The Digestive System

Every time you eat, your body faces a challenge. Food — whether it is a sandwich, an apple, or a bowl of rice — is made of large, complex molecules that your cells cannot use directly. Your cells need tiny, simple molecules: glucose for energy, amino acids to build proteins, fatty acids for membranes and hormones. Digestion is the process of breaking food down from large molecules into small ones that can be absorbed into your blood and delivered to every cell in your body. There are two types of digestion happening at the same time: Mechanical digestion — physically breaking food into smaller pieces. Chewing is the obvious example, but your stomach also churns and mashes food. This increases the surface area available for the next step. Chemical digestion — using chemicals called enzymes to break the molecular bonds in food. Enzymes are like molecular scissors — each one cuts a specific type of bond. Amylase cuts starch. Protease cuts protein. Lipase cuts fat. Without enzymes, digestion would take days instead of hours. The entire digestive system is essentially a long tube — about 9 metres from mouth to anus — with specialised regions at each stage. Food only truly "enters" your body when it crosses the wall of this tube into your blood.

Digestion begins the moment food enters your mouth — before you even swallow. In the mouth: Your teeth chew food into smaller pieces (mechanical digestion). At the same time, your salivary glands release saliva containing salivary amylase — an enzyme that starts breaking starch into smaller sugars. This is why if you chew a plain cracker for long enough, it starts to taste sweet. Swallowing: When you swallow, food moves down the oesophagus (food pipe). The oesophagus does not digest anything — it just transports food to the stomach using waves of muscle contraction called peristalsis. Peristalsis works even upside down — the movement is muscular, not gravity. In the stomach: The stomach is a muscular bag that does two things: 1. Churns food mechanically — the thick walls contract and relax, mashing everything together 2. Releases gastric acid (hydrochloric acid, HCl) — making the stomach extremely acidic (pH 1–2). This kills most bacteria in food and activates pepsin, an enzyme that starts breaking down protein. The stomach also releases a hormone called gastrin that signals it to produce more acid. After 2–4 hours, the churned, acidified food — now called chyme (pronounced "kime") — is released in small amounts through a valve called the pyloric sphincter into the small intestine. Your stomach lining is protected from its own acid by a thick layer of mucus. When this protection breaks down, you get a stomach ulcer — a painful sore in the lining.

Despite its name, the small intestine is where most of the action happens. It is about 6 metres long and divided into three sections: the duodenum, jejunum, and ileum. The duodenum (first section): As acidic chyme arrives from the stomach, the duodenum receives help from two organs: - The pancreas releases pancreatic juice containing: bicarbonate (to neutralise the acid from the stomach), amylase (continues starch digestion), lipase (digests fats), and protease enzymes including trypsin and chymotrypsin (continue protein digestion). - The liver produces bile, stored in the gallbladder and released into the duodenum. Bile does not contain enzymes — it is a detergent that breaks fat droplets into tiny droplets (emulsification), giving lipase more surface area to work on. Without bile, fats cannot be digested properly. Absorption — the ileum: By the time food reaches the ileum (the last and longest section of the small intestine), digestion is complete. Glucose, amino acids, fatty acids, vitamins, and minerals need to be absorbed into the blood. The small intestine is perfectly designed for absorption: - The inner wall is folded into thousands of villi — tiny finger-like projections - Each villus is covered in even tinier microvilli — creating a "brush border" - Together, these folds give the small intestine a total surface area of about 250 square metres — the size of a tennis court — packed into 6 metres of intestine - Each villus has a rich blood supply and a lacteal (a lymph vessel for fat absorption) Glucose and amino acids are absorbed into the blood directly. Fatty acids are absorbed into the lacteals and travel through the lymph system before entering the blood.

By the time material reaches the large intestine (also called the colon), almost all the nutrients have been absorbed. What remains is water, undigested fibre, dead cells shed from the intestinal walls, and billions of bacteria. The large intestine's main job is to absorb water. About 1–2 litres of water are reclaimed each day. Without this, you would lose dangerous amounts of fluid in your faeces — which is exactly what happens in severe diarrhoea. The gut microbiome: The large intestine is home to trillions of bacteria — your gut microbiome. These bacteria are not harmful passengers — they are essential partners. They: - Ferment dietary fibre that your own enzymes cannot digest, producing short-chain fatty acids that feed the colon cells - Produce vitamins, including vitamin K (needed for blood clotting) and some B vitamins - Train and support your immune system - Compete with harmful bacteria, preventing infection Antibiotics kill gut bacteria along with the infection they are treating. This is why antibiotics can cause diarrhoea and why doctors sometimes recommend probiotic yoghurt during antibiotic treatment. Faeces are formed as water is absorbed and bacteria, fibre, and dead cells compact. The final product is stored in the rectum and expelled through the anus under voluntary control. Clinical connection: Bowel cancer (colorectal cancer) is one of the most common cancers in the world. Warning signs include blood in the stool, a change in bowel habits lasting more than 3 weeks, unexplained weight loss, and abdominal pain. Regular screening (colonoscopy) is recommended for people over 50 or those with a family history.

Digestion is not just controlled by nerves — hormones play a huge role in coordinating the whole process. These are released by cells in the gut wall itself. Gastrin — released by the stomach when food arrives. Tells the stomach to produce more acid and keep churning. Also tells the pyloric sphincter to stay closed until food is ready. Secretin — released by the duodenum when acid arrives from the stomach. Tells the pancreas to release bicarbonate to neutralise the acid. Also slows gastric emptying so the small intestine is not overwhelmed. Cholecystokinin (CCK) — released by the duodenum when fat and protein arrive. Tells the gallbladder to release bile. Tells the pancreas to release digestive enzymes. Also signals to the brain that you are full — CCK is one of the hormones of satiety (feeling full). GLP-1 (glucagon-like peptide 1) — released after eating, especially carbohydrates. Tells the pancreas to release insulin. Slows gastric emptying (so glucose enters the blood more slowly). Signals fullness to the brain. A new class of diabetes and weight-loss drugs (semaglutide/Ozempic, liraglutide) mimic GLP-1 — this is why they reduce appetite so powerfully. The digestive system has its own nervous system — the enteric nervous system — sometimes called the "second brain." It contains about 500 million neurons and can control digestion independently of the brain. This is why digestion continues even if the spinal cord is damaged, and why stress (which affects the brain-gut connection) can cause stomach problems.