Physiology of Vomiting

Physiology of Vomiting
Vomiting is the forceful expulsion of contents of the stomach and often, the proximal small intestine. It is a manifestation of a large number of conditions, many of which are not primary disorders of the gastrointestinal tract. Regardless of cause, vomiting can have serious consequences, including acid-base derangments, volume and electrolyte depletion, malnutrition and aspiration pneumonia.
The Act of Vomiting
Vomiting is usually experienced as the finale in a series of three events, which everyone reading this has experienced:
Nausea is an unpleasant and difficult to describe psychic experience in humans and probably animals. Physiologically, nausea is typically associated with decreased gastric motility and increased tone in the small intestine. Additionally, there is often reverse peristalsis in the proximal small intestine.
Retching ("dry heaves") refers to spasmodic respiratory movements conducted with a closed glottis. While this is occurring, the antrum of the stomach contracts and the fundus and cardia relax. Studies with cats have shown that during retching there is repeated herniation of the abdominal esophagus and cardia into the thoracic cavity due to the negative pressure engendered by inspiratory efforts with a closed glottis.
Emesis or vomition is when gastric and often small intestinal contents are propelled up to and out of the mouth. It results from a highly coordinated series of events that could be described as the following series of steps (don't practice these in public):
A deep breath is taken, the glottis is closed and the larynx is raised to open the upper esophageal sphincter. Also, the soft palate is elevated to close off the posterior nares.
The diaphragm is contracted sharply downward to create negative pressure in the thorax, which facilitates opening of the esophagus and distal esophageal sphincter.
Simultaneously with downward movement of the diaphragm, the muscles of the abdominal walls are vigorously contracted, squeezing the stomach and thus elevating intragastric pressure. With the pylorus closed and the esophagus relatively open, the route of exit is clear.
The series of events described seems to be typical for humans and many animals, but is not inevitable. Vomition occasionally occurs abruptly and in the absense of premonitory signs - this situation is often referred to as projectile vomiting. A common cause of projectile vomiting is gastric outlet obstruction, often a result of the ingestion of foreign bodies.
An activity related to but clearly distinct from vomiting is regurgitation, which is the passive expulsion of ingested material out of the mouth - this often occurs even before the ingesta has reached the stomach and is usually a result of esophageal disease. Regurgitation also is a normal component of digestion in ruminants.
There is also considerable variability among species in the propensity for vomition. Rats reportedly do not vomit. Cattle and horses vomit rarely - this is usually an ominous sign and most frequently a result of acute gastric distension. Carnivores such as dogs and cats vomit frequently, often in response to such trivial stimuli as finding themselves on a clean carpet. Humans fall between these extremes, and interestingly, rare individuals have been identified that seem to be incapable of vomiting due to congenital abnormalities in the vomition centers of the brainstem.
Control of Vomition
The complex, almost sterotypical set of activities that culminate in vomiting suggest that control is central, which indeed has been shown to be true. Within the brainstem are two anatomically and functionally distinct units that control vomiting:
Bilateral vomition centers in the reticular formation of the medulla integrate signals from a large number of outlying sources and their excitement is ultimately what triggers vomition. Electric stimulation of these centers induces vomiting, while destruction of the vomition centers renders animals very resistant to emetic drugs. The vomition centers receive afferent signals from at least four major sources:
The chemoreceptor trigger zone (see below)
Visceral afferents from the gastrointestinal tract (vagus or sympathetic nerves) - these signals inform the brain of such conditions as gastrointestinal distention (a very potent stimulus for vomition) and mucosal irritation.
Visceral afferents from outside the gastrointestinal tract - this includes signals from bile ducts, peritoneum, heart and a variety of other organs. These inputs to the vomition center help explain how, for example, a stone in the common bile duct can result in vomiting.
Afferents from extramedullary centers in the brain - it is clear that certain psychic stimuli (odors, fear), vestibular disturbances (motion sickness) and cerebral trauma can result in vomition.
The chemoreceptor trigger zone is a bilateral set of centers in the brainstem lying under the floor of the fourth ventricle. Electrical stimulation of these centers does not induce vomiting, but application of emetic drugs does - if and only if the vomition centers are intact. The chemoreceptor trigger zones function as emetic chemoreceptors for the vomition centers - chemical abnormalities in the body (e.g. emetic drugs, uremia, hypoxia and diabetic ketoacidosis) are sensed by these centers, which then send excitatory signs to the vomition centers. Many of the antiemetic drugs act at the level of the chemoreceptor trigger zone.
To summarize, two basic sets of pathways - one neural and one humoral - lead to activation of centers in the brain that initiate and control vomition. Think of the vomition centers as commander in chief of vomition, who makes the ultimate decision. This decision is based on input from a battery of advisors, among whom the chemoreceptor trigger zone has considerable influence. This straighforward picture is almost certainly oversimplified and flawed in some details, but helps to explain much of the physiology and pharmacology of vomition.
Causes and Consequences of Vomiting
The myriad causes of vomiting are left as an exercise - come up with a list based on personal experience and your understanding of the control of vomition. An important point, however, is that many cases of vomiting are due to diseases outside of the gastrointestinal tract.
Simple vomiting rarely causes problems, but on occasion, can lead to such serious consequences as aspiration pneumonia. Additionally, severe or repetitive vomition results in disturbances in acid-base balance, dehydration and electrolyte depletion. In such cases, the goal is to rapidly establish a definitive diagnosis of the underlying disease so that specific therapy can be instituted. This is often not easy and in many cases, it is advantageous to administer antiemetic drugs in order to suppress vomition and reduce its sequelae.