Why food needs to be broken down
Food is mostly made of large molecules: starches, proteins and fats. The body cannot use them in that form. Digestion breaks them into small units, such as simple sugars, amino acids, fatty acids and glycerol, that can pass through the wall of the gut into the bloodstream. It does this with two kinds of action. Mechanical digestion physically chops and churns food, and chemical digestion uses enzymes, proteins that speed up the chemical reactions that split large molecules apart.
The digestive system is basically a long, winding tube from mouth to anus, with helper organs (the salivary glands, liver, gallbladder and pancreas) that add fluids to it.
The mouth
Digestion starts before swallowing. Teeth cut and grind food into smaller pieces, and the tongue mixes it with saliva. Saliva moistens food and contains an enzyme, amylase, that begins breaking starch into smaller sugars. This is why a piece of bread held in the mouth for a while may start to taste slightly sweet.
The esophagus
Swallowing pushes the softened food, called a bolus, into the esophagus. Waves of muscle contraction called peristalsis squeeze it downward. These waves are automatic, so food reaches the stomach even if you are lying down.
The stomach
The stomach is a muscular bag that churns food and mixes it with gastric juice. This fluid contains hydrochloric acid and the enzyme pepsin, which starts breaking down proteins. The acid is strong, and on the pH scale the stomach contents are typically very acidic. The acid also kills many microbes that arrive with food. A layer of mucus protects the stomach lining from being digested itself.
After a few hours, depending on the meal, the stomach has turned its contents into a thick, soupy mixture called chyme and releases it in small amounts into the small intestine.
The small intestine
Most digestion and nearly all nutrient absorption happen in the small intestine, a coiled tube several meters long. Its first section is the duodenum, followed by the jejunum and then the ileum.
Three helpers add fluids here:
- The pancreas releases enzymes that act on carbohydrates, proteins and fats, along with a fluid that neutralizes stomach acid so the enzymes can work.
- The liver makes bile, which is stored in the gallbladder and released as needed. Bile does not chemically split fat. It breaks large fat globules into tiny droplets, somewhat like the way detergent works in how soap works, so enzymes can reach more fat.
- The intestinal wall itself produces enzymes that finish breaking down small sugars and protein fragments.
How nutrients are absorbed
The inner lining of the small intestine is folded and covered with microscopic finger-like projections called villi, and the cells on them have even tinier projections. This greatly increases the surface area available for absorption, far more than a smooth tube of the same size would have.
Sugars and amino acids pass through the cells into tiny blood vessels. They travel by the portal vein to the liver, which processes and stores many nutrients before they circulate further, eventually reaching every part of the human body by way of the human heart’s pumping. Fats are handled differently: much of the fat is repackaged and enters lymph vessels before joining the blood.
Vitamins and minerals are also absorbed here, and water is absorbed throughout the gut. Some nutrients need specific helpers; for example, vitamin B12 absorption depends on a protein made in the stomach.
The large intestine
What remains, mostly water, fiber and dead cells, moves into the large intestine, or colon. Its main job is absorbing water and some salts, turning the liquid waste into more solid stool. The colon also hosts a very large community of bacteria that ferment some of the fiber we cannot digest ourselves. This is the subject of the separate article on the gut microbiome. Stool is stored in the rectum and eliminated through the anus.
The whole journey usually takes from one to several days, with large differences between people and between meals.
The gut and the nervous system
The gut has its own network of nerve cells, often called the enteric nervous system, which coordinates muscle movements and secretions largely on its own. It also communicates with the brain through nerves and chemical signals. This is one reason emotions and the stress response can influence digestion, such as the feeling of “butterflies” or an upset stomach when nervous. The link between gut and brain is an active area of research, and much about it is still being worked out.
When digestion goes wrong
Problems can range from short-lived upsets, such as an infection or food intolerance, to long-term conditions. Symptoms such as persistent pain, bleeding, unexplained weight loss or changes in bowel habits are good reasons to seek medical attention. This article offers general educational information only and is not medical advice or a way to diagnose anything, and individual cases differ, so a qualified health professional should be consulted about any concerns.