Digestive System

The body’s processing line for food — it breaks meals down into molecules small enough to be absorbed, and disposes of the rest.

The gastrointestinal tract

A continuous tube from mouth to anus, through which food passes:

• Mouth — ingestion, chewing, and the start of chemical digestion.
• Esophagus — carries swallowed food to the stomach.
• Stomach — stores food and breaks it down with acid and churning.
• Small intestine — completes digestion and absorbs nutrients.
• Large intestine — absorbs water and forms waste.

Accessory organs

Organs that aid digestion without food passing through them:

• Liver — produces bile and processes absorbed nutrients.
• Gallbladder — stores and concentrates bile.
• Pancreas — supplies digestive enzymes (and, separately, regulates blood sugar).

What it does

• Ingestion and breakdown — mechanical (chewing, churning) and chemical (enzymes, acid) digestion.
• Absorption — nutrients pass into the blood, mostly across the inner surface of the small intestine.
• Elimination — undigested residue is compacted and expelled.

Key idea

The inside of the digestive tube is, in a sense, still “outside” the body — food only truly enters once it is absorbed across the gut wall. This system is also home to most of the microbiome.