Digestive Anatomy

The digestive system is essentially a long tube — about 9 metres in total — running from mouth to anus. Its job is to break down food into molecules small enough to be absorbed into the bloodstream, extract nutrients and water, and expel waste. The process involves four key activities: 1. Ingestion — taking food in through the mouth 2. Digestion — breaking food down mechanically (chewing, churning) and chemically (enzymes) 3. Absorption — passing nutrients through the gut wall into the blood or lymph 4. Defecation — eliminating undigested waste The digestive system consists of the gastrointestinal (GI) tract (the continuous tube from mouth to anus) plus accessory organs — the liver, gallbladder, and pancreas — that support digestion by producing secretions that enter the GI tract.

The mouth (oral cavity) Digestion begins in the mouth. The teeth mechanically break food into smaller pieces (mastication). The salivary glands produce saliva — a watery secretion containing salivary amylase, an enzyme that begins breaking down starch into sugars. Saliva also lubricates food into a soft ball called a bolus for swallowing. The oesophagus A muscular tube, approximately 25 cm long, that connects the throat to the stomach. It passes through the chest behind the heart and through a hole in the diaphragm (the oesophageal hiatus) to enter the abdomen. The oesophagus does not digest food — it only transports it. This happens through peristalsis — coordinated waves of muscular contraction that push the bolus downward. Peristalsis is so powerful that you can swallow upside down. At the lower end, a muscular ring called the lower oesophageal sphincter (LOS) prevents stomach acid from refluxing back up. When the LOS is weak or relaxed inappropriately, acid enters the oesophagus — causing gastro-oesophageal reflux disease (GORD), felt as heartburn. The stomach A J-shaped, muscular organ in the upper left abdomen. It has several functions: - Storage — can hold up to 1.5 litres, allowing large meals to be digested over time - Mechanical digestion — three layers of muscle churn food into a liquid paste called chyme - Chemical digestion — gastric glands in the stomach lining produce: hydrochloric acid (HCl, pH 1–2) which kills bacteria and activates enzymes; pepsinogen, which is activated by HCl to become pepsin — a protease that begins protein digestion; and intrinsic factor, essential for vitamin B12 absorption - Controlled release — the pyloric sphincter at the stomach exit regulates the release of chyme into the small intestine, preventing the intestine from being overwhelmed The stomach lining protects itself from its own acid with a thick layer of mucus. When this protection fails — due to H. pylori infection or NSAIDs — ulcers develop.

The small intestine is where the vast majority of digestion and absorption takes place. Despite its name, it is about 6–7 metres long — it is "small" in diameter (about 2.5 cm), not in length. It has three sections: 1. Duodenum — the first 25 cm, shaped like a C. This is the most active digestive region. It receives chyme from the stomach, bile from the liver/gallbladder, and pancreatic juice from the pancreas. The mixing of these secretions neutralises the acid from the stomach (bile and pancreatic juice are alkaline) and provides enzymes for fat, protein, and carbohydrate digestion. 2. Jejunum — the middle 2.5 metres. Most absorption occurs here. The walls are highly adapted for absorption with three structural features that massively increase surface area: - Plicae circulares (circular folds) — large folds of the mucosa visible to the naked eye - Villi — finger-like projections of the mucosa, 1 mm tall, visible under a magnifying glass - Microvilli — microscopic projections on each villus cell, forming the brush border Together, these three adaptations increase the absorptive surface area from ~0.5 m² (a flat tube) to ~250 m² — about the size of a tennis court. 3. Ileum — the final 3.5 metres. Absorbs remaining nutrients, including vitamin B12 (bound to intrinsic factor) and bile salts (recycled back to the liver — the enterohepatic circulation). Contains the most lymphoid tissue in the gut (Peyer's patches) for immune surveillance. The small intestine meets the large intestine at the ileocaecal valve — a sphincter that controls flow between them and prevents backflow.

The large intestine (colon) About 1.5 metres long and 6–7 cm in diameter. Its main jobs are: - Absorbing water and electrolytes from undigested material (converting liquid chyme into solid faeces) - Housing the gut microbiome — trillions of bacteria that ferment undigested fibre, produce vitamin K and some B vitamins, and contribute to immune function - Storage and eventual expulsion of faeces Sections: caecum (where the appendix attaches) → ascending colon → transverse colon → descending colon → sigmoid colon → rectum → anal canal → anus. The appendix is a finger-like projection from the caecum. Its function in modern humans is debated — it may serve as a reservoir for gut bacteria. Clinically it is important because it can become inflamed (appendicitis) — a common surgical emergency. The liver The largest internal organ (1.5 kg). The liver has over 500 known functions. Key ones include: - Producing bile — a green-yellow fluid containing bile salts that emulsify fats (break large fat droplets into tiny ones, massively increasing the surface area available for lipase enzymes) - Metabolising nutrients absorbed from the gut — processing glucose, amino acids, and fats - Producing plasma proteins including albumin (maintains blood osmotic pressure) and clotting factors - Detoxifying drugs, alcohol, and other toxins - Storing glycogen (energy reserve), vitamins A, D, B12, and iron All blood from the gut travels to the liver first via the portal vein — giving the liver first access to all absorbed nutrients. The gallbladder A small pear-shaped sac on the underside of the liver. It stores and concentrates bile produced by the liver. When a fatty meal arrives in the duodenum, the hormone CCK (cholecystokinin) triggers gallbladder contraction, squirting concentrated bile into the duodenum via the common bile duct. Gallstones form when bile becomes supersaturated with cholesterol or bilirubin. They can block the bile duct, causing severe colicky right upper quadrant pain (biliary colic) and jaundice. The pancreas A dual-function gland behind the stomach. Its exocrine function (relevant here): produces pancreatic juice — a mixture of bicarbonate (to neutralise stomach acid) and digestive enzymes including amylase (carbohydrates), lipase (fats), and proteases like trypsin (proteins). These enter the duodenum via the pancreatic duct. Its endocrine function — producing insulin and glucagon — is covered in the Endocrinology section.

Appendicitis — inflammation of the appendix, usually due to obstruction and bacterial overgrowth. Classically presents with central abdominal pain migrating to McBurney's point (one third of the way from the right anterior iliac spine to the umbilicus). Untreated, it ruptures causing life-threatening peritonitis. Crohn's disease and ulcerative colitis — inflammatory bowel diseases. Crohn's can affect any part of the GI tract from mouth to anus (with skip lesions); ulcerative colitis affects only the colon and rectum continuously. Both cause diarrhoea, abdominal pain, and weight loss. Colorectal cancer — one of the most common cancers. Usually arises from pre-existing adenomatous polyps in the colon. Symptoms include change in bowel habit, rectal bleeding, and weight loss. Colonoscopy detects and removes polyps before they become cancerous. Cirrhosis — end-stage liver scarring from chronic damage (alcohol, hepatitis B/C). The scarred liver cannot perform its functions — leading to jaundice, ascites (abdominal fluid), coagulopathy, and hepatic encephalopathy. The scarring also obstructs portal blood flow — causing portal hypertension and varices (enlarged, fragile veins in the oesophagus that can rupture catastrophically). Cholecystitis — inflammation of the gallbladder, usually from a gallstone blocking the cystic duct. Causes right upper quadrant pain radiating to the right shoulder tip (referred pain via the phrenic nerve), fever, and nausea. Treatment is cholecystectomy (gallbladder removal).