Sexual intercourse does not necessarily lead to reproduction, but the physiology of reproductive versus non-reproductive sexual arousal is indistinguishable. Sexual arousal has been divided into four stages by Masters and Johnson. These stages are the same whether the arousal results from physical stimulation (such as touch) or mental stimulation (such as reading an arousing book). Hence, arousal can be influenced by personal beliefs, desires, or values. The stages of arousal are: excitement, plateau, orgasm, and resolution.
The male stage of sexual excitement is marked by increased blood flow to the pelvic area and penis. Increased parasympathetic nerve activity causes the blood vessels in the penis to dilate, allowing for vasocongestion which leads to an erection. This may happen in a matter of seconds. Testes size also increases.
The amount of time spent in the plateau phase varies considerably. In this stage, the head of the penis enlarges and darkens from blood pooling. Testes darken, enlarge from vasocongestion, and are lifted back away from the penis. At this point, pre-ejaculatory secretion from the bulbo-urethral gland occurs, and respiration, heart rate, and blood pressure increase.
Male orgasm results from both emission and ejaculation. Emission is the release of the ejaculatory fluid into the urethra. Emission is caused by increased sympathetic nerve stimulation in the ejaculatory ducts and glands which leads to rhythmic contractions that force the fluid out. For ejaculation, rhythmic contractions of the urethra expel the semen (usually 3-5 ml) while the prostate gland closes off the bladder.
In the resolution phase, blood exits the penis and testes, and the penis relaxes. Respiration, blood pressure, and heart rate return to normal, and sexual arousal enters a refractory period. During the refractory period, erection can not occur. The length of refractory period varies from a couple of minutes to several hours and increases with fatigue and age.
- Reproductive System - The Female Reproductive System
- Reproductive System - The Penis
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