Vomiting, also known as emesis, is the forceful expulsion of stomach contents through the mouth. This reflex protects the body by removing harmful substances or irritants. While often uncomfortable, vomiting frequently follows a sensation of nausea, an unpleasant feeling.
The Brain’s Control Center
The command center for the vomiting reflex resides within the brainstem, in the medulla oblongata. This area contains a network of neurons called the “vomiting center.” This center integrates various signals from throughout the body to initiate expulsion.
Near the vomiting center is the chemoreceptor trigger zone (CTZ), found on the floor of the fourth ventricle. The CTZ is outside the blood-brain barrier, allowing it to directly detect circulating toxins, drugs, or hormones in the bloodstream and cerebrospinal fluid. When the CTZ identifies these substances, it sends signals to the vomiting center, which then coordinates vomiting.
The vomiting center also receives input from other bodily systems. Signals can originate from the gastrointestinal tract due to irritation or distension, transmitted via the vagus nerve. The vestibular system in the inner ear, responsible for balance, sends signals that contribute to motion sickness. Additionally, higher brain centers can trigger vomiting in response to sensory inputs like severe pain, unpleasant sights, or strong odors.
The Physical Process of Expulsion
Vomiting involves coordinated muscular actions. It is a forceful expulsion, distinguishing it from passive regurgitation. The process begins with a relaxation of the stomach muscles and the pyloric sphincter, which controls food flow from the stomach to the small intestine.
Simultaneously, the lower esophageal sphincter, a barrier between the esophagus and stomach, relaxes to allow contents to pass upwards. Involuntary contractions of the diaphragm and abdominal muscles then occur. These contractions increase the pressure within the abdomen.
This surge in intra-abdominal pressure forces the stomach’s contents through the relaxed esophagus and out of the mouth. To prevent stomach contents from entering the airways, a deep breath is taken, and the epiglottis, a flap of cartilage, closes over the windpipe. Increased salivation also often precedes vomiting, providing a protective effect for tooth enamel against the acidic stomach contents.
Understanding Vomit’s Contents
Vomit primarily consists of partially digested food mixed with gastric juices. These gastric juices are composed of hydrochloric acid, digestive enzymes, water, and mucus. Hydrochloric acid plays an important role in breaking down food and neutralizing many bacteria that enter the digestive system.
The stomach lining produces mucus, a protective substance that shields its walls from stomach acid. Some of this mucus may be expelled during vomiting. If the stomach is empty or vomiting is prolonged, greenish-yellow bile, produced by the liver and stored in the gallbladder, may also appear in the vomit. The color and consistency of vomit can therefore vary depending on what was recently consumed or the presence of bile.
Common Triggers and When to Be Concerned
Various factors can initiate the vomiting reflex by sending signals to the brain’s control centers. Common triggers include infections, such as viral gastroenteritis or food poisoning, which often cause inflammation in the digestive tract. Motion sickness, certain medications, excessive alcohol consumption, and morning sickness during pregnancy are also frequent causes.
Other potential triggers include migraines, severe pain, or more serious conditions like appendicitis or an intestinal obstruction. While many instances of vomiting are not serious and resolve on their own, certain signs indicate a need for medical attention. It is important to seek immediate care if signs of severe dehydration develop, such as a dry mouth or significantly reduced urination.
Persistent vomiting that lasts for more than 24 to 48 hours, or is accompanied by severe abdominal pain, also warrants medical evaluation. The presence of blood in vomit, which can appear bright red, dark, or like coffee grounds, requires immediate medical attention, as it can signal internal bleeding. Vomiting following a head injury also requires prompt medical assessment, particularly if it is continuous or associated with confusion or a severe headache, as this could indicate a skull fracture or bleeding in the brain.