Final answer:
The inferior one-third portion of the uterus that projects into the vagina is called the cervix, which serves as a protective barrier and the birth canal during labor.
Step-by-step explanation:
The part of the uterus that the student is asking about is called the cervix. The cervix is the inferior one-third portion of the uterus that projects into the vagina. The cervix is approximately 2.5 cm (1 in.) long and connects the uterine cavity with the lumen of the vagina. It plays a crucial role during pregnancy and childbirth, acting as a barrier to protect the uterus from microorganisms and functioning as the birth canal during labor. It is distinct from the other parts of the uterus, such as the fundus, which is the superior portion of the uterus above the opening of the uterine tubes, and the body, which is the middle section.
The cervix's mucus production is influenced by systemic plasma estrogen concentrations, which can thin and become stringy to facilitate sperm movement into the uterus. The mucus also changes in consistency throughout the menstrual cycle to either act as a barrier or aid in sperm transport. This feature, along with its structure and location, serves as a versatile component of the female reproductive system.