Biology · Biochemistry in action

The Human Digestive System

Follow your food from the mouth to the end of the large intestine. For each organ, learn its structure, its job, and which kind of digestion happens there.

The big picture

The path food takes

Food travels through one long muscular tube, the GI (gastrointestinal) tract, and changes form along the way.

  1. 1MouthChews food into a bolus
  2. 2PharynxSwallows the bolus
  3. 3EsophagusSqueezes it down by peristalsis
  4. 4StomachChurns it with acid into chyme
  5. 5Small intestineFinishes digestion; absorbs nutrients
  6. 6Large intestineAbsorbs water; forms solid waste
  7. Rectum & anusWaste leaves the body

Accessory organs* (the liver, gall bladder, and pancreas) help with digestion, but food never goes through them.

Mechanical digestion

Physical changes. Food is broken into smaller pieces but is still the same substance.

  • Teeth grinding food (more surface area)
  • Peristalsis and stomach churning
  • Bile breaking fat into smaller drops

Chemical digestion

Chemical changes. Enzymes and acid break food molecules into new, smaller molecules.

  • Amylase breaking down starch in the mouth
  • Pepsin breaking down proteins in the stomach
  • Pancreatic enzymes finishing digestion in the small intestine
1 mouth salivary glands 2 pharynx 3 esophagus liver* gall bladder* 4 stomach pancreas* 5 small intestine 6 large intestine rectum anus

The whole system

Numbers follow the path food takes; a star marks an accessory organ. Point at an organ to highlight it, or tap it to jump to its notes.

Anatomy illustration: Mariana Ruiz (LadyofHats), public domain.

OrganMechanical digestionChemical digestionMain job
MouthTeeth grind food; saliva moistens itAmylase starts breaking down starchBreak food into smaller pieces; begin digesting polysaccharides
Pharynx & esophagusPeristalsis starts hereNoneMove the bolus to the stomach
StomachChurning (peristalsis); HCl breaks the bolus apartPepsin starts breaking down proteins; HClStart protein digestion; make chyme
Small intestinePeristalsis; bile breaks fat into smaller piecesEnzymes from the pancreas finish digestionFinish digestion; absorb nutrients into the blood
PancreasNoneMakes the enzymes (does not digest anything itself)Send enzymes to the small intestine
Liver & gall bladderLiver makes bile; gall bladder stores itNoneSupply bile to break down fat
Large intestineNone (digestion is finished)None (digestion is finished)Absorb water and salts; form solid waste

Structure · function · digestion

Organ by organ

Each organ in your notes has the same five parts. Learn them in order: structure, function, mechanical, chemical, other.

Where it is

1Food passes through

Mouth

Structure
Teeth, tongue, salivary glands
Function
Break food into smaller pieces and begin the digestion of polysaccharides (like starch)
Mechanical
  • Teeth grind and smash food into smaller parts, which increases surface area.
  • The watery part of saliva adds moisture, so food comes apart more easily.
Chemical
Amylase, an enzyme in saliva, starts breaking starch down into smaller pieces of carbohydrate.
Other
  • The monomer (building block) of carbohydrates is a monosaccharide, a simple sugar like glucose.
  • Three pairs of salivary glands make saliva: parotid, submandibular, and sublingual.
  • Gleeking is when saliva squirts out from the gland under your tongue.

Inside the mouth

The three salivary glands on each side empty saliva into the mouth through small ducts.

Leaves the mouth asa soft, chewed ball of food called a bolus.

Where it is

23Food passes through

Pharynx & Esophagus

Structure
The pharynx (throat) is the muscular area at the back of the mouth. The esophagus is the muscular tube that runs from the mouth to the stomach.
Function
Move the chewed food (bolus) to the stomach
Mechanical
Peristalsis starts here. Peristalsis is the wavelike contractions of the muscles that line the digestive system and move food through the GI tract.
Chemical
None
Other
  • Esophageal sphincter: a valve that keeps stomach contents from coming back up. When they do come back up, that's heartburn.
  • Epiglottis: a small flap at the back of the pharynx that covers the trachea (windpipe) so food doesn't go to the lungs.
  • The larynx is the voice box. Air passing through it makes sound.

The mouth and throat, from the side

teeth tongue salivary glands pharynx epiglottis larynx (voice box) esophagus trachea (windpipe)
The trachea (air) is in front; the esophagus (food) is right behind it. The epiglottis sits where they split.
Breathing or talking

The epiglottis is open. Air goes down the trachea to the lungs.

Swallowing

The epiglottis closes over the trachea. The bolus goes down the esophagus.

Don't talk with your mouth fullTalking opens the epiglottis, and that's how food ends up "going down the wrong pipe."

Where it is

4Food passes through

Stomach

Structure
  • A muscular enlargement of the GI tract (wider than the rest of the tube) with specialized cells and lots of muscle.
  • Lined with cells that make acid and cells that make mucus to protect the stomach from that acid.
Function
Start the digestion of proteins and mix food with acid to break it down into a mixture called chyme
Mechanical
  • Peristalsis (stomach churning)
  • Hydrochloric acid (HCl) helps physically break the bolus into smaller pieces.
Chemical
  • Pepsin, an enzyme activated by low pH (acid), begins breaking down proteins.
  • HCl helps break down the bolus chemically.
Other
  • Stomach acid helps kill any bacteria in the food.
  • The pyloric sphincter, a valve at the bottom of the stomach, keeps food from moving into the small intestine too quickly and keeps chyme from flowing back.
  • The folds lining the inside of the stomach are called rugae.

Two valves, two jobsEsophageal sphincter (top): keeps food from coming back up. Pyloric sphincter (bottom): lets chyme into the small intestine slowly.

Where it is

5Food passes through

Small Intestine

Structure
  • A muscular tube that the pancreas and gall bladder release enzymes and bile into.
  • Lined with finger-like projections (villi) that increase surface area for absorption.
Function
Complete digestion and let nutrients be absorbed into the bloodstream
Mechanical
  • Peristalsis happens throughout.
  • Fat is broken into smaller pieces by bile (made in the liver, stored in and released from the gall bladder).
Chemical
All types of enzymes are released from the pancreas into the small intestine to finish digestion.
Other
About 22 feet long, split into 3 sections. It's called "small" because its opening is only about 2 cm wide.

Three sections, in order

  1. 1stDuodenumEnzymes from the pancreas enter, and digestion happens.
  2. 2ndJejunumDigestion finishes. This is the main place nutrients are absorbed.
  3. 3rdIleumAbsorption of nutrients finishes.

How villi help absorption

microvilli villus capillaries lacteal (absorbs fats) artery vein lymph vessel

The answer to "how do villi aid in absorption?"Villi and the microvilli on them fold the lining over and over, which hugely increases surface area. More surface touching the digested food means more nutrients pass into the capillaries and enter the blood.

Where it is

Accessory organ*Food does not go through this organ

Pancreas

Structure
  • Thick on one end and thin on the other. Its surface is bumpy, like corn on the cob.
  • Located behind the stomach and underneath the liver.
Function
Makes enzymes and releases them into a duct (tube) that flows into the small intestine
Mechanical
None
Chemical
The pancreas does not digest anything itself. It produces the enzymes for chemical digestion in the small intestine.
Other
Works in two body systems: the digestive system and the endocrine system (hormones).
Where they are

Accessory organs*Food does not go through these organs

Liver & Gall Bladder

Structure
The liver is a large, lobed organ under the ribs, near and above the stomach. The gall bladder is a small sac tucked under the liver.
Function
The liver makes bile and sends it to the gall bladder for storage. The gall bladder releases bile into the small intestine to break down fat physically (no chemical change).
Mechanical
None in the liver itself. Its bile physically breaks down fat in the small intestine.
Chemical
None
Other
  • The largest organ in the body. (It's the largest organ inside your body; your skin is bigger.)
  • Filters blood (removes toxins), stores energy, and makes bile.
  • Also makes proteins, hormones, cholesterol, and more.

Don't mix these upThe liver makes bile. The gall bladder stores bile. Bile does its work in the small intestine.

Where it is

6Food passes through

Large Intestine (colon)

Structure
A muscular tube with a valve at the end
Function
  • Receives the liquid waste left after the small intestine finishes digestion and absorption.
  • Absorbs water and salts from the liquid waste back into the bloodstream.
  • Turns what's left into solid waste (feces) and uses its muscles to move it out of the body.
  • Bacteria ferment the leftovers and make vitamins for you.
Mechanical
None. Digestion is finished by this point.
Chemical
None. Digestion is finished by this point.
Other
About 6 feet long, with an opening about 3 inches wide.

Three parts

Colon · about 52–53 inRectum · about 5–6 inAnus · about 2 in

Why is the longer one called "small"?

Length Small intestine 22 ft Large intestine 6 ft Opening small intestine: about 2 cm large intestine: about 3 in Bars and circles are drawn to scale with each other.

Small and large describe width, not lengthThe small intestine is almost four times longer, but its opening is much narrower.

Biochemistry in action

Enzymes & chemicals

An enzyme is a protein that speeds up a chemical reaction, like breaking a big food molecule into smaller ones. Know where each one is made and where it works.

SubstanceMade byWorks inWhat it doesDigestion type
Saliva (water)Salivary glandsMouthMoistens food so it comes apart more easilyMechanical
AmylaseSalivary glands (in saliva)MouthStarts breaking starch, a polysaccharide, into smaller carbohydratesChemical
Hydrochloric acid (HCl)Stomach liningStomachBreaks up the bolus physically and chemically, kills bacteria, and creates the low pH that activates pepsinBoth
PepsinStomachStomachBegins breaking down proteins. Only works at low pH.Chemical
MucusStomach liningStomachProtects the stomach from its own acidProtection
BileLiver (stored in the gall bladder)Small intestineBreaks fat into smaller pieces with no chemical changeMechanical
Pancreatic enzymesPancreasSmall intestineAll types of enzymes. They finish digestion.Chemical

"What is the monomer of carbohydrates?" A monosaccharide, a simple sugar like glucose. Starch is a polysaccharide: a long chain of these sugars linked together, like the beads above. Amylase starts snapping that chain into smaller pieces.

Words to know

Key words

Mechanical digestion
Physical changes: breaking food into smaller pieces
Chemical digestion
Chemical changes: enzymes and acid break molecules into new, smaller molecules
Enzyme
A protein that speeds up a chemical reaction (amylase, pepsin)
GI tract
Gastrointestinal tract: the tube food travels through, from mouth to anus
Peristalsis
Wavelike muscle contractions that move food through the GI tract
Bolus
The ball of chewed food that is swallowed
Chyme
The soupy mix of food and stomach acid that leaves the stomach
Sphincter
A ring-shaped muscle that works as a valve (esophageal, pyloric)
Epiglottis
Flap that covers the trachea when you swallow
Trachea
The windpipe: the tube air takes to the lungs
Larynx
The voice box: air passing through makes sound
Villi
Finger-like projections in the small intestine that increase surface area for absorption
Absorption
Nutrients passing from the small intestine into the blood
Bile
Made by the liver, stored in the gall bladder; breaks fat into smaller pieces
Accessory organ
Helps digestion, but food doesn't pass through it (liver, gall bladder, pancreas)
Polysaccharide
A long chain of sugars, like starch
Monosaccharide
A single simple sugar, like glucose: the monomer of carbohydrates
Duodenum · jejunum · ileum
The three sections of the small intestine, in order