The upper GI tract
People are often encouraged to think about their food and drink choices, yet there is little focus on what happens after these items enter the body.
The passage of food starts when it enters the mouth and ends with the anus, but there are many organs along the way. The gastrointestinal (GI) tract is often thought of as having an upper and lower section the former is concerned with digesting food and preparing it for processing, while the latter looks after the absorption of water and nutrients as well as the expulsion of waste products.
The upper GI tract starts with the mouth and the process of digestion begins with the actions of the tongue and teeth mechanically breaking down food, while salivary glands release saliva – a liquid rich in enzymes – which does the same thing in chemical form.
Sensory organs also play a part. Thinking about, seeing or smelling food can trigger the production of digestive enzymes, but this sensory process also acts as a warning system. For example, the taste buds of the tongue detect potentially hazardous substances, which are often unpalatable.
Partially digested food, known as a bolus, is swallowed, courtesy of the muscular pharynx widening. This also blocks the trachea (windpipe) and nasal cavity, so food doesn’t pass into the respiratory tract and cause choking. Beyond the pharynx lies the oesophagus – a long tube along which food passes thanks to the autonomically controlled waves of movement called peristalsis. A sphincter muscle is situated at each end of the oesophagus, the upper one preventing food moving back (refluxing) into the pharynx, while the lower one allows food to pass into the stomach.
The stomach is an expanding muscular organ, with a volume of 50ml when empty and unfolding to a capacity of around 1,500ml. The muscular exterior of the stomach enables it to physically churn the contents, while stretch receptors embedded in the wall trigger the release of the hormone gastrin, which in turn stimulates the secretion of mucus, pepsin – an enzyme that breaks down proteins – and hydrochloric acid. Gastrin production stops when the pH of the stomach contents drops to a certain level.
Once the stomach has done its work – a process that takes anything from 40 minutes to several hours – the contents, now known as chyme, moves into the last section of the upper GI tract: the duodenum. This uppermost region of the small intestine is where chyme continues to be digested, aided by substances that come from the liver – such as bile, which helps process fats – and the pancreas – including several enzymes that break down carbohydrates, proteins and fats, and also sodium bicarbonate, which neutralises stomach acid.