The blood gets slower and thicker. Your heart rate rises to maintain adequate oxygen levels. Dehydration will result from continuing to engage in strenuous activity without water. All the components in blood plasma dissolve in water, which serves as a solvent. Water is necessary to maintain blood diluted enough to flow through blood arteries. Blood must flow freely to eliminate the waste products of metabolism, such as urea and electrolytes. Moreover, drinking water lowers blood pressure.