In which biological process does menstruation occur?

Menstruation is a crucial part of the female reproductive cycle, specifically linked to the process of reproduction. This cycle involves the monthly preparation of the uterine lining for potential pregnancy. If an egg released during ovulation is not fertilized, the hormonal changes lead to the shedding of the uterine lining, which is menstruation. Therefore, it directly relates to reproductive health and is an indicator of a functioning reproductive system. While excretion, metabolism, and digestion are important biological processes, they do not involve menstruation, which is unique to the reproductive system.