Gastrointestinal system

What the exam asks

Expect a stem that removes or blocks one thing and asks what follows: a bile duct obstruction, a missing brush border enzyme, a drug that raises gastric pH, or a resected segment of bowel. The reasoning is always the same, so practise it rather than memorising organs: name what that structure supplied, ask which step now cannot happen, and follow the consequence to what accumulates and what is lost. The trap that catches most candidates is bile. It is described alongside enzymes, so it gets read as one, and the wrong option in a bile duct question is the one saying fat cannot be chemically broken down. Lipase is still there and still working; what it has lost is surface area, so fat digestion is slow and incomplete rather than absent. The second trap is forgetting that fat-soluble vitamins ride with fat, so anything impairing fat absorption takes vitamins A, D, E and K with it.

The gut is responsible for breaking food into molecules small enough to cross a membrane, absorbing them, and recovering the water it used doing it. The tract is a tube, and the useful way to hold it is as a series of compartments with different conditions: the mouth and stomach break things down, the small intestine finishes digestion and does essentially all the nutrient absorption, and the large intestine reclaims water. The liver and pancreas are not in the tube; they are glands feeding into it, and the exam treats them as such.

Two themes carry most questions. The first is pH: each compartment runs at a pH its own enzymes need, and an enzyme moved into the wrong compartment stops working. Saliva is near neutral, the stomach is strongly acidic, and the duodenum is neutralised by pancreatic bicarbonate because pancreatic enzymes cannot work in acid. The second is surface area, which is why the small intestine is folded and covered in villi and microvilli, and why fat is emulsified before it is digested. Both themes let you predict what a blockage or a missing secretion does.

What you carry in is which organ does what, the pH of each segment, and the reasoning: proteases are secreted as inactive precursors because a tube that digests protein is made of protein, bile emulsifies rather than digests, fat leaves by lymph while sugars and amino acids leave by the portal vein to the liver, and anything left undigested in the lumen holds water there.

What to hold

• Digestion is chemical breakdown into absorbable molecules; absorption is the crossing of the gut wall, and a substance can be digested perfectly and still not absorbed.
• Salivary amylase starts on starch at near neutral pH and is destroyed by stomach acid, so digestion in the mouth stops shortly after swallowing.
• The stomach's parietal cells make hydrochloric acid and its chief cells make pepsinogen, which the acid converts to active pepsin, so pepsin needs a low pH both to be made and to work.
• Neutralise the stomach and pepsin fails on both counts, but protein digestion largely survives because the pancreas supplies proteases downstream.
• Proteases are secreted as inactive zymogens and activated only in the lumen, because a gland that made active protease inside itself would digest itself.
• The pancreas supplies both bicarbonate, which raises duodenal pH so its enzymes can work, and the enzymes for all three food classes.
• Bile is made by the liver, stored and concentrated by the gallbladder, and contains no enzymes at all.
• Bile salts emulsify fat: they break large droplets into many small ones, which increases the surface area lipase can act on without breaking a single bond.
• Almost all nutrient absorption happens in the small intestine, whose folds, villi and microvilli exist to multiply the surface area available for it.
• Sugars and amino acids leave the gut in the blood of the hepatic portal vein and reach the liver before the rest of the body, which is why the liver sees and processes them first.
• Fat leaves differently: absorbed products are repackaged and enter the lacteals as lymph, bypassing the portal vein and so bypassing the liver's first pass.
• The terminal ileum reabsorbs bile salts for reuse and absorbs vitamin B12 bound to intrinsic factor from the stomach, which is why disease at that one site produces two unrelated deficiencies.
• The large intestine absorbs water and electrolytes rather than nutrients, so its failure costs water rather than nutrition.
• Solute left unabsorbed in the lumen holds water there osmotically, which is the mechanism behind most malabsorption diarrhoea.