Penis, scrotum and prostate

Structure

The penis has three cylinders of erectile tissue: two corpora cavernosa (the main erectile chambers) and the corpus spongiosum, which surrounds the urethra and forms the glans. The glans is the most densely innervated region. The frenulum, on the underside where the foreskin attaches, is highly sensitive and is a common site of tearing during dry friction.

The scrotum holds the testes, which produce sperm and roughly 95 percent of circulating testosterone, and the epididymis, where sperm mature. The scrotum sits outside the body because spermatogenesis requires a temperature about 2–4 °C below core; the cremaster muscle raises and lowers the testes to regulate this.

The prostate is a walnut-sized gland below the bladder, palpable through the anterior rectal wall about 5–7 cm in. It contributes fluid to semen and is a source of erotic sensation for many people; there is no requirement that it be.

Size: what the data show

The largest pooled analysis, Veale et al. (2015, BJU International), synthesised 17 studies with 15,521 men measured by clinicians:

About 95 percent of erect lengths fall between roughly 9.8 and 16.4 cm. Self-measured surveys report higher figures and are unreliable. Veale et al. found no correlation between penis size and height, foot size, or body mass index of practical use. A 2023 meta-analysis by Belladelli et al. reported a rise in mean measured erect length across studies since 1942, most plausibly explained by changes in measurement method and sampling rather than biology.

Around 45 percent of men in Veale's associated survey work wanted a larger penis, while partner-satisfaction studies consistently find size is a minor determinant of partner satisfaction relative to communication and technique. Curvature up to about 30 degrees without pain is common and normal.

Erection

Erection is a neurovascular event: parasympathetic signalling releases nitric oxide, smooth muscle in the cavernosa relaxes, arterial inflow rises and venous outflow is compressed. Anything that affects blood vessels, nerves, hormones or mood affects erection. Erectile difficulty is common — the Massachusetts Male Aging Study (Feldman et al., 1994) found some degree of erectile difficulty in 52 percent of men aged 40–70 — and it is an early marker of cardiovascular disease, so new persistent difficulty warrants a medical review, not just a prescription.

Foreskin and circumcision

The foreskin is mobile tissue with its own nerve supply. It is normally non-retractable in childhood; forced retraction causes injury. Three randomised trials in sub-Saharan Africa (Auvert 2005, Bailey 2007, Gray 2007) found voluntary medical male circumcision reduced female-to-male HIV acquisition by roughly 50–60 percent in high-prevalence heterosexual settings; WHO recommends it as one HIV prevention option in those settings. Evidence does not show comparable benefit in low-prevalence settings, and studies of sensitivity outcomes are mixed and methodologically limited. Both circumcised and uncircumcised bodies need normal hygiene: retract if it retracts easily, rinse with water, replace.

Ejaculation and refractory period

Emission and expulsion are distinct reflexes. Semen volume averages 1.5–5 mL. Ejaculation and orgasm are separable events — they can occur independently, and people with spinal cord injury or on certain medications frequently experience one without the other. The post-orgasmic refractory period varies from minutes to more than a day and lengthens with age; there is no reliable way to shorten it.

Testicular self-checks

Testicular cancer is the most common cancer in men aged 15–44 and has a five-year survival above 95 percent when caught early. Check monthly after a warm shower: roll each testis between thumb and fingers. The epididymis at the back is a normal soft ridge. See a clinician for any painless lump, size or heaviness change, or a dull ache. Sudden severe testicular pain is a surgical emergency — torsion needs treatment within about six hours to save the testis.

Sources

• Veale D et al. "Am I normal? A systematic review and construction of nomograms for flaccid and erect penis length and circumference in up to 15,521 men." BJU International, 2015.
• Belladelli F et al. "Worldwide temporal trends in penile length." World Journal of Men's Health, 2023.
• Feldman HA et al. "Impotence and its medical and psychosocial correlates: results of the Massachusetts Male Aging Study." Journal of Urology, 1994.
• Auvert B et al. PLoS Medicine, 2005; Bailey RC et al. Lancet, 2007; Gray RH et al. Lancet, 2007 (randomised circumcision trials).
• WHO/UNAIDS. Voluntary medical male circumcision policy guidance, current edition.
• European Association of Urology. Guidelines on sexual and reproductive health, current edition.

Last reviewed: August 2026.