The area postrema is a circumventricular organ and as such lies outside the blood–brain barrier; it can therefore be stimulated by blood-borne drugs that can stimulate vomiting or inhibit it.There are various sources of input to the vomiting center:

The chemoreceptor trigger zone at the base of the fourth ventricle has numerous dopamine D2 receptors, serotonin 5-HT3 receptors, opioid receptors, acetylcholine receptors, and receptors for substance P. Stimulation of different receptors are involved in different pathways leading to emesis, in the final common pathway substance P appears involved. The vestibular system, which sends information to the brain via cranial nerve VIII (vestibulocochlear nerve), plays a major role in motion sickness, and is rich in muscarinic receptors and histamine H1 receptors. The cranial nerve X (vagus nerve) is activated when the pharynx is irritated, leading to a gag reflex. The vagal and enteric nervous system inputs transmit information regarding the state of the gastrointestinal system. Irritation of the GI mucosa by chemotherapy, radiation, distention, or acute infectious gastroenteritis activates the 5-HT3 receptors of these inputs. The CNS mediates vomiting that arises from psychiatric disorders and stress from higher brain centers. The medulla plays an important role for triggering the vomiting act.The vomiting act encompasses three types of outputs initiated by the chemoreceptor trigger zone: Motor, parasympathetic nervous system (PNS), and sympathetic nervous system (SNS). They are as follows:

Increased salivation to protect tooth enamel from stomach acids. (Excessive vomiting leads to dental erosion.) This is part of the PNS output.
Mechanism
When an arteriovenous fistula is formed involving a major artery like the abdominal aorta, it can lead to a large decrease in peripheral resistance. This lowered peripheral resistance causes the heart to increase cardiac output to maintain proper blood flow to all tissues. The physical manifestations of this typically consist of a relatively normal systolic blood pressure accompanied by decreased diastolic blood pressure, resulting in a wider pulse pressure.Normal blood flow in the brachial artery is 85 to 110 milliliters per minute (mL/min). After the creation of a fistula, the blood flow increases to 400–500 mL/min immediately, and 700–1,000 mL/min within 1 month. A brachiocephalic fistula above the elbow has a greater flow rate than a radiocephalic fistula at the wrist. Both the artery and the vein dilate and elongate in response to the greater blood flow and shear stress, but the vein dilates more and becomes "arterialized". In one study, the cephalic vein increased from 2.3 mm to 6.3 mm diameter after 2 months. When the vein is large enough to allow cannulation, the fistula is defined as "mature".An arteriovenous fistula can increase preload. AV shunts also decrease the afterload of the heart. This is because the blood bypasses the arterioles which results in a decrease in the total peripheral resistance (TPR). AV shunts increase both the rate and volume of blood returning to the heart. Diagnosis
The diagnosis of this condition can be done via ultrasound

See also
References


== External links ==