Human Digestive System

Nutrition is the sum of the processes by which an organism consumes and utilises food substances to obtain energy and the raw materials it needs to grow, repair its tissues and stay alive. The World Health Organisation (WHO) defines nutrition specifically as the intake of food, considered in relation to the body's dietary needs. A healthy human diet must supply carbohydrates, proteins, fats, vitamins, minerals, water and fibre in adequate amounts -- food supplies both the energy for daily activity and the organic material needed for growth and tissue repair, while vitamins and minerals, though needed only in small quantities, are equally essential. Broadly, the term nutrition covers five linked processes: ingestion (taking food into the body), digestion (breaking it down), absorption (taking the useful products into the blood/lymph), assimilation (using those products in the body's own cells) and egestion (eliminating what cannot be used).

Even when a diet already contains every nutrient the body needs, none of it can be used directly in the form it is eaten -- it first has to be processed. Digestion is the process by which the complex, non-diffusible and non-absorbable food substances a person eats are converted into simple, diffusible and assimilable substances that the body can actually take up and use. The human digestive system that carries out this processing is built of two connected parts: a long tube called the alimentary canal, and a set of digestive glands associated with it that pour their secretions into this tube.

The alimentary canal is a long, tubular structure that begins at the mouth and ends at the anus. In an adult it measures roughly 8-10 metres in length. Along its course it forms a series of distinct regions, each specialised for a different stage of digestion -- the mouth (buccal cavity), pharynx, oesophagus, stomach, small intestine, large intestine and anus -- each of which is described in turn below.