Your body may react to these efforts with a reflex that slows the heartbeat. Your blood pressure then decreases and you may faint. Anyone may faint if they have not had enough fluids, especially if they stand up too quickly. This happens more often after illness or working in the hot sun. But it seems that some people are just more prone to having this happen than others.
Some people are especially sensitive to changes in position and prolonged standing.
This type of fainting is called neurocardiogenic syncope. Sometimes heart problems cause fainting. Irregular heart rhythms are the most common. Abnormal heart valves can also cause drops in blood pressure that lead to fainting. Fainting can be a side effect of some medicines.