Step by Step
1
Emulsification by bile salts
Since fats are water-insoluble, bile salts (produced by the liver, stored in the gallbladder, released when triggered by CCK) emulsify large fat globules into tiny droplets, dramatically increasing the surface area available for pancreatic lipase to act on.
2
Lipase and micelle formation
Pancreatic lipase cleaves triglycerides into fatty acids and monoglycerides. These then combine with bile salts to form micelles — tiny soluble packages that ferry the fatty acids and monoglycerides to the enterocyte brush border for absorption.
3
Reassembly and chylomicron formation
Once inside the enterocyte, fatty acids and monoglycerides are reassembled into triglycerides in the smooth ER, then packaged together with cholesterol and apoprotein B-48 into chylomicrons.
4
The unique lymphatic route — bypassing the portal system
Unlike most digested nutrients, chylomicrons enter lacteals (lymphatic capillaries within the intestinal villi) rather than blood capillaries, travel through the thoracic duct, and only then enter the bloodstream — bypassing the portal system entirely. The one exception: short-chain fatty acids are small enough to go directly into the portal blood, just like other digested nutrients.
Applied Walkthrough
1
A fatty meal triggers CCK release, causing the gallbladder to contract and release bile salts into the duodenum, where they emulsify large fat globules into much smaller droplets.
2
Pancreatic lipase acts on these now much more accessible fat droplets, cleaving triglycerides into fatty acids and monoglycerides, which combine with bile salts to form micelles.
3
These micelles ferry the fatty acids and monoglycerides to the enterocyte brush border, where they diffuse in and are reassembled into triglycerides, then packaged into chylomicrons along with cholesterol and apoprotein B-48.
4
Rather than entering a blood capillary like other absorbed nutrients, these chylomicrons enter a lacteal (a lymphatic capillary within the villus), travel through the thoracic duct, and only then join the bloodstream — completely bypassing the portal system that carbohydrates and proteins use.
Exam Application
Exams test whether you can trace the full fat digestion and absorption pathway, and specifically whether you know that chylomicrons enter the lymphatic system (via lacteals) rather than the portal blood, unlike carbohydrates and proteins.
⚠ Common Trap
The most common trap is assuming all absorbed nutrients enter the portal blood like carbohydrates and proteins do — chylomicrons (packaged fats) specifically enter the lymphatic system via lacteals, bypassing the portal system, with only short-chain fatty acids being a direct exception.
✓ Quick Self-Check
1. What role do bile salts play in fat digestion?
They emulsify fat globules into smaller droplets, increasing surface area for lipase to act on.
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2. What are micelles, and what is their function?
Packages formed from bile salts and digested fat products that ferry fatty acids and monoglycerides to the enterocyte for absorption.
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3. What is a chylomicron?
A package of reassembled triglycerides, cholesterol, and apoprotein B-48, formed inside the enterocyte.
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4. How do chylomicrons enter the bloodstream, and how does this differ from carbohydrates and proteins?
Via lacteals (lymphatic capillaries) and the thoracic duct, bypassing the portal system — unlike carbohydrates and proteins, which enter the portal blood directly.
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5. What is the exception to this lymphatic route for fats?
Short-chain fatty acids, which go directly into the portal blood.
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