Fainting – as a symptom of anxiety- can most easily be described as a sudden, temporary, loss of consciousness that is generally caused by insufficient oxygen being delivered to the brain.
Typical symptoms progress through dizziness, clamminess of the skin, a dimming of vision or brownout, possibly tinnitus, complete loss of vision, weakness of limbs to physical collapse.
Simply put, fainting is in effect the brain’s response to a chronic lack of blood. Therefore the brain attempts to prioritise its own needs above any others by commandeering all available blood flow, generally at the expense of all other bodily functions in order to increase the rate of blood oxygenation.
It diverts blood to itself by reducing the supply to most of the rest of the body through constricting the blood vessels and increasing the pulse rate. At the same time it tries to increase oxygenation of the blood by making you breathe faster.
This causes the typical symptoms of fainting such as pale skin, paling face, rapid breathing, and a weakness of the limbs, most notably the legs.
After that you collapse, with yet further symptoms of weakness and nausea.
Possibly that’s a good thing as when your legs get weak that tends to cause you to want to lay down or maybe fall down involuntarily. Which in a way is good as being horizontal is one way of countering low blood pressure, hypotension, in the brain. Fainting, is less common as a panic attack symptom."panic attack"