When a patient faints during a blood draw, it is considered to be an episode of hematemesis, also called blood phobia, which is some mental disturbance in a particular environment. This is because the patient’s fainting from seeing blood and drawing blood is mainly caused by an overactive vagus nerve response. When the vagus nerve is overactive, it causes a series of changes in the human body, such as causing the blood to flow to the legs by slowing down the heart rate and lowering the blood pressure, that means there is not enough blood to enter the human brain, so it can lead to dizziness and even fainting episodes. This condition can actually be called an anomaly, that is, contact with or the sight of blood produces a kind of consciousness, as well as an overreaction of the body, consciousness with panic, palpitations, vertigo, etc. Blood fainting actually originates from a kind of conscious activity in the cerebral cortex.