How the Digestive System Works from Mouth to Intestine

Digestion begins in the mouth. Teeth break food into smaller pieces, increasing its surface area, while saliva moistens the food and contains enzymes that begin breaking down some carbohydrates. The tongue helps form the food into a bolus that can be swallowed.

The bolus moves down the oesophagus by peristalsis, waves of muscular contraction in the gut wall. It then enters the stomach, where muscular mixing combines food with acidic digestive fluid. The stomach begins protein digestion and also helps control how quickly partly digested material passes into the small intestine.

When learning how the digestive system works, separate mechanical digestion from chemical digestion. Mechanical actions physically break and mix food, while enzymes and other chemicals change large molecules into smaller molecules that the body can absorb.

The small intestine receives digestive enzymes from its own lining and from the pancreas. It also receives bile, made by the liver and stored in the gallbladder. Bile helps neutralise acidic material from the stomach and breaks large fat droplets into smaller droplets, giving enzymes more surface area to work on.

By the time digestion has progressed, carbohydrates can be broken into simple sugars, proteins into amino acids, and fats into fatty acids and glycerol-related products. These small molecules can cross the wall of the small intestine and enter blood or lymph vessels for transport around the body.

The lining of the small intestine has many folds and tiny projections called villi. These greatly increase surface area for absorption. A good explanation of how the digestive system works therefore connects structure with function: thin surfaces, rich blood supply, and large area all help nutrients move efficiently into the body's transport systems.

After most nutrients have been absorbed, the remaining material continues through the large intestine. Key jobs include:

• •Absorbing much of the remaining water and some salts.
• •Housing a large community of gut microorganisms.
• •Compacting undigested material into faeces.
• •Moving waste toward the rectum for storage.
• •Removing waste from the body through defecation.

Digestion also depends on enzymes that work on particular molecules. Amylases break down starches, proteases break down proteins, and lipases help break down fats. These reactions happen in different parts of the digestive tract under conditions suited to the enzymes involved. The pancreas is especially important because it releases several digestive enzymes into the small intestine. When revising how the digestive system works, link each organ to a job rather than memorising a long route alone. Ask what is broken down there, what chemical is added, and whether absorption happens at that point.

Much early knowledge of the stomach came from an unusual accident. In 1822, a young fur trader named Alexis St Martin was accidentally shot in the side at a trading post on Mackinac Island in what is now Michigan. The army surgeon William Beaumont treated him, and St Martin survived, but the wound healed in a way that left a permanent opening, called a fistula, from his skin into his stomach. Beaumont realised that he could look directly inside a working human stomach, something no physician had been able to do before.

Over several years Beaumont ran experiments. He tied pieces of food to silk thread, lowered them through the opening and pulled them out at intervals to see how far they had been digested. He also collected gastric juice and showed that it could digest food in a glass outside the body, which supported the idea that digestion is a chemical process rather than just grinding or rotting. He published his observations in 1833. The work is still discussed for its ethics too, because St Martin was poor, dependent on Beaumont, and in no strong position to refuse.

The stomach produces hydrochloric acid strong enough to give its contents a pH of roughly 1.5 to 3.5, together with a protein-digesting enzyme called pepsin. Since the stomach wall is itself made largely of protein, it needs protection. Cells in the lining secrete a thick layer of mucus, and bicarbonate held in that layer neutralises acid right next to the cells. Pepsin is also released in an inactive form, pepsinogen, which only becomes active once it meets acid inside the stomach, so the cells that make it are not attacked. The surface lining is also replaced every few days.

When this protection fails, an ulcer can form. For decades many doctors believed ulcers were caused mainly by stress and rich or spicy food. In the early 1980s two Australian researchers, Barry Marshall and Robin Warren, argued that a bacterium, Helicobacter pylori, which can survive in the stomach lining, was a major cause. Many colleagues were sceptical, so Marshall drank a culture of the bacteria and developed stomach inflammation. Their work showed that many ulcers could be cured with antibiotics, and it earned them the Nobel Prize in Physiology or Medicine in 2005.

Lactose, the sugar in milk, shows how much digestion depends on having the right enzyme. Lactose is a double sugar that must be split by an enzyme called lactase, made by cells lining the small intestine, into glucose and galactose before it can be absorbed. Babies produce plenty of lactase. In most mammals, and in most people worldwide, production falls after early childhood. If lactose then passes undigested into the large intestine, gut bacteria ferment it, producing gas, and the sugar draws water into the bowel, which can cause bloating, cramps and diarrhoea.

Some people keep making lactase throughout adult life, a trait called lactase persistence. It is common in northern Europe and among certain herding peoples of East Africa and the Middle East, and genetic studies have found that it arose through several different mutations in different populations, all affecting the switch that controls the lactase gene. Its spread appears linked to the history of dairy farming, since being able to digest fresh milk provided extra food and fluid. So lactose intolerance is not a disease in the usual sense. It is the ancestral human pattern, and persistence is the newer variation.

How the digestive system works can be followed as a simple route: food is mechanically and chemically broken down, useful molecules are absorbed mainly in the small intestine, water is recovered in the large intestine, and remaining waste leaves the body. Follow the movement of food and ask what each organ adds or absorbs, and digestion becomes a connected process.