The duodenum is the first part of the small intestine, going from the stomach to the jejunum — a C-shaped tube about 25 centimetres long that does the heaviest chemical work of digestion. Acid chyme arrives from the stomach and is immediately neutralised by bicarbonate-rich secretions; then bile from the gallbladder emulsifies fats while the pancreas floods the lumen with enzymes — amylases, lipases, proteases — that dismantle all three macronutrients. Hormones released by the duodenal wall orchestrate the whole downstream process, signalling the gallbladder to contract and the pancreas to secrete. Most chemical digestion is completed here, in this short stretch, before absorption takes over further along. Disorders of the duodenum — ulcers, coeliac damage — strike at digestion’s busiest crossroads.