CorpusHuman Anatomy
ReproductiveFluid

Semen

Semen

Also known as: seminal fluid, ejaculate, semen analysis

The ejaculated mixture of spermatozoa (<5 % of volume) and secretions of the seminal vesicles, prostate and bulbourethral glands that transports, nourishes and protects sperm.

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Overview

Normal volume≈2–5 mL (lower limit 1.4 mL)
pH≈7.2–8.0
Liquefaction15–30 min (PSA)

A normal ejaculate is ≈2–5 mL. Sperm from the epididymis and ampulla contribute only a few per cent of the volume; seminal vesicles add ≈60–70 % (fructose, prostaglandins, semenogelins, alkaline), the prostate ≈20–30 % (PSA, citrate, zinc, acid phosphatase), and the bulbourethral and urethral glands a small mucous fraction. The first fraction of the ejaculate is sperm-rich prostatic fluid; the last is mainly seminal vesicle fluid.

Immediately after ejaculation semen forms a gel (semenogelins), then liquefies within 15–30 minutes as PSA cleaves the semenogelins, releasing motile sperm. Its slightly alkaline pH (≈7.2–8.0) buffers vaginal acidity. Semen also contains immunosuppressive factors and antimicrobial zinc.

Location

Formed in the urethra during emission; expelled at ejaculation.

Function

  • Transports sperm into the female tract.
  • Provides energy (fructose) and buffering (alkaline pH).
  • Coagulates then liquefies to retain sperm near the cervix and then release them.
Physiology

WHO 2021 lower reference limits (5th centile of fertile men): volume 1.4 mL; concentration 16 × 10⁶/mL; total sperm number 39 × 10⁶ per ejaculate; total motility 42 %; progressive motility 30 %; vitality 54 %; normal forms 4 %. Analyse after 2–7 days of abstinence; repeat abnormal results because of high biological variability.

Clinical significance

Interpreting semen analysis

Low volume with acidic pH and no fructose → ejaculatory duct obstruction or congenital absence of the vasa (CF). No ejaculate after orgasm with sperm in post-ejaculatory urine → retrograde ejaculation (diabetic neuropathy, α-blockers, bladder-neck surgery). Haematospermia is usually benign in young men but warrants assessment in older men.

Test yourself

Which accessory gland contributes most of the volume of semen, including fructose?
  1. Prostate
  2. Seminal vesicles
  3. Bulbourethral glands
  4. Epididymis

Answer: Seminal vesicles

The seminal vesicles provide ≈60–70 % of semen volume: an alkaline fluid rich in fructose (sperm fuel), prostaglandins and semenogelins (coagulation). The prostate adds ≈20–30 % (PSA, citrate, zinc).

More reproductive system questions

Compiled from standard anatomy and physiology references — see the Reproductive System sources. For education only; not medical advice.