Science · Body foundations
Digestive System and Nutrient Absorption
Estimated time includes reading and one quiz. Take the time you need.
Start here: key ideas
- Food moves through a continuous digestive tract.
- Mechanical digestion breaks food apart; chemical digestion breaks molecules down.
- Most digestion and nutrient absorption occur in the small intestine.
- The large intestine absorbs water and forms feces.
What you’ll be able to do
- Trace food and accessory secretions.
- Match major enzymes with nutrients.
- Explain absorption in the small and large intestines.
Follow food from mouth to anus and track where molecules are broken down and absorbed.
Optional review
If you’d like to review the basics, visit Anatomical Orientation, Organization, and Homeostasis. You can start this lesson without completing those first.
A continuous tract with accessory helpers
Food passes through the mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, and anus. Peristalsis—waves of muscle contraction—moves it forward. The liver makes bile, the gallbladder stores and releases bile, and the pancreas releases enzymes and bicarbonate into the small intestine. Food does not travel through these accessory organs.
The mouth mechanically breaks food and mixes it with saliva. The stomach stores and churns food while acid unfolds proteins and helps activate pepsin. The small intestine receives bile and pancreatic juice. Bicarbonate neutralizes acidic stomach contents so intestinal and pancreatic enzymes can work.
Production, storage, release, digestion, and absorption are different jobs. The liver produces bile; the gallbladder stores and concentrates it, then releases it through ducts into the small intestine. Bile emulsifies fat but is not an enzyme. Most absorbed water-soluble nutrients enter blood capillaries and travel through the hepatic portal vein to the liver; many lipid products first enter lacteals and lymph. Do not call the portal vessel a duct.
The tract and its accessory organs
Trace food through the mouth, throat, esophagus, stomach, intestines, rectum, and anus. Food does not pass through the liver, gallbladder, or pancreas. These helper organs send substances into the tract. Bile helps break fat into droplets; pancreatic enzymes help digest food.
The tract and its accessory organs — OpenStax College, Anatomy & Physiology (2014 CC BY archive). CC BY 3.0. Original source image reproduced unchanged; explanatory caption added.
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