proteoses/peptones are broken down by trypsin, chymotrypsin and carboxypeptidase into dipeptides; starch and other polysaccharides are hydrolysed by amylase into disaccharides; fats are broken down by lipases, first into diglycerides and then monoglycerides; and nucleic acids are acted on by nucleases to give nucleotides, then nucleosides. The very last stage of digestion happens right at the mucosal epithelial cells, by the enzymes of the succus entericus:

• dipeptides -> [dipeptidases] -> amino acids
• maltose -> [maltase] -> glucose + glucose
• lactose -> [lactase] -> glucose + galactose
• sucrose -> [sucrase] -> glucose + fructose
• nucleotides -> [nucleotidases] -> nucleosides -> [nucleosidases] -> sugars + nitrogenous bases
• di- and monoglycerides -> [lipases] -> fatty acids + glycerol

All of this breakdown happens in the duodenum; the simple substances produced are then absorbed further along, in the jejunum and ileum. Anything left undigested or unabsorbed moves on into the large intestine.

The Large Intestine

No significant further digestion happens here. The large intestine's two jobs are absorbing some water, minerals and certain drugs, and secreting mucus that binds the undigested waste into a mass and lubricates it for easy passage. This undigested material (faeces) enters the caecum through the ileo-caecal valve, which prevents it from flowing back into the small intestine, and is stored temporarily in the rectum until it is expelled.

Coordinating It All …