6.2: Steps of Digestion — Read

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The digestive process follows a sequential pathway from food intake to waste elimination.

This is how an animal "grabs" its food. Different animals have evolved unique strategies:

Horses and cattle use their flexible lips and tongue to pull grass into their mouths
Pigs use their snout to root and their teeth to grab food
Birds use their beaks to peck and scoop
Cats and dogs use their teeth to tear and bite

Think of prehension as the "capture phase"--without it, no digestion can begin.

Once food is in the mouth, chewing begins the physical breakdown:

Teeth crush and grind food into smaller particles
This increases the surface area of the food, making it easier for enzymes to work later
Different teeth serve different purposes:
- Incisors cut and nip
- Molars grind and crush
- Canines (in carnivores) tear meat

Fun Fact: Birds don't have teeth, so they skip this step in the mouth--their gizzard does the grinding later.

Saliva does much more than just make food wet:

Lubrication Functions:

Makes food slippery so it can slide down the esophagus easily
Especially important for horses eating dry, coarse hay

Chemical Functions (varies by species):

Pigs produce salivary amylase, which starts breaking down starches (though this is minimally effective)
Horses and cattle produce saliva with NO enzymes--it's purely for lubrication
Saliva also helps form the food into a bolus--a cohesive ball that's easier to swallow

Fun fact: Cattle can produce 50-80 liters (13-21 gallons) of saliva per day!

Upclose cow muzzle

Image credit: Jan Koetsier from Pexels: https://www.pexels.com/photo/black-a...-head-2647053/

This is the journey from mouth to stomach:

The tongue pushes the bolus to the back of the throat
The epiglottis (a flap of tissue) closes over the windpipe to prevent choking
Muscular contractions called peristalsis push the food down the esophagus
Food reaches the stomach (or in ruminants, the rumen first)

This process is mostly involuntary once swallowing starts--the body takes over.

This is where serious digestion begins:

In the Glandular Stomach (or Abomasum in ruminants):

Hydrochloric Acid (HCl):

Creates an extremely acidic environment (pH 1.5-3)
Denatures proteins, meaning it unfolds their complex structure, exposing bonds that enzymes can break
Kills most harmful bacteria in food
Activates pepsinogen (inactive form) into pepsin (active form)

Pepsin (the enzyme):

Begins breaking down proteins into smaller peptide chains
Works best in very acidic conditions
This is just the START of protein digestion--it's completed in the small intestine

Why this matters: Without the stomach's acid and pepsin, proteins would pass through mostly intact and be wasted.

The small intestine is where the magic happens--nutrients finally enter your bloodstream:

The Three Sections Work Together:

Duodenum (The Chemical Factory):

First 10-12 inches after the stomach
Receives pancreatic juice containing:
- Enzymes for breaking down proteins, fats, and carbohydrates
- Bicarbonate to neutralize stomach acid (pH goes from ~2 to ~7-8)
Receives bile from the liver/gallbladder to emulsify fats (break them into tiny droplets)
Food is chemically broken down into its simplest forms here:
- Proteins → amino acids
- Carbohydrates → simple sugars (glucose, fructose)
- Fats → fatty acids and glycerol

Jejunum (The Absorption Powerhouse):

The longest section (several feet in most animals)
Lined with millions of tiny fingerlike projections called villi (and even tinier microvilli)
These create MASSIVE surface area--like turning a bedsheet into a shag carpet
Most nutrient absorption happens here:
- Amino acids cross into the bloodstream
- Simple sugars enter the blood
- Fatty acids enter the lymphatic system
By the time food leaves the jejunum, most usable nutrients are gone

Ileum (The Final Checkpoint):

Shorter section connecting to the large intestine
Absorbs remaining nutrients, especially:
- Vitamin B12
- Bile salts (recycled back to the liver)
- Any leftover water and electrolytes
Contains lymphoid tissue that helps with immune function

If food moves too quickly through the small intestine (diarrhea), nutrients aren't absorbed, leading to malnutrition despite eating enough food.

After all useful nutrients are extracted, waste must be expelled:

Defecation (Solid Waste):

Undigested material, fiber, dead bacteria, and worn-out cells move to the large intestine
Water is absorbed, forming solid feces
Feces are stored in the rectum until expelled
Feces exit through the cloaca (combined opening) in birds
Feces exit through the anus in mammals

What's in feces?

Undigested fiber (cellulose in herbivores)
Dead bacteria (actually about 50% of fecal mass!)
Bile pigments (give it the brown color)
Sloughed-off intestinal cells

Micturition (Liquid Waste):

This is urination--a separate process from digestion, but important for waste removal
Kidneys filter blood and remove:
- Urea (in mammals)--formed from breaking down excess amino acids
- Uric acid (in birds and reptiles)--the white paste you see in bird droppings
- Excess water, minerals, and metabolic byproducts

Why the difference?

Mammals can afford to use water to flush out urea.
Birds evolved to conserve water (uric acid requires much less water to excrete), which helps them fly.

Poop emoji toy resting on a shelf with similarly shaped desserts.

Image credit: Theodore Nguyen from Pexels: https://www.pexels.com/photo/quirky-...eoul-31736006/

This page titled 6.2: Steps of Digestion was last modified on Sat, 02 May 2026 03:51:12 GMT and is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Deanne Pawlisch.

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