Which structure serves as the passageway for menstrual flow in females?

The vaginal canal is the structure that serves as the passageway for menstrual flow in females. 

During menstruation, the endometrial lining of the uterus sheds and the blood and tissue flow out of the body through the vagina. 

The uterus is the organ responsible for developing and shedding the endometrial lining, while the oviducts (or fallopian tubes) transport eggs from the ovaries to the uterus. 

The ovaries are the reproductive organs that produce eggs and hormones. 

Therefore, the correct answer is the vagina.