Which structure acts as a muscular tube between the external environment and the uterus for both sperm entry and childbirth?

The vagina serves as a muscular tube that connects the external environment to the uterus. Its primary functions are twofold: it provides a passageway for sperm to enter the reproductive system during intercourse, and it serves as the birth canal during childbirth. Structurally, the vagina is designed to accommodate these functions, featuring elastic walls that can stretch to allow for both the entry of sperm and the passage of a newborn during delivery.

In contrast, the urethra is primarily involved in the transport of urine and does not facilitate sperm entry or childbirth. Ova are the female gametes and do not represent a structural tube; they are produced in the ovaries and must be fertilized for reproduction to occur. The ovaries themselves are reproductive organs that produce eggs and hormones, but they are not involved in the direct passage of sperm or the process of childbirth. Therefore, the vagina is uniquely suited to fulfill the role of connecting the external environment to the internal reproductive structures.