BIOLOGY Volume 3 - A Guide to General Biology - 2004

21. REPRODUCTION

21.8. Human Sexual Reproduction

21.8.1. The Sex Act

Internal Fertilization is a vital part of the reproductive cycle in terrestrial organisms. In many species, including humans, this is achieved through a specialized organ—the male Penis—which is inserted into the Vagina to deposit Gametes as deep as possible within the female reproductive tract.

Penile erection occurs due to an increase in Blood pressure within the erectile tissue. Sexual arousal is triggered by stimulation of the parasympathetic Nervous system, which causes constriction of the Veins draining blood from the penis and dilation of the arterioles supplying it with blood. As a result, blood volume and pressure within the penis increase, leading to an erection. In the erect state, the penis can be introduced into the vagina, where friction generated by rhythmic movements during intercourse provides tactile stimulation to sensory receptors in the glans penis. This activates sympathetic Neurons that stimulate the contraction of the internal urethral sphincter, the smooth Muscle of the Epididymis, the vas deferens, and the male accessory sex glands—the Seminal Vesicles, prostate, and Bulbourethral Glands. Consequently, the future components of seminal fluid are propelled into the proximal Urethra, where they mix to form semen. The increased pressure in the proximal urethra triggers reflex activity in motor neurons innervating the Muscles at the Base of the penis. Rhythmic, wave-like contractions of these muscles force the semen through the distal urethra, resulting in ejaculation (the expulsion of semen), which is the climax of the sex act. The physiological and psychological sensations associated with this moment in both men and women are collectively known as the orgasm.

Lubrication during intercourse is provided primarily by the secretions of the vaginal and vulvar glands, and partially by the clear mucus secreted by the male bulbourethral glands following erection.

Sexual arousal also causes erection of the Clitoris in women. The clitoris is considered the female equivalent of the male penis. During orgasm, women experience rhythmic contractions of the vaginal and uterine muscles, and the associated pleasurable sensations subsequently encompass the clitoris.

The secretions of the male accessory glands are alkaline in nature, reducing the normally acidic environment of the vagina to a pH of 6–6.5, which is optimal for sperm motility following ejaculation. An ejaculation releases approximately 3 cm³ of seminal fluid, in which sperm Cells account for only about 10%, yet a single 1 cm³ of semen contains 100 million spermatozoa.