Introduction

We all eat food — skip breakfast and the difference is unmistakable by noon. The food we eat supplies energy and the organic material the body needs for growth and for replacing worn or damaged tissue, and it also helps regulate and coordinate the body's many activities. Our food is made up of carbohydrates, proteins, lipids, vitamins, minerals, fibre and water, drawn from both plant and animal sources — but as eaten, these arrive as large macromolecules that cannot enter our cells directly. They must first be broken down into smaller, absorbable micromolecules, which is exactly the job of a digestive system. Plants, being autotrophs that synthesise their own food, have no need for one; in animals, the digestive system's central role is to carry nutrients, water and electrolytes from the outside environment into every cell of the body through the circulatory system.