In real life the average adult has ten pints / five litres of blood. One gallon is equal to eight pints / 4.55 liters. When this trope is in play, for 'pints', read 'gallons.' Or more: all living things can contain several dozen gallons of blood, stored under incredible pressures. As a result, almost any damage by a cutting or piercing weapon will result in explosive jets of gore squirting out like a firehose. The trope is strong in anime and samurai movies. This may stem from traditional Kabuki theater, where the "blood" was really a long red silk scarf thrown in a great big arc.