11 The Digestive System

The digestive pathway

The digestive system transforms food into molecules small enough to be absorbed, carries nutrients into circulation, and eliminates material the body cannot digest. Its work depends on coordination with the circulatory, lymphatic, nervous, and endocrine systems.

The gastrointestinal (GI) tract is a continuous passage from mouth to anus. Food moves through it by peristalsis, waves of muscular contraction that propel and mix its contents.

Organs and digestive processes

In the mouth, teeth mechanically break food apart, while saliva moistens it and salivary amylase begins digesting starch. Swallowing moves food through the pharynx and into the esophagus. Peristalsis carries it to the stomach, and the lower esophageal sphincter helps prevent stomach contents from flowing backward.

In the stomach, muscular contractions mix food with acid and enzymes. Acid helps denature proteins, and the enzyme pepsin begins protein digestion. The resulting fluid mixture, chyme, passes gradually into the small intestine.

The duodenum, jejunum, and ileum make up the small intestine, where most chemical digestion is completed. The pancreas supplies digestive enzymes and bicarbonate; bicarbonate helps neutralize stomach acid. The liver makes bile, which the gallbladder stores and releases. Bile disperses large fat droplets into smaller ones, helping enzymes digest fats, but it is not itself a digestive enzyme.

The colon, part of the large intestine, absorbs water and electrolytes. Gut microbes ferment some undigested material and produce substances such as vitamin K. The remaining material becomes stool, which is stored in the rectum and expelled through the anus.

The liver, pancreas, and gallbladder are accessory digestive organs: food does not pass through them, but their secretions support digestion.

Breaking down and absorbing nutrients

Digestion combines mechanical processes, such as chewing and stomach mixing, with chemical digestion. In chemical digestion, enzymes split large food molecules into smaller units.

The small intestine completes most chemical digestion and absorbs most nutrients. Its folds, villi, and microscopic microvilli create a large surface for absorption. Sugars and amino acids enter blood capillaries in the intestinal wall. Most dietary fats are absorbed into intestinal cells and packaged into chylomicrons.

Nutrient routes through the body

After absorption, different nutrients follow different routes. Sugars and amino acids enter blood vessels in the intestinal wall and travel through the hepatic portal vein to the liver. Most dietary fats are packaged into chylomicrons, carried first through intestinal lymph vessels, and then enter the bloodstream. The large intestine absorbs remaining water and electrolytes.

Circulation, storage, and nutrient use

Blood from much of the intestine travels first to the liver through the hepatic portal system. The liver processes and stores nutrients, helps regulate nutrient levels in the blood, and releases substances as the body needs them.

After a meal, absorbed glucose can be used by cells or stored as glycogen. Between meals, the liver can release glucose into the blood. When blood glucose rises, the pancreas releases insulin, which promotes glucose uptake and storage in many tissues. When blood glucose falls, glucagon signals the liver to make more glucose available.

Cells use glucose and other absorbed nutrients in metabolism—the chemical reactions that provide energy and building materials for growth, repair, and normal function.

Nervous and hormonal coordination

Signals from the brain and the enteric nervous system help regulate digestive movement and secretions. Hormones released by the stomach and small intestine help coordinate digestion with the arrival of food.

Digestion is therefore not simply food breakdown. It coordinates mechanical and chemical processes, absorption, circulation, and regulation so nutrients become available to the whole body.