Seminal vesicles, semen volume and fructose
The seminal vesicles, a pair of glands behind the bladder, supply most of the volume of semen (the fluid expelled at ejaculation) and the fructose that fuels sperm.
Remember
The seminal vesicles provide most of the volume of semen, about two-thirds, so blocking both of them lowers volume and fructose while sperm production stays normal.
Memory aid
think of the seminal vesicles as the fuel tank of the ejaculate
Use the fuel tank to recall that the vesicles add most of the liquid and the sugar.
Semen is a mixture of sperm and fluid, and most of its volume is fluid rather than sperm. The testes make the sperm, and the epididymis (the coiled tube on the back of each testis) stores them. The fluid around the sperm is added by accessory glands, which make no sperm themselves.
The seminal vesicles are the main contributor. Each one meets the vas deferens (the tube carrying sperm from the testis) to form an ejaculatory duct, which empties into the urethra.
They make about two-thirds of the volume of semen, though texts quote figures from about 60% to 70%. The prostate comes second with about a quarter, and the bulbourethral glands (small glands beside the urethra) add only a small amount of lubricating mucus.
Their fluid is alkaline, the opposite of acidic, so it helps neutralise the acidity of the male urethra and vagina (part of the female reproductive tract) and helps sperm to survive. It is also rich in fructose, a sugar that gives sperm energy.
Blockage of both glands, or their congenital absence (the glands failing to form), lowers the semen volume and the fructose. Sperm are made in the testes, so sperm production stays normal. A fructose test on a semen sample can give indirect evidence of seminal vesicle dysfunction.
Key facts
- Largest share
- the seminal vesicles, about two-thirds of the volume of semen (texts quote about 60% to 70%)
- Runner-up
- the prostate, about 25% of the volume, with enzymes that keep semen fluid
- Smallest share
- the bulbourethral glands add only a small amount of lubricating mucus
- Seminal vesicle fluid
- alkaline and rich in fructose, a sugar that gives sperm energy
- Blockage pattern
- low semen volume, low pH and low or absent fructose suggest obstruction or congenital absence of the seminal vesicles
- Sperm production
- stays normal with this pattern, because sperm are made in the testes
Exam traps
- The prostate is the runner-up, not the main contributor, because it adds about a quarter of the volume, which is less than the seminal vesicles supply.
- Blocking both seminal vesicles does not stop sperm being made, because sperm are made in the testes and stored and matured in the epididymis while the vesicles add fluid.
- The bulbourethral glands release only a little lubricating mucus, so a fault in them would not remove fructose or markedly lower the volume.
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