8 The Digestive System

The digestive system at a glance

The digestive system takes in food, breaks it into absorbable molecules, transfers nutrients and water into the body, and eliminates undigested material. It includes the gastrointestinal (GI) tract, a continuous passage from mouth to anus, and accessory organs, which assist digestion by producing or storing secretions. Food travels through the GI tract; it does not pass through the accessory organs.

From the mouth to the stomach

Food first enters the mouth, where teeth mechanically break it into smaller pieces. The tongue mixes the food with saliva and shapes it into a bolus for swallowing. Salivary amylase begins starch digestion.

Swallowing moves the bolus through the pharynx into the esophagus. Coordinated muscular waves called peristalsis propel it toward the stomach. A ring of muscle at the lower end of the esophagus helps prevent stomach contents from flowing back upward.

Stomach digestion

The stomach’s muscular contractions churn food and mix it with gastric juice. Acid unfolds proteins and helps kill many microbes, while the enzyme pepsin begins protein digestion. This produces a semifluid mixture called chyme, which the stomach releases gradually into the small intestine.

The small and large intestines

The small intestine begins with the duodenum, which receives chyme and mixes it with bile and pancreatic juice. The jejunum and ileum continue digestion and absorb most nutrients. Folds, villi, and microscopic microvilli greatly increase the absorptive surface area of the intestinal lining.

The cecum and colon receive material remaining after most digestion and absorption. The colon absorbs water and electrolytes, and gut bacteria use some of the remaining substances. As water is absorbed, the contents become increasingly solid and form stool.

How accessory organs help

Accessory organs support digestion by supplying or storing substances needed along the digestive tract. Their secretions contribute to the breakdown of food and the processing of nutrients.

Digestive secretions

• Salivary glands release saliva, which moistens food and contains enzymes that begin carbohydrate digestion.

• Liver produces bile and processes and stores many nutrients absorbed from the intestine.

• Gallbladder stores and concentrates bile, then releases it into the duodenum. Bile disperses large fat globules into smaller droplets through emulsification, helping digestive enzymes act on fats. Bile is not itself a digestive enzyme.

• Pancreas releases pancreatic juice into the duodenum. Bicarbonate helps neutralize acidic chyme, while pancreatic enzymes digest carbohydrates, proteins, fats, and nucleic acids.

Digestion and movement

Digestion combines mechanical processes that break food apart with chemical processes that split large molecules into smaller ones. Chewing begins mechanical digestion, and the stomach continues it by churning food. In the small intestine, segmentation—localized contractions that mix intestinal contents—brings chyme into contact with digestive juices and the absorptive surface. Peristalsis propels contents forward through the tract.

The main nutrient molecules are broken down into smaller components:

• Carbohydrates are broken down into simple sugars, such as glucose.

• Proteins are broken down into amino acids and small peptides.

• Fats (triglycerides) are broken down into fatty acids and monoglycerides.

How nutrients and water are absorbed

Absorption is the movement of digested substances across the intestinal lining into blood or lymph. It occurs mainly in the small intestine. Sugars, amino acids, many vitamins, minerals, and other water-soluble substances enter blood capillaries and travel first to the liver through the hepatic portal circulation.

Most long-chain fats enter intestinal lymphatic vessels called lacteals before they reach the bloodstream. The large intestine absorbs additional water and electrolytes, helping conserve body fluids and form stool.

Storage and elimination

Material that is not digested or absorbed, along with bacteria, shed cells, and water, forms feces. Feces move through the colon into the rectum, where stool is temporarily stored.

Stretching of the rectum contributes to the defecation reflex. When conditions are appropriate, coordinated muscular activity and sphincter relaxation allow stool to pass through the anus. This elimination removes undigested residues from the body.