In the stomach, alcohol gets mixed with stomach acid and any other stomach contents.
A small percentage of alcohol is broken down in the stomach by the enzyme alcohol dehydrogenase (ADH) into a substance called acetaldehyde.
Women have rather less ADH in their stomachs than men, so less alcohol is broken down in their stomachs. For this and other reasons, women who drink the same amount of alcohol as men end up with a higher alcohol concentration in their blood.