Digestion

The Digestive Tract

Food travels through the body inside a continuous muscular tube known as the digestive tract.

• Food moves through these organs in order: mouth, oesophagus, stomach, small intestine, large intestine, rectum, and anus.
• Ingestion means taking food in through the mouth, while egestion means expelling undigested waste through the anus.
• The liver, gall bladder, and pancreas act as accessory organs.
• Food never enters these accessory organs, but they supply vital digestive juices to the tract.

Physical versus Chemical Digestion

Your body uses two distinct methods to break down food before absorption can happen.

• Physical digestion breaks food into smaller pieces by chewing or muscular churning without changing any chemical structures.
• Chemical digestion uses biological chemicals called enzymes to break large food molecules into simple units.
• Each enzyme is specific and breaks down only one nutrient type.

Vitamins, minerals, and water are already small enough to pass into the blood without digestion. Fibre cannot be digested, but it provides bulk to keep waste moving.

The Upper Digestive Tract: Mouth to Stomach

Digestion starts straight away once you take a bite of food.

• In the mouth, teeth chew food into small pieces while saliva moistens the food.
• Saliva contains salivary amylase, an enzyme that starts breaking down cooked starch into simple sugars.
• The oesophagus is a muscular pipe that pushes food downward using wave-like muscular contractions called peristalsis.
• The stomach churns food with gastric juice for one to four hours.
• Gastric juice contains pepsin, an enzyme that begins splitting proteins into simpler units.
• Gastric juice also contains hydrochloric acid, which kills bacteria and provides the acid conditions pepsin requires.
• A thick layer of mucus lines the stomach wall so the acid cannot damage the organ.

The Lower Digestive Tract: Small and Large Intestines

Most chemical digestion and nearly all absorption take place in the small intestine.

• The liver makes bile, which is stored in the gall bladder and emulsifies fats into tiny droplets.
• The pancreas sends pancreatic juice into the small intestine to break down starches, proteins, and fats.
• Millions of tiny, finger-like projections called villi line the small intestine wall to absorb nutrients into the blood.
• The large intestine absorbs water and dissolved minerals from the remaining waste.
• Harmless gut bacteria in the large intestine produce vitamins B and K for the body.
• Semi-solid waste gathers in the rectum before leaving the body through the anus.

Key terms

Digestion

The breakdown of food into simple, soluble substances that can be absorbed into the bloodstream.

Enzymes

Biological chemicals that speed up the breakdown of specific nutrients into their simplest forms.

Peristalsis

The wave-like muscular contractions that push food along the digestive tract.

Absorption

The movement of digested nutrients through the wall of the small intestine into the bloodstream.

Villi

Tiny, finger-like projections lining the small intestine that create a large surface area for nutrient absorption.

Bile

A fluid made by the liver and stored in the gall bladder that breaks fats into small droplets.

Salivary amylase

An enzyme in saliva that begins the breakdown of starch into sugars in the mouth.

Pepsin

An enzyme in gastric juice that begins breaking down proteins in the stomach.