The normal process of ingestion of food or fluids occurs via the oral cavity, which opens into the gastrointestinal tract. Swallowing is a complex activity with voluntary and reflexive components. It is usually described as having four stages and requires intact anatomy and sensorimotor functioning of the cranial nerves (Bass [18], Butler and Leslie [27], Logemann [87]). It starts with the oral preparatory stage, which is influenced by the sight and smell of food. Food or liquid is placed in the mouth and the lips are closed. After chewing and mixing with saliva, the food forms a cohesive mass (bolus) that is held on the centre of the tongue. The second (oral) stage of the swallow is initiated as food is moved through the oral cavity towards the pharynx. These swallowing stages are voluntary.

The third stage of swallowing is pharyngeal and is involuntary (Figure 8.11); this occurs as the food bolus crosses the mandible/tongue base as the palate closes, sealing entry to the nasal cavity and reducing the risk of nasal regurgitation. Movement of the tongue base and posterior pharyngeal wall squeezes the bolus down the pharynx. Involuntary movements of the larynx (voice box), vocal cords and epiglottis protect the airway and open the cricopharyngeus. In the fourth stage, the bolus travels from the cricopharyngeus to the gastro‐oesophageal junction and peristaltic action then transfers the bolus down the oesophagus (Atkinson and McHanwell [6], Butler and Leslie [27]).

The gastrointestinal tract is the site where food is ingested, digested and absorbed, thus enabling nutrients to be used by the body for growth and maintenance of body functions. Food ingested is moved along the gastrointestinal tract by peristaltic waves through the oesophagus, stomach, small intestine and large intestine. The passage of food and fluid through the gastrointestinal tract is dependent on the autonomic nervous system, gut hormones (such as gastrin and cholecystokinin), the function of exocrine glands (such as the parotid, pancreas and liver), and psychological aspects (such as anxiety). Sphincters situated between the stomach and duodenum, the ileocaecal valves and the anal sphincter also regulate the rate of passage of food and fluids through the tract (Barrett [17]).

Before food can be absorbed, it must be digested and broken down into molecules that can be transported across the intestinal epithelium, which lines the gastrointestinal tract. This process is dependent on digestive enzymes (secreted by the pancreas and lining of the intestinal tract) that act on specific nutrients. Bile, from the liver, is required to emulsify fat, thus enabling it to be broken down by digestive enzymes. The absorption of nutrients is dependent on an active, or energy dependent, transport across the intestinal epithelium lining the digestive tract. Villi (finger‐like projections) increase the surface area of the small intestine to aid absorption.

Most nutrients are absorbed from the small intestine, although some require specific sites within the gastrointestinal tract; for example, vitamin B₁₂ is absorbed in the terminal ileum. Mucosa shed into the lumen of the intestine is broken down and absorbed along with fluid and electrolytes secreted into the lumen during the process of digestion. The volumes of fluid secreted into the gut are large and may amount to 8–9 L per day when combined with an oral intake of 1.5–2 L daily. The majority of fluid is reabsorbed in the small intestine with the remainder being absorbed in the large intestine. Bacteria in the large intestine metabolize non‐starch polysaccharides (dietary fibre), increasing faecal bulk and producing short‐chain fatty acids, which are absorbed and metabolized for energy (Fuller et al. [57]).