You become unable to process information efficiently. Alcohol is a depressant and, after it hits the cerebral cortex, it dulls your pain and inhibition receptors. The limbic system (the body's emotional centre) is also hit, giving you that loving feeling.
Ninety per cent of alcohol is absorbed into the bloodstream through the stomach and intestines, affecting the whole body. Fizzy drinks accelerate this process as the carbon dioxide opens the intestines' pyloric valve to speed up absorption.
The bladder fills with more water than it should because alcohol inhibits the antidiuretic hormone vasopressin, which stimulates the kidneys to reabsorb water so you don't dehydrate. Hence so much time spent in the gents after 11pm.
Alcohol is broken down into the toxic chemical acetaldehyde, which is further broken down into water and carbon dioxide. The liver metabolises about one unit an hour, so the last drink stays with you until the next day.
It beats faster because alcohol is a vasodilator, relaxing your blood vessels and lowering blood pressure. Your heart rate increases to get enough blood to your organs. Either that, or you've finally noticed that girl at the bar.