Vomiting is the process of expelling the upper gastrointestinal contents in a forceful manner. Also known as emesis, vomiting occurs when the nervous supply to the pharynx, esophagus, stomach and duodenum of the small intestine is overstimulated. This often occurs as a result of overdistention or irritation of these areas. Less frequently, vomiting may be triggered by chemicals, disease processes not related to the gut, motion and emotional experiences.
How does vomiting occur?
Physiology of Vomiting
Peristalsis is the rhythmical contractions of the wall of the gastrointestinal tract to propel food downwards from the esophagus to the colon. In vomiting, the coordination of these contractions are disturbed in a manner that pushes the gut contents upwards – from the small intestine or stomach to the mouth. This is referred to as antiperistalsis.
Vomiting rarely occurs on its own with no stimulus. Usually stimuli in the upper tract, especially the pharynx, esophagus and stomach, sends impulses via the parasympathetic and sympathetic afferent nerve fibers to the vomiting center in the brain stem. This center is actually multiple nuclei that are located in the area around the medulla and pons. These impulses to the vomiting center from the gut may also trigger the sensation of nausea.
Other areas of the brain can also stimulate the vomiting center, either directly or indirectly. This may account for vomiting in association with certain thoughts, experiences and medical disorders that are not related to the gut. One of these associated areas is the chemoreceptor trigger zone in the medulla. When stimulated this zone will trigger the vomiting center and it is often associated with drug side effects which leads to vomiting. The trigger zone may also be stimulated indirectly by other parts of the brain like in a case of motion sickness. This cross stimulation of areas often triggers the sensation of nausea as well.
Before the Act of Vomiting
Once these areas are stimulated, the vomiting center sends motor nerve impulses to the gastrointestinal tract, diaphragm and abdominal muscles. These impulses travel via the cranial nerves, CN V (5), VII (7), IX (9), X (10) and XII (12), to the upper gut. The diaphragm and abdominal muscles receive impulses via the spinal nerves.
These nerve impulses prepare the upper gastrointestinal tract for the process of vomiting which is also known as the vomiting act. Antiperistaltic waves push the gut contents into duodenum, stomach and partially into the esophagus. These waves can start from as low as the end portions of the small intestine known as the ileum and send gut contents all the way up to the duodenum within a few minutes.
As the contents collect in the duodenum and stomach, it causes overdistension of these organs. This further stimulates the vomiting center. The LES (lower esophageal sphincter) partially relaxes allowing the contents to partly fill the esophagus.
Process of Vomiting
Once the vomiting act has been started, it is difficult to stop. It occurs as follows :
- The person takes a deep breath which is followed by the opening of the upper esophageal sphincter.
- The trachea and posterior nares close off by the movement of the glottis and soft palate respectively. This prevents the vomitus from traveling into the lungs or out through the nose.
- The diaphragm and abdominal muscles then contact thereby squeezing the stomach.
- Once the pressure within the stomach is sufficiently high, the lower esophageal sphincter (LES) opens completely and the vomitus is expelled out into the mouth.