The digestive tract
The digestive system breaks food into molecules small enough to absorb, from a 9-metre tube running from mouth to anus.
Functions of digestion
The digestive system carries out ingestion (taking in food), propulsion (moving it along), mechanical digestion (chewing, churning), chemical digestion (enzymes breaking bonds), absorption (moving nutrients into blood and lymph), and defecation (eliminating waste).
The wall of the tract
From the oesophagus to the anus, the tract has four layers: the mucosa (inner lining that secretes and absorbs), submucosa (blood vessels, nerves), muscularis (smooth muscle layers that move food), and serosa (outer covering). Waves of muscle contraction called peristalsis push food along, even if you are upside down.
The mouth
Teeth tear and grind food, increasing its surface area. Saliva from three pairs of salivary glands moistens food and contains salivary amylase, which starts breaking starch into sugars. The tongue shapes food into a ball (bolus) and pushes it back for swallowing.
Swallowing and the oesophagus
Swallowing begins voluntarily, then becomes an automatic reflex: the soft palate closes off the nasal cavity, the epiglottis covers the larynx, and peristalsis carries the bolus down the oesophagus. A ring of muscle, the lower oesophageal sphincter, keeps stomach acid out; when it is weak, acid reflux causes heartburn.
The stomach
The stomach stores food and churns it with gastric juice into a thick liquid called chyme. Parietal cells secrete hydrochloric acid (pH about 1.5-2), which kills microbes and activates pepsin, an enzyme that starts protein digestion. A thick mucus layer protects the stomach wall from its own acid. Little absorption occurs here, except for some water, alcohol, and certain drugs.
Peptic ulcers
Ulcers are sores in the stomach or duodenum lining. Most are caused by infection with the bacterium Helicobacter pylori or by regular use of anti-inflammatory painkillers. This was proven by Barry Marshall and Robin Warren, who won the 2005 Nobel Prize; ulcers are now usually cured with antibiotics and acid-reducing drugs.
The small intestine
The small intestine, about 6 metres long, has three parts: the duodenum, jejunum, and ileum. It completes digestion with enzymes from the pancreas and its own lining, aided by bile. Its surface is folded into circular folds, finger-like villi, and microscopic microvilli, giving an absorptive area of roughly 30 square metres.
The large intestine
The large intestine (colon), about 1.5 metres long, absorbs water and electrolytes, turning liquid waste into faeces. Its trillions of bacteria ferment fibre, produce vitamins K and some B vitamins, and influence immunity. The appendix is attached near its start. Waste is stored in the rectum and eliminated through the anus.
Key takeaways
Digestion includes ingestion, propulsion, mechanical and chemical digestion, absorption, and defecation. Peristalsis moves food through a four-layered tube. Saliva starts starch digestion; the stomach's acid and pepsin start protein digestion. The small intestine completes digestion and absorbs nutrients across villi and microvilli. The large intestine absorbs water and hosts gut bacteria.
Flashcard
Waves of muscle contraction
Check your understanding
Why does the stomach not digest itself?