Digestive System

Digestion is more than food moving through a tube. Muscles move and mix it, while digestive juices break large food molecules into smaller parts. The intestinal lining absorbs useful substances, and supporting organs add bile and enzymes.

The digestive system includes a tract and supporting organs The digestive tract is a continuous route from the mouth to the anus. It includes the mouth, throat, oesophagus, stomach, small intestine, large intestine, rectum and anus. Food passes through these hollow organs. The salivary glands, liver, gallbladder and pancreas support digestion without food passing through them. Digestion has several linked jobs. Movement carries and mixes food. Mechanical digestion breaks it physically, while chemical digestion uses enzymes to split large molecules. Absorption moves useful substances into blood or lymph. Passing stool removes material that was not absorbed. The mouth and oesophagus begin the journey Chewing cuts and crushes food. Saliva moistens it and begins the chemical digestion of starch. Swallowing moves food into the oesophagus. Peristalsis then uses coordinated waves of muscle to move it towards the stomach. Rings of muscle open and close between some sections of the tract. They help control direction and timing, but they are not perfect one way valves. The stomach stores, mixes and starts protein digestion The stomach stores food for a time, churns it and mixes it with acid and enzymes. This begins protein digestion and produces a semi liquid mixture called chyme. Protection depends on a mucosal barrier, not mucus alone. Mucus and bicarbonate, tightly joined lining cells, good blood flow and rapid cell renewal all contribute. The stomach releases chyme gradually into the small intestine. Emptying time varies with meal size, composition, health conditions and medicines. The small intestine completes most digestion and absorption The small intestine receives bile from the liver and gallbladder. It also receives digestive enzymes and bicarbonate from the pancreas. Most chemical digestion and most nutrient absorption occur here. Millions of villi and microscopic surface folds create a large area for absorption. Most sugars and amino acids enter small blood vessels and travel to the liver before reaching the wider circulation. Much absorbed dietary fat enters lymph before joining the bloodstream. Water, vitamins and minerals use different routes according to their properties. Food does not follow one fixed transit time through the small intestine. The large intestine handles what remains The large intestine absorbs remaining water and electrolytes. It forms and moves stool towards the rectum, which stores stool before a bowel movement. The gut microbiome includes bacteria, fungi, viruses and other microorganisms living mainly in the large intestine. Some microbes ferment fibre and produce substances that the bowel lining can use. Microbiome composition varies between people and can change with diet, medicines, illness and other exposures. A measured change does not explain a symptom or prove disease by itself. The liver, gallbladder and pancreas provide support The liver receives much of the blood draining the digestive tract. It processes absorbed nutrients, makes bile and many proteins, and changes or removes selected substances. The gallbladder stores and concentrates bile before releasing it into the small intestine. Bile helps disperse fat into smaller droplets, but it is not a digestive enzyme. One part of the pancreas sends digestive enzymes and bicarbonate into the small intestine. Other pancreatic cells release hormones, including insulin and glucagon, into the blood. These pancreatic functions are distinct but not completely independent. Some conditions affect mainly one function, while others can affect both. Nerves and hormones coordinate digestion The enteric nervous system is a nerve network within the digestive tract wall. It helps coordinate movement, secretion and local blood flow. The brain, autonomic nerves, hormones and local signals also influence digestion. Peristalsis mainly moves contents forward, while other contractions mix them with digestive juices. Immune tissues in the gut lining help monitor microbes and support the barrier. This normally happens without treating food or every resident microbe as a threat. Symptoms and tests provide clues Abdominal pain, nausea, vomiting, bloating, swallowing difficulty and bowel changes have many possible causes. Their location or timing provides clues but does not identify one organ. Examination can assess tenderness, swelling, hydration and general illness. Bowel sounds are one observation and cannot measure intestinal function completely. Blood tests may show patterns involving inflammation, bilirubin, liver injury, bile flow or pancreatic enzymes. Tests called liver function tests do not directly measure every liver function or diagnose a cause. Imaging creates pictures of organs and ducts. Endoscopy uses a camera to inspect the lining of selected digestive organs and can take a small tissue sample. Results need interpretation with the symptoms and examination. A normal test may not exclude every digestive problem. Jaundice has several possible causes Jaundice is yellowing caused by a build up of bilirubin. It can result from increased red cell breakdown, liver disease or blocked bile flow. Blocked bile flow can arise in the bile ducts or near the pancreas. Jaundice does not point only to the liver or gallbladder. Yellow skin may be less noticeable on brown or black skin. Yellowing of the whites of the eyes, dark urine, pale stool or itching can provide further clues. Ask for an urgent GP appointment or contact NHS 111 if your skin or the whites of your eyes look yellow. This applies even when you have no pain. Call 999 or go to A&E for abdominal pain that starts very suddenly, is severe, or hurts when your abdomen is touched. The same applies if you vomit blood, pass black sticky stool, collapse or have difficulty breathing. Do not drive yourself to A&E.

Digestion combines movement, mechanical and chemical breakdown, absorption and waste removal. Different organs overlap in these jobs, while nutrients enter blood or lymph through different routes.