Final answer:
The narrow lower portion of the uterus is called the cervix(a). It connects the uterine cavity to the vagina and plays crucial roles in both fertilization and childbirth, including mucus secretion that aids in sperm movement.
Step-by-step explanation:
The narrow lower portion of the uterus is known as the cervix(a). The cervix is approximately 2.5 cm long and extends downward into the vagina. It has a small canal that connects the uterine cavity with the lumen of the vagina, allowing for the passage of semen into the uterus, and during childbirth, it serves as the birth canal for the baby to pass from the uterus into the vagina. The cervix also secretes mucus that changes in consistency, serving as a barrier to microorganisms or as a medium to assist sperm movement during ovulation. Above the cervix is the body of the uterus, or corpus, and the uppermost part is called the fundus. The uterus itself is a muscular organ lined with a tissue called the endometrium, which supports a developing baby.