Vomiting (or emesis) is the forceful expulsion of the contents of one's stomach through the mouth. Although it probably evolved as a mechanism for expelling ingested poisons, it may be due to many causes not related to poisoning, ranging from gastritis to brain tumors. The feeling that one is about to vomit is called nausea. It usually, but not necessarily, precedes vomiting, nor does it always lead to vomiting. Antiemetics are sometimes necessary to suppress nausea and vomiting.
The medical branch investigating vomiting, emetics and antiemetics is called emetology.
Vomiting is co-ordinated in the vomiting center in the medulla. Receptors on the floor of the fourth ventricle of the brain represent a chemoreceptor trigger zone , stimulation of which can lead to vomiting. The chemoreceptor zone lies outside the blood-brain barrier, and can therefore be stimulated by blood-borne drugs which can stimulate vomiting, or inhibit it.
The vomiting act encompasses:
- Increased salivation to protect the enamel of teeth from stomach acids (excessive vomiting does lead to caries).
- Retroperistalsis , starting from the middle of the small intestine, sweeping up the contents of the digestive tract into the stomach, through the relaxed pyloric sphincter.
- A lowering of intrathoracic pressure (by inspiration against a closed glottis), coupled with an increase in abdominal pressure as the abdominal muscles contract, propels stomach contents into the esophagus without involvement of retroperistalsis. The lower esophageal sphincter relaxes.
- Vomiting is ordinarily preceded by retching. The purpose of retching is to build up the pressure needed to expel the stomach contents from the body. In retching, the body makes movements similar to vomiting. These spasms build up pressure within the chest cavity. When a sufficient amount of pressure has been created, the diaphragm transfers the pressure from the chest to the abdomen, and this pressure then results in actual vomiting.
The neurotransmitters that regulate vomiting are poorly understood, but inhibitors of dopamine, histamine and serotonin are all used to suppress vomiting, suggesting that these play a role in the initiation or maintenance of a vomiting cycle. Vasopressin and neurokinin may also participate.
As the stomach secretes acid, vomit contains a high concentration of hydronium and is thus strongly acidic. The potential physiological complications associated with excessive vomiting are mainly metabolic alkalosis (increased blood pH), hypokalemia (potassium depletion) and hypochloremia (chlorine depletion). The hypokalemia is an indirect result of the kidney compensating for the loss of acid.
The content of the vomitus (vomit) may be of medical interest. Fresh blood in the vomit is termed hematemesis ("blood vomiting"). Old blood bears resemblance to coffee grounds (as the iron in the blood is oxidized), and when this matter is identified the term "coffee ground vomiting" is used. Bile can enter the vomit during subsequent heaves due to duodenal contraction if the vomiting is severe. Fecal vomiting is often a consequence of intestinal obstruction , and is treated as a warning sign of this potentially serious problem.
Vomiting may be due to a large number of causes, and protracted vomiting has a long differential diagnosis.
Causes in the digestive tract:
Sensory system and brain
Causes in the sensory system:
Causes in the brain:
Metabolic disturbances (these may irritate both the stomach and the parts of the brain that coordinate vomiting):
Opioids, many chemotherapy drugs and a host of other drugs may cause nausea and vomiting.
An emetic, such as Syrup of Ipecac, is a substance that induces vomiting when administered orally or by injection. An emetic is used medically where a substance (typically poison) has been ingested and must be expelled from the body immediately. Inducing vomiting can remove the substance before it is absorbed into the body.
An antiemetic is a drug that is effective against vomiting and nausea. Antiemetics are typically used to treat motion sickness and the side effects of some opioid analgesics and chemotherapy directed against cancer.
Most people try to contain their vomit by vomiting into a sink, toilet, or trash can, as both the act itself and the vomit are widely considered embarrassing. On airplanes and boats, special bags are supplied for sick passengers to vomit into. Alternatively, a special disposable bag is available containing absorbent material that solidifies the vomit quickly, making it convenient and safe to keep (leakproof, puncture resistant, odorless) until there is an opportunity to dispose of it conveniently.
People who vomit chronically (e.g. as part of an eating disorder) may devise various ways to hide this abnormality.
As with other physiological processes involving body wastes, vomiting has taboo aspects. This is shown by the large number of colourful euphemisms for vomiting. (see: toilet humour)
In other animals
Whales vomit regularly (every 7 to 10 days) as a means of the ordinary digestive process, to expel inedible things they have swallowed.
The domestic cat is well known for its tendency to vomit, particularly when attempting to dislodge hairballs from its throat or upper gastrointestinal tract. Dogs also vomit often (frequently after eating grass) and are also known for eating their own vomit, a fact even cited in the Bible.
Some adult birds regurgitate food to feed their young, triggered by a feather or a beak of their young. The food can be either incompletely digested or partially predigested, depending on the species. Some bird species may also use regurgitation as a form of defense, vomiting when wounded or molested. When an intruder or a predator comes near a fulmar on its nest, the bird vomits oil up to 3 feet at the enemy.
Some large mammals, including horses, rarely vomit. Many rodents lack the ability to vomit, which is why mice and rats are easily killed by poisoning.