Digestion and the gastrointestinal tract

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Digestion: how the gastrointestinal tract works - from mouth to anus

Digestion is the process by which the body breaks down food and drink into smaller molecules that can be absorbed into the blood and used for energy, building blocks or signalling. The gastrointestinal tract is around 8-9 metres long from mouth to anus and contains more than 100 trillion bacteria - our gut microbiome. A typical Western diet transits the tract in 24-72 hours [1].

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The five main sections

Section	Length	Main function
Mouth and oesophagus	~25 cm	Chewing, salivary amylase, transport
Stomach	~25 cm (holds 1-4 L)	Acid, pepsin, mechanical breakdown, storage
Small intestine (duodenum, jejunum, ileum)	6-7 m	Main site of digestion and absorption
Large intestine (colon)	1.5 m	Water resorption, electrolytes, bacterial fermentation
Rectum and anal canal	15-20 cm	Storage and evacuation of stool

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The four phases of digestion

1. Oral cavity - digestion begins:

• Chewing doubles surface area for digestive enzymes.
• Salivary amylase starts starch breakdown.
• Lingual lipase initiates fat digestion in infants.
• Saliva also contains lysozyme (antibacterial) and IgA.

2. Stomach - acid environment and storage:

• Produces 1.5-3 L of gastric acid daily (pH 1.5-3).
• Pepsin breaks down proteins.
• Intrinsic factor is required for B12 absorption in the ileum.
• Gastric emptying takes 2-4 hours, depending on meal composition (fat delays it).

3. Small intestine - the main stage:

• Duodenum receives bile from the gallbladder and enzymes from the pancreas.
• Jejunum and ileum absorb 95 % of all nutrients via villi and microvilli (total surface area ~250 m² - tennis-court size).
• Bile acids emulsify fat; lipase, amylase, proteases complete breakdown.
• B12 and bile salts are resorbed in the terminal ileum.

4. Large intestine - water and bacteria:

• Absorbs 1-1.5 L water daily.
• The microbiome ferments undigested fibre into short-chain fatty acids (butyrate, propionate, acetate) - energy for colonocytes.
• Produces vitamin K and biotin.
• Stool concentrates from liquid (caecum) to formed (sigmoid).

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Key accessory organs

Organ	Contribution
Liver	Produces bile, detoxifies blood from gut (portal vein)
Gallbladder	Stores and concentrates bile
Pancreas	Produces digestive enzymes + bicarbonate + insulin/glucagon

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The gut-brain axis

The gut contains around 500 million nerve cells - the "enteric nervous system" or "second brain". It produces 95 % of the body's serotonin. The gut-brain axis explains why stress can trigger IBS, bloating or diarrhoea, and why microbiome imbalance is linked to depression and anxiety [2].

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What can go wrong?

Level	Examples
Oesophagus	GERD, eosinophilic oesophagitis, dysphagia
Stomach	Peptic ulcer, H. pylori, gastric cancer
Small intestine	Coeliac, SIBO, Crohn's, bile acid malabsorption
Large intestine	IBD, IBS, polyps, colorectal cancer, diverticula
Rectum/anus	Hemorrhoids, anal fissure, anal fistula

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How to keep digestion healthy?

• Fibre intake 25-35 g/day (whole grains, vegetables, legumes)
• 2-2.5 L of fluid daily
• Regular physical activity - increases motility
• Limit red and processed meat (WHO recommendation)
• Probiotics after antibiotics or for IBS
• Stress management - meditation, sleep, routine
• Avoid smoking and excess alcohol

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Investigation at Kirurgen.dk

We investigate the entire GI tract with oral and nasal gastroscopy, colonoscopy and sigmoidoscopy. See also: Peptic ulcer, Bloating, IBD overview, IBS.

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References

1. Asnicar F, Leeming ER, Dimidi E, et al. Blue poo: impact of gut transit time on the gut microbiome using a novel marker. Gut 2021;70:1665-74.
2. Mayer EA, Tillisch K, Gupta A. Gut/brain axis and the microbiota. J Clin Invest 2015;125(3):926-38.