Semen has two parts: sperm cells and the liquid they swim in. That liquid is seminal plasma. It is built from secretions of the testes, the epididymis and the accessory sex glands, and contains ions, hormones, proteins and peptides (including cytokines and enzymes), cholesterol, and DNA and RNA, often packaged inside tiny extracellular vesicles1.
The seminal vesicles provide about 65% of the volume, the prostate about 25%, the testes and epididymis around 10%, and the bulbourethral glands about 1%2. So when people talk about increasing semen volume, they are really talking about seminal plasma.
Seminal plasma is not the same as pre-ejaculate, which is a separate, much smaller secretion released during arousal, before ejaculation.
What does seminal plasma do?
It carries sperm, gives them energy, and briefly holds them in a gel before releasing them. It also interacts with the female reproductive tract in ways that may support conception.
Its main jobs, as described in the research:
- Transport: it is the vehicle that gets sperm out of the body and into the vagina.
- Gel and release: seminal vesicle proteins form a clot that prostate enzymes, chiefly PSA, then dissolve2.
- Sperm support: seminal vesicle secretions contribute to sperm motility, the stability of sperm DNA packaging and suppression of immune activity in the female tract3.
- Chemical signalling: zinc and citrate from the prostate help coordinate the chain of events triggered by ejaculation4.
- Preparing the female tract: seminal plasma triggers changes in the female genital tract that may help prepare for pregnancy1.
What is in seminal plasma?
A mix of sugars, proteins, minerals and enzymes, with each gland adding its own signature ingredients. Together they make up the bulk of every ejaculate.
The main contributions by source:
- Seminal vesicles: fructose, semenogelins, fibronectin, prostaglandins and cytokines2.
- Prostate: zinc, citrate and kallikrein enzymes, including PSA2,4.
- Testes and epididymis: the sperm, in a small volume of fluid2.
- Bulbourethral glands: a little mucus, which acts as a lubricant2.
Do sperm need seminal plasma to fertilise an egg?
Not strictly. Sperm taken directly from the epididymis, or ejaculated sperm washed free of seminal plasma, can still fertilise an egg.
This is why assisted reproduction can use washed sperm successfully1. In natural conception, though, seminal plasma does real work, from protecting sperm on the journey to influencing the environment they arrive in1,3.
Researchers are still working out how much each ingredient matters, and much of the detailed evidence comes from animal studies1.
Can seminal plasma tell doctors anything?
Yes. Because each gland adds its own ingredients, measuring them can show which gland is underperforming or where a blockage might be.
Fructose is the classic marker of seminal vesicle function, although it is affected by sperm count and needs careful interpretation3. Zinc and citrate reflect prostate activity4. Prostate inflammation and infection can also affect sperm function and fertility4.
If a semen analysis shows low volume, a doctor may order these tests to work out why. Talk to a doctor if you have ongoing low volume, pain on ejaculation or blood in your semen.
Common questions
Is seminal plasma the same as semen?
Not quite. Semen is seminal plasma plus sperm. Seminal plasma is the liquid part only, and it makes up most of the volume, so most of what a lab records as semen volume is really seminal plasma.
Where does seminal plasma come from?
Mostly from the seminal vesicles (about 65%) and the prostate (about 25%), with smaller amounts from the testes, epididymis and bulbourethral glands. Each adds different proteins, sugars and minerals.
Is there still seminal fluid after a vasectomy?
Yes. A vasectomy blocks the tubes that carry sperm from the testicles, but the seminal vesicles and prostate, which make about 90% of the fluid, are untouched. The ejaculate simply no longer contains sperm once any remaining sperm have cleared.
Read more on this
Sources
- Rodriguez-Martinez H, Martinez EA, Calvete JJ, et al. Seminal plasma: relevant for fertility? Int J Mol Sci. 2021;22(9):4368. PubMed
- Anamthathmakula P, Winuthayanon W. Mechanism of semen liquefaction and its potential for a novel non-hormonal contraception. Biol Reprod. 2020;103(2):411-426. PubMed
- Gonzales GF. Function of seminal vesicles and their role on male fertility. Asian J Androl. 2001;3(4):251-258. PubMed
- Verze P, Cai T, Lorenzetti S. The role of the prostate in male fertility, health and disease. Nat Rev Urol. 2016;13(7):379-386. PubMed
Not medical advice. Written by Eric Stephens from the research cited above; not reviewed by a doctor. If something worries you, see a GP.