Nutrition and Diet Therapy
Module 5 (Digestion and Absorption)
Anatomy of the Digestive Tract
A. The Digestive Organs
1. Mouth
Digestion starts here.
Teeth crush and soften the food.
Saliva makes the food wet and easier to swallow.
The tongue helps with tasting, chewing, and moving food.
Once the food is chewed and swallowed, it’s called a bolus.
2. Pharynx (Throat)
Shared by both the digestive and breathing systems.
Food goes from the mouth to the pharynx before entering the
esophagus.
3. Epiglottis
A small flap that covers the windpipe during swallowing.
It keeps food from going into the lungs.
4. Esophagus
A long tube that carries food to the stomach.
The upper sphincter opens to let food in.
The lower sphincter closes behind the food to stop it from coming back
up.
5. Stomach
The bolus mixes with stomach juices and becomes chyme (a thick liquid).
Chyme slowly leaves the stomach through the pyloric sphincter and
enters the small intestine.
6. Small Intestine (about 10 feet long)
Has three parts:
o Duodenum – gets bile from the gallbladder and enzymes from
the pancreas.
o Jejunum
o Ileum
Digestion finishes here, and most nutrients are absorbed.
7. Ileocecal Valve
Connects the small intestine (ileum) to the large intestine.
Controls the flow of food and prevents it from going backward.
8. Large Intestine (Colon)
Starts in the lower right side of the belly.
Path: Up the right side → across → down the left side → to the back and
rectum.
Absorbs water from the waste.
Waste becomes more solid.
The appendix is a small pouch near the start of the large.
9. Rectum
Stores solid waste .
Muscles hold the waste until it’s time to go to the bathroom.
10. Anus
The last part of the digestive tract.
Opens to let the waste out during a bowel movement.
B. The Involuntary Muscles and the Glands
Gastrointestinal Motility
After swallowing, involuntary muscular contractions move food through
the GI tract.
These movements are collectively known as gastrointestinal motility.
Two main types of GI movements:
Peristalsis
Propels or pushes contents forward.
Wave-like muscular contractions that move food from one section to the
next.
Segmentation
Mixes the contents and pushes them gradually.
Circular muscles contract in segments to mix food with digestive juices
and enhance nutrient absorption.
Liquefying Process
Muscles also help liquefy food into chyme to allow digestive enzymes
better access to nutrients.
Steps in liquefying food:
o Mouth:
Chewing (mechanical breakdown)
Mixing with saliva
Stirring with the tongue
o Stomach:
Further mixes and kneads food into a semi-liquid mass
(chyme)
C. The Process of Digestion
Digestion in the Mouth
Starts carbohydrate digestion.
Salivary glands secrete saliva containing:
o Water
o Salts
o Enzymes (especially salivary amylase which breaks starch into
smaller chains)
Functions of saliva:
o Begins starch breakdown
o Protects tooth surfaces
o Protects linings of the mouth, esophagus, and stomach
Digestion in the Stomach
Gastric juice is secreted by gastric glands; it contains:
o Water
o Enzymes
o Hydrochloric acid (HCl) – very acidic
Functions of gastric acid:
o Kills most bacteria
o Prevents bacterial growth
Mucus protects the stomach lining from acid
Main digestive activity:
o Initial breakdown of proteins (via pepsin and stomach acid)
Minor digestive events:
o Some fat digestion by gastric lipase
o Slight breakdown of sucrose
o Vitamin B12 binds to a protein carrier for absorption later
Digestion in the Small Intestine
Pancreatic juice contains:
o Digestive enzymes (for carbs, proteins, fats)
o Sodium bicarbonate – neutralizes stomach acid, making chyme
alkaline
Enzymes from the intestinal wall assist digestion
Bile:
o Produced by the liver
o Stored and concentrated in the gallbladder
o Released into the duodenum when fat is present
o Not an enzyme, but an emulsifier (breaks fats into small droplets
so enzymes can act)
Result: All three energy nutrients (carbs, fats, proteins) are digested here
Digestion in the Large Intestine
No significant enzymatic digestion
Absorbs water and some minerals
Houses gut bacteria (intestinal flora):
o Produce vitamins like biotin and vitamin K
o Protect the host by competing with harmful microbes
Rate of Digestion
Depends on meal composition:
o High sugar → faster digestion
o High fat → slower digestion
⎯ The many folds, villi, and microvilli of the small intestine increase
D. The Absorptive System
⎯ This large surface area allows for maximum nutrient absorption.
surface area greatly.
⎯ Nutrients enter the body through the cells of the villi.
⎯ Depending on the type of nutrient:
o Water-soluble nutrients and small fat fragments → enter the
bloodstream.
o Fat-soluble nutrients (including large fats and fat-soluble
vitamins) → enter the lymphatic system.
⎯ Nutrients absorbed into the blood:
E. Absorption of Nutrients and Transport of Nutrients
⎯ Nutrients absorbed into the lymphatic system:
o Routed first to the liver before reaching the rest of the body.
o Eventually enter the vascular (blood) system, bypassing the liver
initially.
Very-Low-Density Lipoproteins (VLDL)
Made primarily by liver cells.
Function: Transport lipids (mainly triglycerides) to various body tissues.
Composition: Mostly triglycerides.
Low-Density Lipoproteins (LDL)
Formed from VLDL after triglycerides are removed.
Function: Carry cholesterol and triglycerides from the liver to body cells.
Composition: Primarily cholesterol.
High LDL = Increased risk of heart disease.
High-Density Lipoproteins (HDL)
Function: Transport cholesterol from peripheral tissues back to the liver.
Composition: Primarily protein.
High HDL = Protective against heart disease.
