What digestion has to accomplish
Food is made of large molecules: starches, proteins and fats, along with vitamins, minerals, fiber and water. The body cannot use most of these directly. Digestion breaks them into small pieces that can be taken across the lining of the gut, and absorption moves those pieces into the blood or lymph. What cannot be absorbed continues along the tract and is eventually eliminated.
Two kinds of breakdown work together. Mechanical digestion is chewing, churning and mixing. Chemical digestion relies on enzymes, protein catalysts that split molecules at specific points, along with acid and bile.
The digestive tract is essentially a long, muscular tube from mouth to anus, with accessory organs, the salivary glands, liver, gallbladder and pancreas, that add secretions along the way.
Mouth and esophagus
Digestion begins before swallowing. Chewing breaks food into smaller pieces and mixes it with saliva, which moistens it and contains an enzyme, amylase, that begins splitting starch. The tongue forms the food into a soft mass called a bolus, which is swallowed.
The esophagus carries it to the stomach by peristalsis, waves of muscular contraction that push contents forward. This is why swallowing works even when someone is upside down. A ring of muscle at the lower end of the esophagus normally keeps stomach contents from flowing back up.
The stomach
The stomach is a stretchy, muscular sac. Its lining releases hydrochloric acid, which makes the contents strongly acidic, helps denature proteins and kills many microbes. It also secretes enzymes, including pepsin, which begins breaking down proteins, and a layer of mucus that protects the lining from the acid.
Muscular churning turns the food into a thick liquid called chyme, which is released in small amounts into the small intestine. The time food spends in the stomach depends on the meal; fatty meals typically leave more slowly.
The small intestine: the main event
Most chemical digestion and nearly all nutrient absorption happens here. The small intestine is several meters long in a living person and divided into three parts: the duodenum, jejunum and ileum.
As chyme enters the duodenum, secretions arrive from two accessory organs:
- The pancreas releases enzymes that digest carbohydrates, proteins and fats, as well as bicarbonate that neutralizes the stomach acid.
- The liver makes bile, stored in the gallbladder and released into the duodenum. Bile does not contain digestive enzymes; instead it breaks large fat droplets into smaller ones, giving the fat-digesting enzymes more surface to work on.
Cells in the intestinal wall have their own enzymes on their surfaces that finish the job, producing simple sugars, amino acids and fatty acids.
The inner wall is built to absorb. It is folded, covered in tiny finger-like projections called villi, and each of these cells is in turn covered with microscopic projections. This arrangement hugely increases the surface area available. Sugars and amino acids enter capillaries and travel via the portal vein to the liver, which processes and regulates what reaches the rest of the body. This links to blood sugar regulation, since the liver and pancreas help control sugar levels after a meal. Digested fats are repackaged and mostly enter lymph vessels first. Vitamins and minerals are absorbed here too, and certain ones need special help, as with vitamin B12, which requires a protein made in the stomach for absorption.
The large intestine
What remains, mostly water, fiber and indigestible residue, moves into the large intestine (colon). Its main roles are absorbing water and some salts and forming stool. Transit through the colon is slow, often taking many hours to days, and varies from person to person.
The colon is also home to the densest community of microbes in the body. These microbes ferment some fibers that human enzymes cannot digest, producing short-chain fatty acids, which the colon’s lining uses for energy. This ecosystem is the subject of its own article on the microbiome. The final section of the tract, the rectum, stores stool until it is released.
Control of the system
The gut has its own extensive network of nerves, sometimes called the enteric nervous system, which can coordinate movement and secretion with considerable independence, while also communicating with the brain. Hormones released by the gut lining regulate secretions, appetite and the speed of emptying. That is why the sight and smell of food, stress and emotions can affect digestion.
Common variation and problems
Intolerances and allergies involve different mechanisms. Lactose intolerance, for example, is caused by low levels of the enzyme that breaks down milk sugar, whereas food allergic reactions involve the immune system. Reflux, ulcers, gallstones and bowel disorders are among the many conditions that affect parts of the tract, with causes that vary.
Eating patterns affect what reaches each stage. Understanding macronutrients and micronutrients helps explain what the gut is absorbing, and a balanced diet supplies a mix of them.
This is a general overview and not medical advice. Persistent digestive symptoms are best discussed with a healthcare professional.