Final answer:
The prostate gland secretes several enzymes and citrate during ejaculation, making up about 30% of the semen. Additionally, the bulbourethral glands release a fluid that lubricates and neutralizes the urethra prior to ejaculation.
Step-by-step explanation:
The walnut-shaped prostate gland secretes several enzymes and citrate into the urethra during ejaculation. This glandular secretion is a thin, milky fluid that contains citrate, enzymes, and prostate specific antigen (PSA). PSA, a proteolytic enzyme, plays a role in liquefying semen after its release, and the prostate's secretions make up about 30 percent of the bulk of semen.
Moreover, the paired bulbourethral glands, also known as Cowper's glands, contribute to the final composition of semen. They secrete a thick, salty fluid that not only lubricates the urethra but also neutralizes any acidic residue left from urine, ensuring a protective environment for the sperm. This bulbourethral secretion occurs prior to ejaculation and represents a smaller proportion of the total ejaculation compared to the contributions of the prostate gland and seminal vesicles.