How dilated veins reach the sperm
A varicocele is an abnormal dilation of the pampiniform plexus, the network of veins that drains the testis. When the valves in these veins work poorly, blood pools and the veins enlarge, more often on the left because of how that vein connects to the renal vein. The leading explanation for its effect on fertility is that this pooling raises scrotal temperature, and sperm production is sensitive to even small temperature increases.
Pooled blood may also expose the testis to higher oxidative stress and altered metabolic conditions. Together these are thought to impair sperm count, motility, and morphology in some men, though many men with a varicocele have entirely normal fertility.
Why treatment is selective rather than automatic
Because most varicoceles cause no problem, repair is generally considered only when there is a palpable varicocele together with abnormal semen parameters or fertility difficulty. Studies of whether repair improves pregnancy rates show mixed results, so the decision is individualised rather than routine.
This is general educational information, not medical advice. Evaluation and any decision to treat a varicocele belong with a qualified clinician.