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🫀 Science · Human Anatomy and Physiology · Lesson 9 of 12 · 6 min

Digestive system: path, organs, enzymes and absorption

🍎 A sandwich travels several meters of muscular tubing before it is done. The TEAS asks where each piece gets broken down and where it is absorbed.

💡 Key idea

Starch starts in the mouth, protein in the stomach, fat in the small intestine, and most absorption happens in the small intestine.

🧠 The idea

The alimentary canal runs mouth, pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine, rectum and anus. Food is pushed along by PERISTALSIS, waves of smooth muscle contraction. Digestion is mechanical (chewing, churning) and chemical (ENZYMES). Salivary amylase starts starch digestion in the mouth. The stomach adds hydrochloric acid and pepsin to begin protein digestion. In the duodenum, the pancreas adds amylase, trypsin and lipase plus bicarbonate to neutralize acid, and BILE, made by the liver and stored in the gallbladder, emulsifies fat. Most nutrients are absorbed in the small intestine, whose villi and microvilli give it a huge surface area; fats enter lymph vessels called lacteals. The large intestine mainly absorbs water and electrolytes and houses bacteria that make some vitamin K. The liver, gallbladder and pancreas are ACCESSORY ORGANS: food never passes through them.

✏️ Worked example

A patient has had the gallbladder removed. Which nutrient is hardest to digest? Step 1: the gallbladder stores and concentrates bile. Step 2: bile emulsifies fat into small droplets so lipase can work. Step 3: without a store of bile, a large fatty meal is handled poorly. Answer: fat. Carbohydrate and protein digestion depend on enzymes that do not need bile.

📌 Takeaways

  • Mouth → esophagus → stomach → duodenum → jejunum → ileum → large intestine → rectum → anus.
  • Amylase = starch, pepsin and trypsin = protein, lipase = fat, bile = emulsifies fat (not an enzyme).
  • Small intestine absorbs most nutrients; large intestine absorbs mostly water.

✅ Check yourself

Question 1 of 3

Which organ produces bile?

  1. Liver
  2. Duodenum
  3. Gallbladder
  4. Pancreas
Show the answer and rationale

Answer: A

The liver makes bile. The gallbladder only stores and concentrates it, which is the classic trap. The pancreas makes digestive enzymes and bicarbonate, and the duodenum is where bile is delivered.

Question 2 of 3

In which organ is most of the absorption of nutrients completed?

  1. Stomach
  2. Small intestine
  3. Esophagus
  4. Large intestine
Show the answer and rationale

Answer: B

The small intestine, lined with villi and microvilli, absorbs most nutrients. The stomach absorbs very little (some water, alcohol), the large intestine absorbs mainly water and electrolytes, and the esophagus only transports food.

Question 3 of 3

Which enzyme begins the chemical digestion of protein in the stomach?

  1. Salivary amylase
  2. Lipase
  3. Lactase
  4. Pepsin
Show the answer and rationale

Answer: D

Pepsin, activated by stomach acid, starts breaking proteins into smaller peptides. Salivary amylase acts on starch, lipase on fats, and lactase on the milk sugar lactose in the small intestine.

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