Final answer:
The liquid portion of semen comes from the seminal vesicles, the prostate gland, and the bulbourethral glands, which together produce a fluid rich in nutrients, enzymes, and substances that facilitate sperm motility and viability in the acidic vaginal environment.
Step-by-step explanation:
The liquid portion of semen is secreted by three critical accessory sex glands in the male reproductive system: the seminal vesicles, the prostate gland, and the bulbourethral glands. The seminal vesicles contribute about 60-70% of the seminal fluid, and their secretion is thick, yellowish, and alkaline which helps provide an environment conducive to sperm motility by reversing the acidity of the vaginal environment. This fluid includes nutrients such as fructose to nourish the sperm, as well as mucus, coagulating enzymes, ascorbic acid, and prostaglandins.
The prostate gland secretes an alkaline fluid that makes up close to 30% of semen. This fluid contains zinc, which is vital for sperm health and motility. Lastly, the bulbourethral glands secrete a smaller volume of fluid compared to the other glands, but it plays a crucial role in lubricating the urethra and neutralizing traces of acidic urine before the semen is passed during ejaculation.