Overview
The digestive system converts food into small molecules the body can absorb, then eliminates what is left. It has two parts: the gastrointestinal (GI) tract, a continuous tube from mouth to anus, and the accessory organs (teeth, tongue, salivary glands, liver, gallbladder, and pancreas) that aid digestion without food passing through them. It supplies the raw materials that every other system depends on.
The GI Tract Path
Food follows a one-way route, moved along by peristalsis (waves of smooth-muscle contraction).
| Organ | Main role |
|---|---|
| Mouth | Chewing (mechanical); salivary amylase begins starch digestion |
| Esophagus | Carries the food bolus to the stomach |
| Stomach | Stores food; acid and pepsin begin protein digestion; forms chyme |
| Small intestine | Completes digestion and absorbs most nutrients |
| Large intestine (colon) | Absorbs water and electrolytes; forms and stores feces |
| Rectum and anus | Store and eliminate waste |
The small intestine has three parts: the duodenum (where bile and pancreatic juice arrive), the jejunum, and the ileum. Its lining is folded into millions of villi and microvilli, creating a vast surface for absorption.
The Stomach in Detail
The stomach is a stretchy, J-shaped sac that both stores a meal and mixes it. Its lining contains gastric glands with several cell types: parietal cells secrete hydrochloric acid, chief cells secrete the protein-digesting precursor pepsinogen, and mucous cells produce a protective mucus layer that shields the stomach wall from its own acid. Powerful muscular churning combines with acid and enzymes to turn food into chyme, which is released a little at a time through the pyloric valve into the duodenum. This slow release protects the intestine and gives digestion time to work.
Accessory Organs
- Salivary glands release saliva containing amylase.
- Liver produces bile and processes absorbed nutrients.
- Gallbladder stores and concentrates bile, releasing it when fatty food arrives.
- Pancreas secretes digestive enzymes and bicarbonate; it also produces hormones, linking digestion to the endocrine system.
Chemical Digestion and Enzymes
Large food molecules are chemically split by enzymes into absorbable units.
| Nutrient | Key enzymes | End products |
|---|---|---|
| Carbohydrates | Amylase (saliva, pancreas) | Simple sugars (e.g., glucose) |
| Proteins | Pepsin (stomach), trypsin (pancreas) | Amino acids |
| Fats | Lipase (pancreas), aided by bile | Fatty acids and glycerol |
Stomach acid (about pH 2) activates pepsin and kills many microbes; pancreatic bicarbonate then neutralizes the chyme so intestinal enzymes can work. Digestion actually begins in the mouth, where chewing shreds food and salivary amylase starts on starch, and continues chemically along the whole tract.
The liver deserves special mention because it does far more than make bile. All nutrient-rich blood from the intestines passes through it first, where it stores glucose as glycogen, builds blood proteins, detoxifies drugs and alcohol, and processes fats. In this way the liver is a hub connecting digestion to the body’s overall metabolism.
Absorption
Digested nutrients cross the intestinal lining into the blood or lymph. Sugars and amino acids enter capillaries and travel to the liver through the hepatic portal vein; most fats enter lymph vessels called lacteals. The absorbed water and nutrients ultimately reach every cell through the cardiovascular system, and remaining wastes pass to the colon. The large intestine is also home to the gut microbiome, trillions of helpful bacteria that ferment undigested fiber, produce certain vitamins (such as vitamin K), and help protect against harmful microbes. Of the roughly 9 liters of fluid entering the tract each day from food, drink, and secretions, the intestines reabsorb nearly all of it, so only a small amount is lost in feces.
Clinical Relevance
Gastroesophageal reflux disease (GERD) occurs when the muscular valve between the esophagus and stomach weakens, letting acid back up and causing heartburn. Peptic ulcers, often caused by Helicobacter pylori infection or NSAID use, are sores in the stomach or duodenal lining. Gallstones can block bile flow and cause severe pain, while appendicitis is a surgical emergency. Because the digestive tract absorbs all nutrients and much of the body’s water, prolonged vomiting or diarrhea can rapidly cause dehydration, a key concern in emergency and nursing care.