The Digestive System

What the digestive system is

The digestive system breaks the food and drink you consume into small molecules your body can absorb and use for energy, growth, and cell repair. It is a coordinated network of hollow organs joined in a long, twisting tube called the gastrointestinal (GI) tract, plus several solid accessory organs (the liver, pancreas, gallbladder, and salivary glands) that add digestive juices.

In a single trip, food travels through the mouth, esophagus, stomach, small intestine, and large intestine, ending at the anus. Along the way it is mixed, broken down chemically, and stripped of nutrients and water. In an adult the small intestine averages about 22 feet (roughly 7 meters) in length, while the large intestine adds about 5 feet (roughly 1.5 meters).

How food moves and breaks down

Digestion combines two processes that work together throughout the GI tract.

• Motility (peristalsis): Rings of muscle in the organ walls contract behind the food and relax in front of it, squeezing contents forward in a wave. This is why you can swallow and digest food even while lying down.
• Chemical digestion: Digestive juices containing enzymes, acid, and bile break large molecules (carbohydrates, proteins, fats) into absorbable units (sugars, amino acids, fatty acids).

Digestion is regulated by hormones released by cells lining the stomach and small intestine and by the enteric nervous system, a dense web of nerves in the gut wall sometimes called the "second brain."

The main organs and their roles

Mouth. Digestion begins in the mouth. Teeth grind food while saliva from the salivary glands moistens it and begins breaking down starches with the enzyme amylase. Swallowing pushes the food into the esophagus.

Esophagus. The esophagus is a muscular tube that carries food from the throat to the stomach using peristalsis. A ring of muscle (the lower esophageal sphincter) opens to let food in and closes to keep stomach contents from flowing back up.

Stomach. The stomach stores food, secretes strong acid and enzymes, and churns everything into a thick liquid called chyme. The acidic environment kills many ingested microbes and begins protein digestion.

Small intestine. The small intestine is the longest part of the GI tract and the site of most digestion and nutrient absorption. Its three sections, the duodenum, jejunum, and ileum, receive enzymes from the pancreas and bile from the liver. Millions of finger-like villi create a vast surface area to absorb sugars, amino acids, fatty acids, vitamins, and minerals into the bloodstream.

Large intestine (colon). The large intestine absorbs water and electrolytes, turning liquid waste into solid stool. It is home to trillions of microbes (the gut microbiome) that ferment leftover fiber and help produce some vitamins, including vitamin K.

Rectum and anus. The rectum stores stool until nerves signal the need to empty, and the anus controls its release from the body.

Accessory organs

• Pancreas: Makes a digestive juice rich in enzymes that break down carbohydrates, fats, and proteins, and it produces insulin to help regulate blood sugar.
• Liver: Produces bile to help digest fats and certain vitamins, and processes nutrients absorbed from the intestines.
• Gallbladder: Stores and concentrates bile between meals, then releases it into the small intestine when you eat fatty food.
• Salivary glands: Secrete saliva to begin starch digestion and lubricate food.

The gut microbiome

The large intestine hosts a community of bacteria, viruses, and fungi collectively called the gut microbiome. These microbes help break down dietary fiber, produce short-chain fatty acids that nourish colon cells, and support immune function. A diverse, balanced microbiome is increasingly linked to overall health, though research into the gut-body connection is still evolving and many findings are not yet definitive.

Common digestive conditions

Digestive diseases are common in the United States. The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) reports that 60 to 70 million people in the U.S. are affected by all digestive diseases combined.

• Gastroesophageal reflux disease (GERD): Stomach acid repeatedly flows back into the esophagus, causing heartburn and other symptoms. NIDDK estimates that about 20 percent of people in the United States have GERD.
• Irritable bowel syndrome (IBS): A common functional disorder causing abdominal pain, bloating, and changes in bowel habits.
• Gallstones: Hardened deposits in the gallbladder that can block bile ducts.
• Constipation and diarrhea: Frequent issues often tied to diet, hydration, medications, or underlying conditions.
• Inflammatory bowel disease (IBD), celiac disease, and colorectal cancer are more serious conditions that require medical evaluation.

This section is for general information only. See a clinician for warning signs such as persistent abdominal pain, unexplained weight loss, rectal bleeding, black stools, difficulty swallowing, or a lasting change in bowel habits.

How to keep your digestive system healthy

Several everyday habits are associated with better digestive health.

• Eat enough fiber. The Dietary Guidelines for Americans recommend roughly 22 to 34 grams per day for adults (varying by age and sex), yet most people fall short. Fiber supports regularity and feeds beneficial gut bacteria. Increase it gradually and drink plenty of fluids to limit gas and bloating.
• Stay hydrated to help the colon form soft, easy-to-pass stool.
• Move regularly. Physical activity supports healthy gut motility.
• Limit ultra-processed foods, excess alcohol, and smoking, all linked to digestive problems.
• Manage stress and sleep, which influence the gut-brain connection.
• Get screened. Both the U.S. Preventive Services Task Force and the American Cancer Society recommend that average-risk adults begin colorectal cancer screening at age 45.

This article is for general education and is not a substitute for personalized medical advice. Talk with a qualified healthcare provider about your individual symptoms and before making changes to your health routine.

Sources

• [NIDDK — Your Digestive System and How It Works](www.niddk.nih.gov/health-information/digestive-diseases/digestive-system-how-it-works)
• [NIDDK — Digestive Diseases Statistics for the United States](www.niddk.nih.gov/health-information/health-statistics/digestive-diseases)
• [NIDDK — Acid Reflux (GER & GERD) in Adults: Definition & Facts](www.niddk.nih.gov/health-information/digestive-diseases/acid-reflux-ger-gerd-adults/definition-facts)
• [Dietary Guidelines for Americans, 2020–2025 — Food Sources of Dietary Fiber](www.dietaryguidelines.gov/resources/2020-2025-dietary-guidelines-online-materials/food-sources-select-nutrients/food-sources-fiber)
• [U.S. Preventive Services Task Force — Colorectal Cancer: Screening](www.uspreventiveservicestaskforce.org/uspstf/recommendation/colorectal-cancer-screening)
• [American Cancer Society — Colorectal Cancer Screening Guidelines](www.cancer.org/cancer/types/colon-rectal-cancer/detection-diagnosis-staging/acs-recommendations.html)