7 The Digestive System

How digestion works

Digestion transforms food into molecules small enough to cross the gut lining and be used by the body. It combines mechanical digestion, the physical breakdown and mixing of food, with chemical digestion, in which enzymes and other secretions break large molecules into smaller ones. The small intestine is the main site of digestion and nutrient absorption.

Digestion along the tract

Mouth and stomach

Digestion begins in the mouth. Chewing breaks food into smaller pieces and mixes it with saliva. Salivary amylase begins starch digestion, and the swallowed mass, or bolus, travels down the esophagus to the stomach.

In the stomach, muscular contractions churn food and mix it with gastric juice, forming a semifluid mixture called chyme. Hydrochloric acid creates an acidic environment, denatures proteins, and helps activate pepsin, an enzyme that digests proteins. The stomach releases chyme gradually into the small intestine.

Small intestine

Most chemical digestion is completed in the small intestine. Pancreatic enzymes digest carbohydrates, proteins, and fats; bile helps disperse fats into small droplets; and enzymes on the intestinal surface complete nutrient breakdown. Carbohydrates become simple sugars, proteins become amino acids, and fats are processed into absorbable lipid components.

Absorption and nutrient transport

Absorption is the movement of digested nutrients and water from the gut lumen across the intestinal lining into the body. Folds, villi, and microscopic microvilli in the small intestine provide a large surface area for absorption.

Most sugars, amino acids, water-soluble vitamins, minerals, and other water-soluble nutrients enter blood capillaries and travel first to the liver through the hepatic portal circulation. Most absorbed fats are packaged into particles that enter lymphatic vessels called lacteals before reaching the bloodstream.

The large intestine absorbs additional water and electrolytes. As this occurs, its contents become increasingly solid and feces form.

For example, after a meal containing bread and butter, starch is digested into simple sugars and absorbed mainly into blood. Fat is digested with the help of bile and pancreatic enzymes; much of its absorbed product enters lacteals before joining the blood circulation.

Movement through the digestive tract

Motility is the movement and mixing of contents through the digestive tract. Peristalsis consists of coordinated waves of contraction and relaxation that propel contents forward; for example, it moves a swallowed bolus down the esophagus. Segmentation consists of alternating local contractions, especially in the small intestine, that mix contents with digestive juices and bring them into contact with the absorptive lining. Segmentation mainly mixes rather than propels contents over long distances.

The stomach also churns food. Sphincters regulate passage between regions; for example, the lower esophageal sphincter helps limit stomach contents from flowing backward into the esophagus. The enteric nervous system coordinates many local movements and secretions, while autonomic nerves and digestive hormones modify this activity.

Digestive secretions and regulation

Digestive secretions act at different points in the tract to support digestion and protect the lining:

• Saliva, produced by the salivary glands, moistens and lubricates food; salivary amylase begins starch digestion.

• Gastric juice, produced by stomach glands, contains hydrochloric acid, which supports protein digestion and activates pepsin. Pepsin digests proteins, and mucus helps protect the stomach lining.

• Pancreatic juice supplies enzymes for digesting carbohydrates, proteins, and fats. Its bicarbonate helps neutralize acidic chyme entering the small intestine.

• Bile, produced by the liver and stored and concentrated in the gallbladder, emulsifies fats. This increases the surface available to digestive enzymes; bile is not an enzyme.

• Intestinal secretions provide fluid and enzymes that support the final stages of digestion at the small-intestinal surface.

• Mucus, made by cells lining the digestive tract, lubricates contents and helps protect the lining.

Secretions are coordinated with food intake. The intestinal hormone secretin promotes pancreatic bicarbonate secretion, while cholecystokinin (CCK) stimulates pancreatic enzyme secretion and gallbladder contraction. These responses help the small intestine digest incoming chyme while limiting excessive acidity.