The sperm entering the fallopian tubes pass through multiple barriers from the vagina, such as cervical mucus, uterine cavity, and at the climax of intercourse, sperm are drawn into the isthmus through the interstitial part of the fallopian tube by the relaxation of the funnel-shaped uterine horn sphincter and the attraction of muscle peristalsis. The utero-tubal junction is another barrier for sperm to enter the fallopian tubes, regulating the number of sperm entering the tubes, and progesterone also affects sperm transport. The isthmus of the fallopian tube has contracted retroperistaltic segmental waves that have the dual function of transporting sperm toward the ovary to the site of fertilization and transporting the fertilized egg in the reverse direction to the uterine cavity. The tubal fluid acts as a carrier for sperm transport and provides nutrition for both sperm and fertilized eggs. The main stream of the tubal fluid flows from the junction of the uterus and the fallopian tubes toward the abdominal cavity and is a driving force to propel the sperm forward in the fallopian tubes. After entering the vagina and passing through the uterus to reach the fallopian tubes, most of the sperm are blocked at the proximal end of the isthmus of the fallopian tubes where they are energized and undergo the acrosome reaction, waiting for the opportunity to ovulate and fertilize. Only a small percentage is transported to the fallopian tube umbilicus within a few minutes, which may be related to saturation of the storage site in the reproductive tract. Once ovulation has occurred, sperm are slowly released from the isthmus into the abdomen and provided with the most viable number of sperm for fertilization. The mechanisms by which the isthmus controls sperm release and promotes sperm capacitation are unknown and may be related to the following factors: 1. Increased concentrations of progesterone, androstenedione, estradiol and PGF2a in the proximal blood of the fallopian tube during ovulation may regulate the contraction and permeability of the isthmus smooth muscle. 2. The secretory function of the isthmus secretory cells is also most active during ovulation, secreting a variety of proteins such as 33.8% albumin, 44.4% globulin, 1.8% r-globulin and various enzymes such as amylase and lactate dehydrogenase. 3. Carbonic anhydrase is secreted from the cell membrane of the uterine oviduct junction and isthmus, which regulates the acid-base balance of the lumen. The pH of the fallopian tube increases from 7.1-7.3 to 7.5-7.8, which is favorable to the activity of sperm. 4. Potassium ions in the isthmus inhibit and stimulate the synthesis of pyruvate, which also has a role in sperm motility. 5. The content of catecholamines, such as dopamine, norepinephrine and epinephrine, in the lumen of the isthmus during ovulation is higher than in the abdomen, thus regulating the tension of smooth muscle in the isthmus to control the release of stored sperm.