The Digestive System: Breaking Down and Absorbing Food

1Overview

A sandwich is made of molecules far too large to enter a cell. Starch, protein and fat must be reduced to glucose, amino acids, fatty acids and glycerol before they can cross a membrane, and the leftovers must be removed. The digestive system is a long tube with helpers that carries out this disassembly and then absorbs the products into the blood and lymph, supplying the building blocks and fuel of Lessons 1 and 2. This lesson covers its structure, the six processes of digestion, the role of each organ, the control of digestive activity and its place in homeostasis.

2Organization and processes of the digestive system

The alimentary canal (gastrointestinal or GI tract) is a one-way tube about 7.6 m long in a living person, running from mouth through the pharynx, esophagus, stomach, small intestine and large intestine to the anus. Accessory organs assist but are not part of the tube: teeth and tongue (mechanical digestion), salivary glands (chemical digestion) and the liver, gallbladder and pancreas, which release bile and enzymes into the small intestine through ducts.

Six processes

1. Ingestion: food enters the mouth, where chewing and saliva begin the breakdown.
2. Propulsion: swallowing and peristalsis, waves of alternating contraction and relaxation of smooth muscle, push food along.
3. Mechanical digestion: chewing, stomach churning and segmentation (localized contractions of circular muscle in the small intestine that mix contents back and forth) shrink food and increase its surface area without changing its chemistry.
4. Chemical digestion: enzymes, acids and salts hydrolyze large molecules to their building blocks (Lesson 1); most is completed in the small intestine.
5. Absorption: nutrients pass into blood and lymph, mainly across the small intestine lining.
6. Defecation: undigested material leaves as feces.

Wall of the GI tract

From the esophagus to the anus the wall has four layers, from inside to outside: the mucosa (epithelium, a connective tissue layer rich in blood and lymph vessels and immune tissue, and a thin muscle layer), the submucosa (loose connective tissue with vessels and nerves), the muscularis (smooth muscle that mixes and moves food) and the serosa, the visceral peritoneum. Two nerve networks in the wall form the enteric nervous system, with about 100 million neurons: the myenteric plexus controls motility and the submucosal plexus controls secretion.

3From mouth to stomach

In the mouth, teeth and tongue break food into smaller pieces mixed with saliva. Salivary amylase begins starch digestion and lingual lipase, activated by acid, begins fat digestion. Saliva also contains lysozyme that attacks bacteria. The tongue forms a mass (bolus) that is swallowed; the epiglottis closes the airway (Lesson 10) and peristalsis carries the bolus down the esophagus.

The stomach

The stomach has a cardia, fundus, body and pylorus, and the pyloric sphincter controls emptying into the duodenum. It is fist-sized when empty but can stretch to about 4 L, and folds called rugae flatten as it fills. An extra oblique layer of muscle gives vigorous churning, with mixing waves about every 20 seconds that turn food into chyme, a semi-liquid mixture released a few millilitres at a time. The stomach empties in 2 to 4 hours; carbohydrate-rich meals leave fastest, protein-rich ones next and fatty meals stay 6 hours or more.

Gastric secretion has three overlapping phases. In the cephalic phase the sight, smell, taste or thought of food triggers a reflex. In the gastric phase (3 to 4 hours) stretch and partly digested proteins stimulate acid and gastrin release. In the intestinal phase chyme in the duodenum inhibits secretion and closes the pyloric sphincter through the enterogastric reflex, so the intestine is not overwhelmed. Very little absorption occurs in the stomach, except for alcohol and some drugs such as aspirin.

4The small intestine and its accessory organs

The small intestine has three parts: the duodenum (about 25 cm), the jejunum and the ileum, which meets the large intestine at the ileocecal valve. It is the main site of both chemical digestion and absorption.

Accessory secretions

• Pancreas. It makes more than a litre per day of pancreatic juice. Sodium bicarbonate makes the juice slightly alkaline (pH about 7.1 to 8.2), which neutralizes acidic chyme and inactivates pepsin so that intestinal enzymes can work. The enzymes digest carbohydrate (amylase), fat (lipase), nucleic acid (nuclease) and protein (trypsin and others, secreted inactive to prevent self-digestion and activated in the intestine by the brush-border enzyme enteropeptidase).
• Liver and bile. The liver, the largest gland, makes about a litre of bile per day. Bile is alkaline and contains bile salts, phospholipids, cholesterol and the pigment bilirubin from the breakdown of old red blood cells (Lesson 8). Bile does not digest chemically: its salts emulsify fat globules into tiny droplets, increasing surface area for lipase. Bile salts are reabsorbed in the ileum and reused (enterohepatic circulation).
• Gallbladder. It stores bile between meals and concentrates it up to tenfold, then contracts to release it when fatty chyme arrives.
• Intestinal glands. Brunner's glands in the duodenum secrete bicarbonate-rich mucus, and the intestinal juice is slightly alkaline.

Absorption and the villus

Three adaptations together raise the absorptive area more than 600-fold: circular folds (which spiral the chyme and slow it), villi (finger-like projections 0.5 to 1 mm long) and microvilli (about 1 micrometre extensions of the cell membrane that form the brush border and carry enzymes that finish digestion). Each villus holds a capillary bed and a lacteal. Sugars and amino acids pass into the capillaries (by facilitated diffusion and sodium-linked symporters, Lesson 3) and travel in the hepatic portal vein to the liver, which processes all absorbed nutrients, drugs and toxins before they reach the general circulation. Lipid breakdown products enter the lacteals and reach the blood through the lymphatic system (Lesson 9). The small intestine absorbs about 90% of ingested water.

5The large intestine and elimination

The large intestine (cecum, colon, rectum and anal canal) has no villi. It finishes reclaiming water and salts, forms feces and houses a large community of bacteria. Of roughly 500 mL of residue entering the cecum each day, about 150 mL becomes feces. The microbiota, trillions of mostly harmless bacteria from more than 700 species, ferment leftover carbohydrates (producing gas) and synthesize vitamins including biotin, pantothenic acid and vitamin K. A mucosal immune system keeps them out of the body (Lesson 9). Bilirubin from bile is converted by bacteria into stercobilin, which colours stool.

Defecation begins when mass movements push feces into the rectum. Stretch triggers a spinal reflex that relaxes the involuntary internal anal sphincter, while the external sphincter is skeletal muscle under voluntary control. A related article describes the microbiome in more detail.

6Regulation of digestion

Digestion is regulated by nerves and hormones. Short reflexes use the enteric nervous system within the gut wall (stomach stretch increases secretion). Long reflexes involve the brain and autonomic nerves: the sight or smell of food prepares the stomach. Parasympathetic activity ("rest and digest") increases motility and secretion, whereas sympathetic activity ("fight or flight") decreases both (Lesson 6). Hormones from cells of the stomach and duodenum act through the blood.

• Gastrin (stomach) stimulates acid secretion in response to food.
• Secretin (duodenum) is released when acidic chyme arrives and stimulates the pancreas to secrete watery bicarbonate, which neutralizes the acid. The pancreas secretes just enough bicarbonate to offset stomach HCl, which keeps blood pH stable.
• Cholecystokinin (CCK) (duodenum) is released when proteins and fats arrive and stimulates enzyme-rich pancreatic juice and gallbladder contraction.
• Gastric inhibitory peptide (GIP) slows gastric secretion, emptying and motility; it also stimulates early insulin release after a meal (Lesson 7).

Putting digestion together: following a meal

A meal of bread, cheese and an apple shows the stages in order. Chewing and salivary amylase begin on the starch. In the stomach, acid denatures the cheese protein and pepsin cuts it into shorter chains, while the fat in the cheese is only partly digested. Chyme then enters the duodenum, where acid triggers secretin and fat triggers CCK. Bicarbonate neutralizes the acid, bile emulsifies the fat and pancreatic enzymes finish the starch, protein and lipid breakdown. Brush-border enzymes release glucose and amino acids, which enter capillaries and travel to the liver, while fatty acids move into lacteals. The apple's fibre passes unchanged into the colon, where bacteria ferment part of it and the rest adds bulk to the feces.

When digestion fails

Many common disorders reflect a failure of one step. In celiac disease an immune reaction to gluten damages villi, shrinking absorptive area and causing nutrient deficiencies. Gastroesophageal reflux occurs when the lower esophageal sphincter lets acid rise into the esophagus. Constipation follows excess water reclamation or low fibre, whereas diarrhea follows too little reclamation and risks dehydration. Each case links a structural change to a functional change, the central theme of this course. Diet matters as well: the World Health Organization recommends limiting free sugars to under 10% of energy intake and eating at least 400 g of fruit and vegetables daily, advice that supports both the gut and the microbiota that live there. Lesson 14 returns to these recommendations.

7Interdependence, homeostasis and health

The digestive system contributes to homeostasis by supplying water, ions and nutrients, by buffering pH (HCl is neutralized by bicarbonate so that acid-base balance is not disturbed), and by helping to maintain blood glucose through the liver. Loss of fluid through severe diarrhea or vomiting disturbs fluid balance and acid-base balance (Lesson 1).

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