Syncope occurs when the part of your nervous system that regulates heart rate and blood pressure malfunction in response to a trigger, such as the sight of blood. Your heart rate slows and the blood vessels in your legs widen. This allows blood to pool in your legs, which lowers your blood pressure. This drop in blood pressure and slowed heart rate quickly diminish blood flow to your brain, and you faint.
William P. Cheshire, Syncope, Mayo Clinic, https://mayoclinic.pure.elsevier.com/en/publications/syncope
i. Neurocardiogenic Syncope
Syncope is defined as a transient loss of consciousness associated with a loss of postural tone, in which the patient recovers spontaneously without the need for electrical or pharmacological cardioversion. Although frequently thought of as a condition with a neurological origin, it’s actually a cardiovascular problem-as such, a neurologic work-up is seldom rewarding.
Neurocardiogenic syncope is caused by an abnormal or exaggerated autonomic response to various stimuli, of which the most common are standing and emotion. The two main causes of syncope are cardiac arrhythmias and neurocardiogenic (vasovagal, vasodepressor) syndromes.
Carol Chen-Scarabelli and Tiziano M Scarabelli, Neurocardiogenic Syncope, NCBI, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC506859/
ii. Vasovagal Syncope
Vasovagal syncope occurs when you faint because your body overreacts to certain triggers, such as the sight of blood or extreme emotional distress. The vasovagal syncope trigger causes your heart rate and blood pressure to drop suddenly. This leads to reduced blood flow to your brain, causing you to briefly lose consciousness.
Symptoms include pale skin, lightheadedness, tunnel vision, nausea, feeling warm, a cold and clammy sweat, yawning, and blurred vision. During an episode, bystanders may notice jerky, abnormal movements, a slow, weak pulse, and dilated pupils.
Vasovagal Syncope. Mayo Clinic. https://www.mayoclinic.org/diseases-conditions/vasovagal-syncope/symptoms-causes/syc-20350527