Dehydration can occur as a result of drinking too much alcohol. Alcohol is a diuretic, which means that it makes you urinate more. If this fluid isn’t replaced you may become dehydrated, so drinking alcohol should be balanced with drinking non-alcoholic fluids, such as water.
Excess alcohol consumption can cause dehydration in a variety of ways.Alcohol decreases the body’s production of anti-diuretic hormone, which is used by the body to reabsorb water. With less anti-diuretic hormone available, your body loses more fluid than normal through increased urination.
The digestive tract passes alcohol to the blood stream, which is why your brain can be affected. As your blood volume increases, your kidneys work to reduce the volume, but they take out whatever mix of liquids you’ve got. (Your liver works to metabolize the poisonous alcohol, which is why it gets damaged.) If I understand correctly, one of alcohol’s properties in the blood is that it doesn’t carry hemoglobin, the transporter for oxygen, so you’re starving the system (and brain) of oxygen. As the liver finally begins to get rid of the alcohol, your blood volume will be reduced, probably challenging your heart muscle but lowering your O₂ level even more. In the morning, you’re going to wake up (or after some hours, if you happen to stay awake, you’ll find yourself to be) dehydrated, and with a massive headache from oxygen starvation (just like the folks who suffer from sleep apnea.)
You can counteract some of the dehydrating effect by drinking a large volume of water that will dilute the alcohol volume in your blood. Think in terms of multiple glasses of water for every alcoholic drink you had, although it still isn’t going to flush it all out. And consider taking an aspirin, AlkaSeltzer or other remedy for the headache pain that’s coming.