Digestion, GI Motility & Liver Functions

Digestion is the breakdown of food into molecules small enough to absorb: mechanical digestion (chewing, churning, muscular squeezing called motility) and chemical digestion (enzymes and acid cutting molecules apart). The liver supports the process by making bile, processing absorbed nutrients, and clearing toxins from the blood.

This matters because nearly every drug you swallow depends on this system, and its failures are among the most common diseases — acidity, gallstones, fatty liver, and jaundice. Where each nutrient is digested explains why some medicines must be taken with food and why liver tests look the way they do.

Key Concepts

• Peristalsis: waves of contraction pushing food forward; segmentation is the back-and-forth mixing of the small intestine.
• Digestive phases: cephalic (sight/smell trigger juices), gastric (food in the stomach), intestinal (chyme in the duodenum) — switching secretions on and off.
• Key enzymes: salivary amylase starts starch digestion; pepsin (activated by stomach acid) digests protein; pancreatic lipase, amylase, and trypsin finish in the duodenum; brush-border enzymes like lactase complete carbohydrate digestion.
• Bile salts: made by the liver, stored in the gallbladder; they emulsify fat into droplets so lipase can attack. Bile contains no enzymes.
• Liver functions: bile production, albumin and clotting-factor synthesis, glycogen storage, ammonia-to-urea conversion, bilirubin processing, drug and alcohol detoxification.
• Portal circulation: gut blood flows first to the liver through the portal vein, so the liver inspects everything absorbed before it reaches the body.

How It Works

Follow a meal. Chewing and saliva start starch digestion; the esophagus moves the bolus down in seconds. In the stomach, acid (pH 1.5–3.5) kills germs and unfolds proteins while pepsin cuts them, and churning turns food into chyme. The pylorus squirts chyme into the duodenum, where CCK triggers gallbladder contraction and pancreatic enzyme release, and secretin triggers bicarbonate neutralising the acid. Enzymes reduce the meal to fatty acids, sugars, and amino acids, which cross into blood or lymph. Everything absorbed travels the portal vein to the liver — storing glucose as glycogen, making plasma proteins, converting ammonia to urea. Leftovers reach the colon, which absorbs water and salts, and the rest leaves as stool about a day later.

Worked Example

How much acid does the stomach make in a day? About 1.5 litres of gastric juice at roughly pH 2. Step 1: pH 2 means [H⁺] = 10⁻² mol/L = 0.01 mol/L. Step 2: 0.01 × 1.5 = 0.015 mol of H⁺ per day — about 15 millimoles of pure acid manufactured daily by parietal cells, which is exactly why proton-pump inhibitors are so effective against acidity.

Common Misconceptions

• The stomach does most digestion. It only starts protein digestion; the small intestine does the bulk of chemical digestion and nearly all absorption.
• The liver makes digestive enzymes. It makes bile — zero enzymes, only fat emulsification. Digestive enzymes come from salivary glands, stomach, pancreas, and gut wall.
• The gallbladder is essential. It merely stores and concentrates bile; after removal the liver drips bile continuously and digestion adapts within weeks.

Real-World Applications

• Antacids and proton-pump inhibitors treat acidity by neutralising or blocking that daily acid load.
• Gallstones form when bile cholesterol crystallises; removal of the gallbladder cures the attacks.
• Liver function tests (bilirubin, albumin, ALT) read out the liver's synthetic and processing jobs.
• Oral drugs are dosed around meals because food changes gastric emptying and first-pass liver metabolism.

Digestion pairs mechanical motility with chemical attack by acid and enzymes, finishing in the small intestine while the liver filters everything through the portal vein. The stomach makes about 15 millimoles of acid a day, bile emulsifies fat without enzymes, and the liver stores, builds, and detoxifies.