The mouth and pharynx are the first parts of the digestive system; they are the principal areas of the gastrointestinal system of interest to dental students and practitioners and are fully described in Chapters 25 and 28. The anatomy of the remainder of the system is described briefly to provide a working knowledge for applications in other aspects of undergraduate dental courses such as nutrition.
In essence, the digestive tract is a long convoluted tube illustrated in Figure 6.1. It extends from the mouth, via the pharynx, oesophagus, stomach, small intestine, and large intestine to the anal canal. It is formed along most of its length by longitudinal and circular layers of smooth muscle. It is lined throughout by epithelium which shows marked structural differences from region to region to match different functional requirements of secretion of digestive enzymes, absorption of nutrients, and excretion of waste products.
The liver and pancreas are large organs essential to the function of the gastrointestinal system.
Food is ingested through the mouth and then prepared for swallowing by being broken up and mixed with saliva by the chewing action of the teeth. Saliva has a major lubricant and minor digestive function. The food is formed into a pellet or bolus and is then swallowed by being moved back by the tongue into the pharynx. Once food is in the pharynx, swallowing becomes a reflex mechanism designed to coordinate contraction of muscles to push the food through the pharynx and oesophagus to the stomach as well as ensuring food and drink do not enter the lower respiratory tract. Swallowing is complex, involving several sets of muscles in the head and neck and is described in more detail in Section 29.1. The passage of food along the remainder of the digestive tract is achieved by regular contractions (peristalsis) of the smooth muscle layers in its walls.
The oesophagus is a muscular tube about 25 cm in length. It begins in the neck as a continuation of the pharynx and lies posterior to the trachea as it enters the thorax. It passes through the thorax, lying slightly to the left against the vertebral column before passing through the muscular part of the diaphragm and entering the stomach below. The diaphragmatic muscle acts as an external sphincter which prevents gastric contents from regurgitating into the oesophagus.
If gastric reflux occurs regularly, the strong stomach acid causes the pain known as ‘heartburn’ and can/>