Science · Body foundations
Digestive System and Nutrient Absorption
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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.
Large molecules must be broken into smaller units before cells can use them.
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.
OpenStax College, Anatomy & Physiology (2014 CC BY archive) · CC BY 3.0 · https://github.com/philschatz/anatomy-book/blob/eadaa6b2992f4e4010f45d58ad4db065e398093e/resources/2401_Components_of_the_Digestive_System.jpg · https://creativecommons.org/licenses/by/3.0/ · Original source image reproduced unchanged; explanatory caption added.
Match substrate, product, and site
Digestion breaks food into small molecules that can be absorbed. Salivary and pancreatic amylase break starch into smaller sugars. Pepsin begins protein digestion in the acidic stomach; pancreatic and intestinal enzymes continue it to amino acids. Lipase breaks fats into fatty acids and monoglycerides. Bile is not an enzyme. It separates fat into droplets so lipase has more surface to reach.
Enzymes are specific. Amylase acts on carbohydrate, proteases act on protein, and lipase acts on fat. If pancreatic secretion is blocked, all three major nutrient groups are affected. Digestion may begin earlier, but most chemical digestion is completed in the small intestine.
Villi route nutrients
Most nutrients enter the body through the small intestine. Folds and tiny fingerlike projections called villi create a large surface. The cells on each villus have even smaller projections called microvilli. Sugars and amino acids enter blood capillaries. Most fat products enter lymph vessels called lacteals before reaching the blood. Keep the two routes separate when tracing an absorbed nutrient.
Capillary blood from the intestine travels first to the liver through the hepatic portal vein. The liver processes and stores absorbed nutrients. Fats packaged into chylomicrons enter lacteals because those particles are too large to enter ordinary blood capillaries easily.
Servier Medical Art by Servier · CC BY 4.0 · https://smart.servier.com/smart_image/intestinal-villi/ · https://creativecommons.org/licenses/by/4.0/ · Original source image reproduced unchanged; explanatory caption added.
Recover water and form feces
The large intestine receives material that was not digested or absorbed. It absorbs water and electrolytes. Gut bacteria make some vitamins, and some of these are absorbed. The remaining material becomes feces, which is stored in the rectum before leaving through the anus.
Worked example: A person loses a large amount of small-intestine surface. Starch may still be digested normally, yet less glucose enters blood because fewer villi remain for absorption. This separates digestion, which breaks molecules down, from absorption, which moves them into the body.
Key terms
- peristalsis: Wave-like muscle contractions that move food.
- enzyme: A protein that speeds a chemical reaction.
- bile: A fluid that breaks fat into small droplets.
- villus: A fingerlike fold that increases absorption area.
- pancreas: An organ that sends enzymes and bicarbonate to the small intestine.
- large intestine: The organ that absorbs water and forms feces.
Summary
- The stomach begins strong protein digestion.
- The pancreas supplies enzymes and bicarbonate; the liver makes bile.
- Villi increase small-intestine absorption area.
Small intestine: nutrients. Large intestine: remaining water.
Sources
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