Vomiting (Emesis): Definition and Mechanism | Reflex | Human | Biology

In this article we will discuss about:- 1. Definition of Vomiting 2. Events takes Places During Vomiting 3. Mechanism.

Definition of Vomiting:

Vomiting is a reflex which serves to relieve the upper G.I. tract by forcible expulsion of gastric contents through the mouth. This may occur either because the contents are irritating or because the organs themselves or the nerves that supply them are more irritable than normal. Excessive distention and compression or irritation of the intestine, appendix, bile ducts and other abdominal viscera can also initiate this movement. This is a reflex movement (Fig. 9.49).

Events takes Places During Vomiting:

A number of events take place during vomiting, more or less, in the following order:

1. Nausea:

At the onset a feeling of nausea is experienced, followed by excess salivation.

2. Glottis becomes closed and the nasopharynx is also shut off by raising the soft palate:

The purpose is to prevent the entry of any vomited material into the trachea and nose.

3. The body of the stomach, the cardiac sphincter and the oesophagus relax:

The pylorus contracts and press­es its contents into the relaxed stomach. Weak contraction and antiperistalsis may take place in the stomach but they are not important, because vomiting normally takes place even if stomach is replaced by a blad­der. Pyloric sphincter remains closed.

4. Intra-abdominal pressure sharply rises:

This is caused by retching during which expiratory muscles, abdominal muscles and diaphragm contract. These contractions increase the intra-abdominal pressure so that the relaxed stomach is forcibly compressed. This increased intra-abdominal pressure is the chief motive force of vomiting. Gastric contents are pressed into oesphagus and then from the latter ejected out through the mouth. Antiperistalsis may take place in oesphagus helping ejection.

5. The process continues till the stomach is empty:

Towards the end diaphragm ascends (relaxes) and the expiratory muscles contract. Glottis being closed it raises the intrapulmonary pressure and compress the oesphagus. This helps to expel the last remnants of the vomitus from the oesophagus.

Mechanism of Vomiting:

Vomiting is a reflex process. Straightforward vomiting is governed by a vomiting centre which is situated in the dorsal part of the lateral reticular formation of the medulla lying ventral to the solitary tract and its nucleus.

It forms one of the components of the complex visceral centres which include salivation defaecation and vasomotor centre and vestibular nuclei. So there is consistent relationship of vomiting with salivation defae­cation, respiration and vasomotion. This centre can be directly stimulated (central vomiting) by certain drugs (apomorphine, etc.), certain toxins (such as those of uraemia), increased intracranial pressure (brain tumour, asphyxia, meningitis, etc.) and such others. It can be reflexly stimulated in various ways.

There are two pathways by which the vomiting centre is affected:

1. Nervous path.

2. Vascular path.

The nervous path lies in the various afferent pathways coming from various organs especially digestive tract. The most sensitive part is in the first part of duodenum. The afferent impulses may also arise in the throat (tickling—sensory nerves V and IX), stomach (irritation), intestine or other organs outside the gastro­intestinal tract like heart, kidney, uterus or semicircular canals.

Chemical substances (emetics) pass via body fluids and act on chemosensitive area called chemoreceptor trigger zone in the floor of IV^th ventricle and cause vomiting. Destruction of this area results loss of response when emetics placed directly on the receptor site in chemoreceptor trigger zone.

The efferent impulses—both excitatory and inhibitory, are carried in the phrenic chiefly and vagus and the sympathetic.

The commonest cause of vomiting is gastric irritation and its purpose is to drive out the irritant from the stomach.

Projectile Vomiting:

The central vomiting caused by stimulation of central nervous system (e.g., trauma and tumour in brain and irritation in meninges etc.) is projectile type having little feeling of nausea and absence of participation of voluntary muscles.

Persistent Type of Pernicious Vomiting of Pregnancy:

In this type the excitability of the centre is in­creased by metabolic disturbances (e.g., carbohy­drate starvation and dehydration with ketosis).

Loss of water, and both H⁺ and Na⁺ ions due to excessive vomiting might be harmful.

Emetics are substances that cause vomiting (such as drinking of sodium chloride and warm water).