What happens to food in the small intestine — absorption explained

The small intestine is about 6–7 metres long and divided into three parts:

• Duodenum (~25 cm): The first part, where most digestion occurs; receives chyme from stomach, bile from liver, and pancreatic juice from pancreas
• Jejunum (~2.5 m): Main site of nutrient absorption
• Ileum (~3.5 m): Absorbs remaining nutrients; specific site for vitamin B₁₂ and bile salt reabsorption

Structural adaptations for absorption:

• Villi (finger-like projections of mucosa): Increase surface area 5–7×
• Microvilli (brush border on villi epithelial cells): Increase surface area further ~20× — together villi + microvilli give total surface area of ~200–250 m²
• Rich blood supply and lacteals (lymph capillaries) inside each villus

From the pancreas (pancreatic juice, pH 7.8–8.2):

• Trypsin (from trypsinogen activated by enterokinase): Breaks peptide bonds — endopeptidase
• Chymotrypsin (from chymotrypsinogen): Cleaves peptide bonds adjacent to aromatic amino acids
• Carboxypeptidase: Exopeptidase — removes C-terminal amino acids
• Pancreatic amylase: Hydrolyses starch → maltose + dextrins
• Pancreatic lipase: Breaks down fats into fatty acids + glycerol (requires bile salts for emulsification)
• Nucleases: Hydrolyse nucleic acids (DNA, RNA) → nucleotides

From the liver (bile, stored in gallbladder, pH 7.7):

• Bile salts (not enzymes!): Emulsify fats — break large fat droplets into tiny ones (increasing surface area for lipase action)
• Bile pigments (bilirubin, biliverdin): Products of haemoglobin degradation — excreted in faeces; give stool its brown colour

From intestinal glands (intestinal juice / succus entericus):

• Enterokinase: Activates trypsinogen → trypsin
• Maltase: Maltose → 2 glucose
• Sucrase: Sucrose → glucose + fructose
• Lactase: Lactose → glucose + galactose
• Aminopeptidase and dipeptidase: Final breakdown of small peptides → amino acids

Carbohydrates must be fully broken down to monosaccharides (glucose, fructose, galactose) before absorption.

Mechanism:

• Glucose and galactose: Absorbed by secondary active transport (co-transport with Na⁺ via SGLT1 transporter). Na⁺ gradient (maintained by Na⁺/K⁺ ATPase) drives glucose into the cell.
• Fructose: Absorbed by facilitated diffusion (via GLUT5) — passive, no energy needed
• Once inside the enterocyte, all monosaccharides exit into the bloodstream through GLUT2 (facilitated diffusion)
• Enter portal vein → liver → general circulation

Undigested carbohydrates (dietary fibre) pass to the large intestine.

Proteins:

• Absorbed as amino acids and small di/tripeptides
• Amino acids: Absorbed by Na⁺ co-transport (similar to glucose)
• Di/tripeptides: Absorbed via PepT1 transporter, then hydrolysed inside the enterocyte
• Enter portal vein → liver

Fats:

• Pancreatic lipase breaks fats into fatty acids and monoglycerides (with help of bile salt micelles)
• Long-chain fatty acids and monoglycerides: Enter enterocytes by simple diffusion; re-assembled into triglycerides inside the cell; packaged with cholesterol and proteins into chylomicrons (lipoprotein particles)
• Chylomicrons are too large for blood capillaries — they are released into lacteals (lymph capillaries) → enter lymph (thoracic duct) → reach blood via subclavian vein
• Short-chain fatty acids: Directly absorbed into blood (portal vein) — they’re water-soluble enough

Fat-soluble vitamins (A, D, E, K): Follow the same path as fats — absorbed with fatty acids, packaged in chylomicrons, enter lymph.

Water-soluble vitamins (B, C): Absorbed in the jejunum by specific transporters; enter portal blood directly.

Special cases:

• Vitamin B₁₂: Requires intrinsic factor (secreted by stomach) for absorption; absorbed in the ileum
• Iron: Absorbed in the duodenum; requires an acidic environment (hence stomach acid is important for iron absorption)
• Calcium: Absorbed in duodenum; requires vitamin D (increases Ca²⁺ binding protein synthesis)