Calcium ion is the key regulator of human sperm function.

Calcium ions have an apparently paradoxical effect on sperm motility. In the epididymis, calcium ions stimulate immature sperm, whereas, in ejaculated semen, calcium ions inhibit sperm motility. Maturation processes change the response of sperm to calcium ions. Calcium binding substances and calcium transport inhibitors are secreted by male accessory sexual organs and mixed with sperm during ejaculation. In the female genital tract sperm acquire full capacity of fertilise the ovum. Calcium binding substances and calcium transport inhibitors are removed during the process known as capacitation. Finally, calcium ions trigger the acrosome reaction and facilitate sperm penetration into the ovum.