Why do you feel faint or dizzy whenever you stand up very quickly?

All people may not always feel dizzy when they stand up quickly.However some people may experience this sensation some times depending on their health condition and, how quickly they stand up, and their body condition just before the standing up. This happens because of sudden and temporary reduction of blood supply to the brain.

When we are sitting or lying down, our body is relaxed and the body consumes minimum energy and oxygen. Thus the heart also has to supply less blood to the body and therefore beats at lower rate. Also, when the we are lying down, the heart and brain being at almost same level, the heart has to make less effort to pump blood to the brain. However when we suddenly stand up from a relaxing position the body reacts internally, preparing for action. Thus the oxygen consumption of the entire body, along with that of the brain increases. With this the heart also need to immediately start pumping additional blood. But the heart may take some time to adjust to the requirement of increased blood supply. During this adjustment period, the brain, being located a higher level than heart, may not receive adequate blood and, with that, oxygen supply. This lack of enough oxygen to the brain makes a person feel dizzy.