What It Does
The prostate makes and secretes prostatic fluid, one of the five major fluids that make up semen. Semen is the ‘bodyguard’ for sperm. Sperm make up only about 10% of the semen. The other 90% is, in effect, protection and nourishment for the sperm. The prostatic fluid gives ejaculate its typical milky colour. It makes up about a third of the entire ejaculate volume.
Prostatic fluid
Prostatic fluid is slightly alkaline. This helps motility and fertility of the sperm once it gets where it’s going. The average pH of the combined semen is about 7.5, the alkaline prostatic fluid having more than neutralised the mild acidity of the other portions of the semen.
Sperm and prostatic fluid are ejaculated first, followed by fructose rich fluid from the seminal vesicles, though you won’t be able to tell this by just looking!
A clotting enzyme from the prostatic fluid makes the ejaculate clot weakly. This holds the semen in the deeper regions of the vagina where the cervix lies. In the early minutes after ejaculation the sperm remains relatively immobile within this clot. After 15-30 minutes the clot begins to dissolve. As it dissolves, the sperm become highly motile. This has an obvious advantage in reproduction – the sperm are in the best place to increase the chances of fertilisation.
PSA, a constituent of prostatic fluid is responsible for dissolving the clot, liberating the sperm, and so making them much more active.
Apart from the clotting and dissolving enzymes prostatic fluid contains, amongst other things, calcium, citrate, and phosphate. Citrate is one of the major constituents – and the prostate has the highest concentration of any tissue in the human body. But we don’t really know why this is, or what it does. Scientists infer a role in reproduction but what it is remains obscure.
Zinc is also found in the prostate at high concentrations. Again its role remains mysterious but scientists think it has some role in fighting bacterial infection in the prostate.
Also...
Some of the prostate is made of muscle. It contracts, which helps secrete prostatic fluid into the urethra and this contraction may also help expel the ejaculate. The prostate may also make sure that men don’t pee at orgasm, but as men can have their prostate removed and still not pee during sex there’s some uncertainty about how true this is.
And though the prostate is part of the male reproduction system it’s not clear how much of a role the prostate gland may play in fertility.
Does anything else have a prostate?
All mammals have some kind of prostate though they differ considerably. The most similar to human prostate is found in the dog. Indeed the dog is the only other animal which is at risk of significant levels of benign prostate enlargement and prostate cancer.