The semen sample consists of the spermatozoa and the seminal plasma, the sum of secretions that originate mainly from the seminal vesicles, the prostate and the epididymis. These secretions come together at ejaculation.
If an obstruction exists in the male reproductive system the spermatozoa are not found in the ejaculate and the sample is azoospermic. An obstruction also blocks the flow of the secretions from the glands. An obstruction at the ejaculatory ducts will prevent the secretions from all the glands to go through whereas if the obstruction is at the level of the epididymis only the secretion of the epididymis will be blocked.
Semen biochemistry is needed when obstruction in the male reproductive system is suspected and can also determine the site of the obstruction based on the presence or absence of certain molecules.