🌀 Digestive System Lesson

"Dirty Jobs In": the three small intestine regions

The small intestine isn't one uniform tube — its three regions each specialize in a different part of digestion and absorption.

Duoden.

Jejunum

Ileum

📖 Full Breakdown

Duodenum, Jejunum, Ileum — length, function, and what each absorbs

Position in the sequence directly predicts what each region is responsible for.

Duodenum

~25 cm — where digestion accelerates

Receives chyme from the stomach along with bile from the liver/gallbladder and enzymes from the pancreas, entering at the ampulla of Vater. Most chemical digestion occurs here, but relatively little absorption.

Jejunum

~2.5 m — the main site of absorption

Where the bulk of carbohydrate, protein, fat, and vitamin absorption takes place, aided by villi and microvilli that dramatically increase surface area.

Ileum

~3.5 m — the specialist

The only site where vitamin B12 (with intrinsic factor) and bile salts are absorbed. Connects to the large intestine at the ileocecal valve.

🩺 Clinical / Exam Application

A patient who has had their terminal ileum surgically removed (due to Crohn's disease) develops vitamin B12 deficiency years later, despite eating a normal diet rich in B12. Because the ileum is the only site in the entire digestive tract where B12 can be absorbed — and intrinsic factor from the stomach is required for that absorption to happen there — no other region of the small intestine can compensate for its loss.

⚠️ Exam Alert

A frequently tested fact: intrinsic factor (from stomach parietal cells) is required for B12 absorption, but the actual absorption site is the terminal ileum, not the stomach. Exams test both halves of this two-step process.

🚧 Common Trap

Don't assume the duodenum does the most absorption just because digestion is most intense there. The duodenum is primarily a mixing and enzyme-delivery zone — the jejunum is the true absorption workhorse for most nutrients.

✅ Quick Check

Which region of the small intestine would be most affected if a patient could not absorb vitamin B12, and why can't the jejunum compensate?

📝 Exam Prep

Common Exam Questions

❓ What are the three regions of the small intestine in order?

✅ Duodenum (~25 cm, receives bile and pancreatic enzymes), Jejunum (~2.5 m, main site of nutrient absorption), and Ileum (~3.5 m, absorbs vitamin B12 and bile salts).

❓ Why is the ileum uniquely important for vitamin B12 absorption?

✅ The terminal ileum is the only site in the digestive tract with the specific receptors needed to absorb the vitamin B12–intrinsic factor complex — no other region of the small or large intestine can absorb B12.