Swallowing is the mechanism by which food is transported from the mouth to the stomach. Part of the mechanism is under active control while the rest is under autonomic control.
This article shall consider the process of swallowing and some clinical conditions that may result from the process going wrong.
Phases of Swallowing
Voluntary Phase
Mastication leads to a bolus of food being produced, during this stage the back of the tongue is elevated and the soft palate pulled anteriorly against it. This keeps the food within the oral cavity and allows the airway to remain open. The duration of this stage varies.
Following this, inspiration is inhibited and the bolus of food is moved to the pharynx by the tongue. This leads to the stimulation of the swallowing reflex.
Pharyngeal Phase
Once the bolus has been moved to the pharynx, pressure receptors are activated in the palate and anterior pharynx. This signals the swallowing centre in the brain stem which:
- Inhibits respiration
- Raises the larynx
- Closes the glottis
- Opens the upper oesophageal sphincter
The soft palate is elevated to close the nasopharynx to allow passage of food. In addition to this, the true vocal cords close to prevent aspiration.
After this, the bolus is moved towards the oesophagus via peristalsis of the pharyngeal constrictor muscles. Gravity makes very little contribution to this process and the main factors affecting the speed of this are the viscosity and volume of the bolus.
Oesophageal Phase
The upper third of the oesophagus is voluntary skeletal muscle and the lower two thirds are involuntary smooth muscle. Further information on the anatomy of the oesophagus can be found here.
At the beginning of this phase, the larynx lowers, returning to its normal position. The cricopharyngeus muscle then contracts to prevent reflux and respiration begins again.
The bolus is moved down the oesophagus via peristalsis, which is coordinated by extrinsic nerves. Each area of muscle systematically relaxes to allow food through and contracts afterwards to propel it further. The bolus is propelled at a rate of around 3-5 cm per second and so the transit time to the stomach takes around 9 seconds.
Fig 1 – Diagram showing the process of swallowing.
Clinical Relevance – Dysphagia
Dysphagia is the term for difficulty swallowing. It can affect swallowing of both solid and liquid substances depending on the cause. In addition to this, it can be due to motility problems (an error of peristalsis) or obstruction of the tube.
Common causes include:
- Stroke
- Alzheimer’s disease
- Tumours
- Xerostomia
- Oesophageal strictures
- Hiatus hernia
The presence of dysphagia is typically identified via a barium swallow study and then further tests, such as CT scans, can be used to identify causes.
The treatment depends on the cause and severity of the patient’s difficulty. They can include swallowing therapy, dietary changes, surgery or in extreme cases nasogastric tubes. Complications of dysphagia can include aspiration, dehydration and weight loss.
Fig 2 – A normal barium swallow.
Fig 2 - A normal barium swallow.[/caption]