Chapter 3 – Digestion and Absorption
🔹 What is Digestion?
Digestion is the process of breaking down food into nutrients that the body can absorb and use for
energy, growth, and repair. This happens through:
1. Mechanical digestion – Physical breakdown (chewing, stomach churning).
2. Chemical digestion – Enzymes and acids break down food molecules.
Together, they transform complex food into simple molecules like:
● Amino acids (from proteins),
● Fatty acids and glycerol (from fats),
● Monosaccharides (from carbohydrates),
● Vitamins, minerals, and water.
🔹 The GI Tract (Gastrointestinal Tract)
The GI tract is a muscular tube that runs from the mouth to the anus. Food moves through it by a
wave-like motion called peristalsis. The major organs of the GI tract are:
● Mouth
● Esophagus
● Stomach
● Small intestine
● Large intestine
In addition, accessory organs help with digestion:
● Pancreas
● Liver
● Gallbladder
🔹 Phases of Digestion
1. Cephalic Phase – Begins before food is even in your mouth. Just thinking about or smelling
food triggers saliva and gastric secretions.
2. Gastric Phase – Starts when food reaches the stomach. The stomach releases acid and
enzymes to begin digestion.
3. Intestinal Phase – Begins when chyme enters the small intestine. Hormones and enzymes
continue digestion and absorption.
The Mouth and Esophagus
, ● Mouth:
Chewing breaks food down.
○ Saliva contains salivary amylase, which starts breaking down starch (a
carbohydrate).
○ Food becomes a soft ball called a bolus.
○ Lingual lipase for fat breakdown, mostly active in stomach
● Esophagus:
○ A muscular tube that moves the bolus to the stomach using peristalsis.
○ The lower esophageal sphincter (LES) opens to let food into the stomach and then
closes to prevent acid reflux.
🔹 The Stomach
The stomach mixes food with gastric juices, forming chyme (a semi-liquid substance). It has several
key digestive functions:
● Mechanical digestion: Churns the food.
● Chemical digestion:
○ Hydrochloric acid (HCl) lowers the pH to around 2 → kills bacteria and
unfolds proteins.
○ Pepsinogen is activated to pepsin, which starts digesting proteins.
● The stomach also releases intrinsic factor, needed to absorb vitamin B12 later in the small
intestine.
Minimal nutrient absorption happens here — only small amounts of water, alcohol, and some drugs
are absorbed in the stomach.
🔹 The Small Intestine – Main Site of Digestion and Absorption
The small intestine is where most digestion and absorption take place. It’s divided into three parts:
1. Duodenum – First 25–30 cm. Receives:
○ Chyme from the stomach,
○ Bile from the gallbladder (helps digest fats),
○ Pancreatic juice (contains enzymes and bicarbonate).
2. Jejunum – Middle section. Most absorption of macronutrients occurs here.
3. Ileum – Last section. Absorbs vitamin B12, bile salts, and any remaining nutrients.
🔹 Intestinal Structure: Villi and Microvilli
● The inside of the small intestine is lined with villi (finger-like projections).
● Each villus is covered in microvilli, forming the brush border.
● These structures increase the surface area, making absorption super efficient.
● Each villus contains:
○ A capillary network (absorbs carbs & proteins),
○ A lacteal (absorbs fats into the lymph).
🔹 Pancreas: Enzyme Powerhouse
The pancreas releases pancreatic juice into the duodenum. It contains:
● Bicarbonate – Neutralizes stomach acid.
● Enzymes:
○ Pancreatic amylase – breaks down starch.
○ Pancreatic lipase – breaks down fats.
○ Proteases like trypsin and chymotrypsin – break down proteins.
These enzymes work best at a neutral to slightly basic pH, which is why bicarbonate is crucial.
,🔹 Liver and Gallbladder
● Liver:
○ Produces bile, made of bile salts, cholesterol, and pigments.
○ Bile emulsifies fats → breaks large fat globules into tiny droplets,
increasing surface area for lipase.
● Gallbladder:
○ Stores and concentrates bile.
○ Releases bile into the duodenum in response to the hormone cholecystokinin
(CCK), which is triggered by fat in the chyme.
🔹 Fat Digestion and Absorption
● Emulsification: Bile breaks fat into smaller droplets.
● Digestion: Lipase breaks triglycerides into monoglycerides and free fatty acids.
● Micelle formation: These lipids combine with bile salts to form micelles.
● Absorption:
○ Micelles carry lipids to the intestinal wall, where they enter cells.
○ Inside, they’re reassembled into triglycerides and packed into chylomicrons.
○ Chylomicrons enter the lymphatic system, not the bloodstream right away.
🔹 Carbohydrate Digestion and Absorption
Step 1 – Digestion
● Starts in the mouth with salivary amylase, breaking starch into maltose.
● In the small intestine, pancreatic amylase continues the breakdown of starch into
disaccharides (e.g. maltose, sucrose, lactose).
● The brush border enzymes (in microvilli) finish digestion:
○ Maltase → maltose → 2 glucose
○ Sucrase → sucrose → glucose + fructose
○ Lactase → lactose → glucose + galactose
, Step 2 – Absorption
● Monosaccharides (glucose, fructose, galactose) are absorbed into enterocytes:
○ Glucose & galactose: via active transport with sodium (Na+).
○ Fructose: via facilitated diffusion.
● All three sugars enter the bloodstream through the portal vein, going to the liver.
🔹 Protein Digestion and Absorption
Step 1 – Digestion
● Starts in the stomach
○ Pepsin (activated from pepsinogen) breaks proteins into peptides.
● Continues in the small intestine:
○ Pancreatic proteases (trypsin, chymotrypsin, carboxypeptidase) further break
peptides into smaller peptides or amino acids.
○ Brush border enzymes (peptidases) break peptides into single amino acids.
Step 2 – Absorption
● Amino acids are absorbed into enterocytes via active transport (Na+-dependent).
● Some dipeptides and tripeptides can be absorbed and are broken down inside the cell.
● Amino acids enter the bloodstream via the portal vein and go to the liver.
🔹 Water and Electrolyte Absorption
● About 8 liters of fluid enter the GI tract daily (via food, drink, and secretions).
● Most of it (~7 liters) is absorbed in the small intestine, the rest in the large intestine.
● Water absorption is passive and follows osmotic gradients.
● Electrolytes like sodium, potassium, and chloride are absorbed actively or passively
depending on their concentration.
🔹 Vitamin Absorption
● Fat-soluble vitamins (A, D, E, K):
○ Absorbed like fats: enter micelles → absorbed with lipids → packed into
chylomicrons → enter lymph.
● Water-soluble vitamins (B-complex, C):
○ Absorbed by diffusion or specific transporters.
● Vitamin B12:
○ Requires intrinsic factor, produced in the stomach.
○ Absorbed in the ileum.
🔹 Mineral Absorption
● Calcium:
○ Requires vitamin D for absorption.
○ Mostly absorbed in the duodenum and jejunum.
○ When calcium intake is high, it’s absorbed passively; when intake is low, it’s absorbed
via active transport (regulated by vitamin D).
● Iron:
○ Found in two forms: heme iron (from animal foods) and non-heme iron (plant
sources).
○ Heme iron is absorbed more efficiently.
○ Iron absorption is regulated by body iron stores — when iron levels are low,
absorption increases.
● Zinc, magnesium, copper, selenium:
○ Each has its own absorption mechanism (active or passive).
🔹 What is Digestion?
Digestion is the process of breaking down food into nutrients that the body can absorb and use for
energy, growth, and repair. This happens through:
1. Mechanical digestion – Physical breakdown (chewing, stomach churning).
2. Chemical digestion – Enzymes and acids break down food molecules.
Together, they transform complex food into simple molecules like:
● Amino acids (from proteins),
● Fatty acids and glycerol (from fats),
● Monosaccharides (from carbohydrates),
● Vitamins, minerals, and water.
🔹 The GI Tract (Gastrointestinal Tract)
The GI tract is a muscular tube that runs from the mouth to the anus. Food moves through it by a
wave-like motion called peristalsis. The major organs of the GI tract are:
● Mouth
● Esophagus
● Stomach
● Small intestine
● Large intestine
In addition, accessory organs help with digestion:
● Pancreas
● Liver
● Gallbladder
🔹 Phases of Digestion
1. Cephalic Phase – Begins before food is even in your mouth. Just thinking about or smelling
food triggers saliva and gastric secretions.
2. Gastric Phase – Starts when food reaches the stomach. The stomach releases acid and
enzymes to begin digestion.
3. Intestinal Phase – Begins when chyme enters the small intestine. Hormones and enzymes
continue digestion and absorption.
The Mouth and Esophagus
, ● Mouth:
Chewing breaks food down.
○ Saliva contains salivary amylase, which starts breaking down starch (a
carbohydrate).
○ Food becomes a soft ball called a bolus.
○ Lingual lipase for fat breakdown, mostly active in stomach
● Esophagus:
○ A muscular tube that moves the bolus to the stomach using peristalsis.
○ The lower esophageal sphincter (LES) opens to let food into the stomach and then
closes to prevent acid reflux.
🔹 The Stomach
The stomach mixes food with gastric juices, forming chyme (a semi-liquid substance). It has several
key digestive functions:
● Mechanical digestion: Churns the food.
● Chemical digestion:
○ Hydrochloric acid (HCl) lowers the pH to around 2 → kills bacteria and
unfolds proteins.
○ Pepsinogen is activated to pepsin, which starts digesting proteins.
● The stomach also releases intrinsic factor, needed to absorb vitamin B12 later in the small
intestine.
Minimal nutrient absorption happens here — only small amounts of water, alcohol, and some drugs
are absorbed in the stomach.
🔹 The Small Intestine – Main Site of Digestion and Absorption
The small intestine is where most digestion and absorption take place. It’s divided into three parts:
1. Duodenum – First 25–30 cm. Receives:
○ Chyme from the stomach,
○ Bile from the gallbladder (helps digest fats),
○ Pancreatic juice (contains enzymes and bicarbonate).
2. Jejunum – Middle section. Most absorption of macronutrients occurs here.
3. Ileum – Last section. Absorbs vitamin B12, bile salts, and any remaining nutrients.
🔹 Intestinal Structure: Villi and Microvilli
● The inside of the small intestine is lined with villi (finger-like projections).
● Each villus is covered in microvilli, forming the brush border.
● These structures increase the surface area, making absorption super efficient.
● Each villus contains:
○ A capillary network (absorbs carbs & proteins),
○ A lacteal (absorbs fats into the lymph).
🔹 Pancreas: Enzyme Powerhouse
The pancreas releases pancreatic juice into the duodenum. It contains:
● Bicarbonate – Neutralizes stomach acid.
● Enzymes:
○ Pancreatic amylase – breaks down starch.
○ Pancreatic lipase – breaks down fats.
○ Proteases like trypsin and chymotrypsin – break down proteins.
These enzymes work best at a neutral to slightly basic pH, which is why bicarbonate is crucial.
,🔹 Liver and Gallbladder
● Liver:
○ Produces bile, made of bile salts, cholesterol, and pigments.
○ Bile emulsifies fats → breaks large fat globules into tiny droplets,
increasing surface area for lipase.
● Gallbladder:
○ Stores and concentrates bile.
○ Releases bile into the duodenum in response to the hormone cholecystokinin
(CCK), which is triggered by fat in the chyme.
🔹 Fat Digestion and Absorption
● Emulsification: Bile breaks fat into smaller droplets.
● Digestion: Lipase breaks triglycerides into monoglycerides and free fatty acids.
● Micelle formation: These lipids combine with bile salts to form micelles.
● Absorption:
○ Micelles carry lipids to the intestinal wall, where they enter cells.
○ Inside, they’re reassembled into triglycerides and packed into chylomicrons.
○ Chylomicrons enter the lymphatic system, not the bloodstream right away.
🔹 Carbohydrate Digestion and Absorption
Step 1 – Digestion
● Starts in the mouth with salivary amylase, breaking starch into maltose.
● In the small intestine, pancreatic amylase continues the breakdown of starch into
disaccharides (e.g. maltose, sucrose, lactose).
● The brush border enzymes (in microvilli) finish digestion:
○ Maltase → maltose → 2 glucose
○ Sucrase → sucrose → glucose + fructose
○ Lactase → lactose → glucose + galactose
, Step 2 – Absorption
● Monosaccharides (glucose, fructose, galactose) are absorbed into enterocytes:
○ Glucose & galactose: via active transport with sodium (Na+).
○ Fructose: via facilitated diffusion.
● All three sugars enter the bloodstream through the portal vein, going to the liver.
🔹 Protein Digestion and Absorption
Step 1 – Digestion
● Starts in the stomach
○ Pepsin (activated from pepsinogen) breaks proteins into peptides.
● Continues in the small intestine:
○ Pancreatic proteases (trypsin, chymotrypsin, carboxypeptidase) further break
peptides into smaller peptides or amino acids.
○ Brush border enzymes (peptidases) break peptides into single amino acids.
Step 2 – Absorption
● Amino acids are absorbed into enterocytes via active transport (Na+-dependent).
● Some dipeptides and tripeptides can be absorbed and are broken down inside the cell.
● Amino acids enter the bloodstream via the portal vein and go to the liver.
🔹 Water and Electrolyte Absorption
● About 8 liters of fluid enter the GI tract daily (via food, drink, and secretions).
● Most of it (~7 liters) is absorbed in the small intestine, the rest in the large intestine.
● Water absorption is passive and follows osmotic gradients.
● Electrolytes like sodium, potassium, and chloride are absorbed actively or passively
depending on their concentration.
🔹 Vitamin Absorption
● Fat-soluble vitamins (A, D, E, K):
○ Absorbed like fats: enter micelles → absorbed with lipids → packed into
chylomicrons → enter lymph.
● Water-soluble vitamins (B-complex, C):
○ Absorbed by diffusion or specific transporters.
● Vitamin B12:
○ Requires intrinsic factor, produced in the stomach.
○ Absorbed in the ileum.
🔹 Mineral Absorption
● Calcium:
○ Requires vitamin D for absorption.
○ Mostly absorbed in the duodenum and jejunum.
○ When calcium intake is high, it’s absorbed passively; when intake is low, it’s absorbed
via active transport (regulated by vitamin D).
● Iron:
○ Found in two forms: heme iron (from animal foods) and non-heme iron (plant
sources).
○ Heme iron is absorbed more efficiently.
○ Iron absorption is regulated by body iron stores — when iron levels are low,
absorption increases.
● Zinc, magnesium, copper, selenium:
○ Each has its own absorption mechanism (active or passive).
