Water is highly underrated by most of us. It does far more than take care of our thirst. It enhances the sympathetic nervous system, making us more alert.
"David Robertson, MD, and colleagues first observed water's curious ability to increase blood pressure about 10 years ago, in patients who had lost their baroreflexes – the system that keeps blood pressure within a normal range. The observation came as a complete surprise, said Robertson, professor of Medicine, Pharmacology and Neurology."
Water does not make a noticeable difference in blood pressure for healthy people. But in those who have a tendency to experience pooling of blood in the extremities, drinking more water can make them less prone to fainting.
The American Red Cross is very interested in this newly discovered impact that drinking water has on the fainting response. They speculate that having blood donors drink water beforehand may make the experience of donating blood more pleasant.