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Alcohol dilates peripheral blood vessels, but has been known to actually. constrict some blood vessels in the brain, lowering the oxygen content. Alcohol dilates peripheral blood vessels, but has been known to actually  constrict some blood vessels in the brain, lowering the oxygen content.
This is because alcohol acts as a diuretic, which means that it dehydrates the drinker. A person who is dehydrated may subsequently become constipated, and they will need to strain when opening their bowels. Binge drinking can also raise blood pressure, which in turn adds pressure to the veins in the anal area.