Introduction to the Human Digestive System

Every living organism, from the simplest single-celled protist to a complex multicellular animal, must take in nutrients from its surroundings to survive. Nutrients serve two broad purposes: they are oxidised to release the energy an organism needs for every activity, from muscle contraction to nerve conduction, and they supply the raw building blocks — amino acids, fatty acids, simple sugars, minerals — from which the organism constructs and continually repairs its own tissues. In a human being, this nutrient supply comes from food, but the food as eaten is very largely unusable in its raw form. Boiled rice, a slice of bread, a piece of meat or a spoon of ghee are all made up chiefly of large, complex, often insoluble macromolecules — starch, proteins, triglycerides — and no cell of the body, and no blood vessel, is built to absorb or transport a molecule of that size and complexity. Before the nutrients locked inside food can be of any use to the body, the food itself has to be broken down into far smaller, water-soluble units.

This breakdown, together with everything that leads up to it and follows from it, is the subject of the present chapter, and it is useful to separate it, right at the outset, into five distinct though continuous events. The first is ingestion, simply the taking of food into the body through the mouth. The second is digestion itself: the mechanical breakdown of food into smaller pieces (by chewing, and later by churning in the stomach and intestine) together with its chemical breakdown, region by region, by a whole series of enzymes, each specific to one class of nutrient, until what remains are molecules small enough to cross a cell membrane — monosaccharides from starch, amino acids from protein, and fatty acids and monoglycerides from fat. The third event is absorption: the passage of these simple, digested end-products across the lining of the small intestine into the bloodstream or, for most fats, into the lymphatic system. The fourth is assimilation, the utilisation of these absorbed nutrients by the cells and tissues of the body — for energy release, for growth, and for the continual repair and replacement of worn-out tissue. The fifth and final event is egestion, the elimination from the body, as faecal matter, of the portion of the food that could not be digested or absorbed at all — chiefly plant fibre in a typical Indian diet.

Carrying out these five events requires a dedicated organ system: the human digestive system, made up of a long, continuous, highly differentiated tube called the alimentary canal, running from the mouth to the anus, together with a set of associated glands — the salivary glands, the liver and the pancreas — whose secretions are poured into the canal at particular points along its length to assist digestion. The sections that follow take up, in turn, the structure of this alimentary canal and its regions, the arrangement of the teeth, the associated glands and their secretions, the movements (peristalsis) that carry food along the canal, the enzymes and gastrointestinal hormones that control digestion, the digestion and absorption of carbohydrates, proteins and fats specifically, the process of egestion, and finally the common nutritional and digestive disorders that arise when any part of this system fails to work properly — an outline that maps directly onto the topics named in the WBCHSE Class 11 syllabus for this chapter.