If commercial television is any indication, humans are completely obsessed by the workings of their digestive system. Reality television cooking shows occupy almost as much airtime as news or current affairs and advertisements for food, food outlets, oral health products, cures for indigestion, laxatives and toilet paper act as a constant reminder of the need to attend to this important physiological system.
Cells throughout the body require a lot of energy and various substrates to perform the functions that they are responsible for. For the most part, these are provided by the 2 - 3 Kg of food and liquid that we eat and drink every day.
The food that we eat is rarely in a form that can be used cells. For example there is very little that a neurone in the cerebral cortex can do with a slice of pepperoni from a mega meatlovers pizza. Before the energy and other substances found in food can be used by cells, the complex macromolecules that it contains has to be broken down into their molecular building-blocks. This process is what we refer to as digestion and it is one of the basic functions of the digestive system.
Through digestion, proteins are broken down into peptides and amino acids, complex carbohydrates into monosaccahrides and fats into glycerol and fatty acids. These small molecules (together with other small molecule dietary elements such water, electrolytes and vitamins) are then able move out of the digestive system into the blood and become available for cellular processes.
The importance of the digestive system is highlighted by the fact that so many illnesses are a consequence of digestive system dysfunction. From mumps, to gastric ulcers through lactose intolerance to diarrhoea; our digestive system has many weird and wonderful ways of letting us know when something is wrong.