Causes of a vasovagal response include pain, sensory stressors and straining coughing, bowel movement or urination, states MedicineNet. This causes the heart to slow down, the blood vessels in the legs to widen, and the blood pressure to drop, causing fainting episodes due to insufficient supply of oxygenated blood to the brain. A vasovagal response is most common in young adults and children, and most people who experience this condition have no underlying nerve, brain or heart problem, notes WebMD.
A vasovagal response occurs from the malfunctioning of the part of the nervous system that regulates blood pressure and heart rate, states Mayo Clinic. Stimulation of the vagus can cause an excessive release of acetylcholine, slowing the heart rate and dilating blood vessels, making it hard for blood to defeat gravity, explains MedicineNet. Blood tends to move to the legs instead of the head. Situations that can cause this response include observing an operation, hearing bad news and experiencing the sight of needles or blood.
Standing up quickly from a lying position can cause postural hypotension to some people, which is the inability of the body to regulate blood pressure appropriately, causing dizziness, lightheadedness and fainting, reports WebMD. Fainting from postural hypotension is more common in people with poor muscle tone, especially older people.