Vomiting activity is coordinated in the vomiting centre in medulla oblongata (part of the brain).
Stimulation of the chemoreceptor trigger zone (receptors on the floor of the fourth ventricle of the brain) causes vomiting.
- the trigger zone lies outside the blood-brain barrier, therefore it can be stimulated by blood-borne drugs (emetics).
- increased salivation in anticipation, to protect teeth from stomach acids.
- retroperistalsis (peristalsis backwards), from the middle of small intestine towards the stomach, through a relaxed pyloric sphincter. This is why you can sometimes see bile.
- lowering of intrathoracic pressure (by breathing in against a closed glottis), coupled with raising of abdominal pressure (by contraction of abdominal muscles), causes stomach contents to be propelled into the oesophagus without retroperistalsis; lower oesophageal sphincter relaxes.
- upper oesophageal sphincter relaxes to allow stomach contents (vomitus) to be expelled. Bile may enter vomit during subsequent heaves due to contraction of the duodenum, if the vomiting is severe.
In principle, it is possible to vomit faeces, because rectally-inserted pills have been vomited up before.
Retch = “vomiting” with the upper oesophageal sphincter contracted; contents drain back into the stomach. Upper oesophageal sphincter usually relaxes after a few retches.
Vomiting is elicited by:
- stretched stomach.
- genitourinary pain
- touching the back of the throat
Vomiting causes the following physiological complications:
- metabolic alkalosis
- hypokalaemia à muscle weakness
- teeth damage