Before a man's sexual arousal begins he is in a relaxed state. His muscle tension, blood pressure, heart rate and respiratory rate are at levels normal for him.

During this time, in the penis, the arterial blood vessels are constricted and the venous blood vessels are uncompressed, giving a balance of blood flow in and out of the corpora cavernosa.

Excitement Phase.

The excitement phase of a man's sexual response is begun by one or more of at least two distinct mechanisms, which interact in normal sexual activity. Central psychogenic erections begin in response to stimulus from the brain; imagination, memories or pr0n, for the most part. Reflexogenic erections begin in response to stimulus of the penis. Sensory receptors on the penis receive the stimuli which, through spinal interactions, cause somatic and parasympathetic efferent actions.

During the excitement phase, blood flow to the corpora cavernosa increases dramatically. This increase is due to  smooth muscle relaxation in the trabeculae and arterial vasculature, and causes  the cavernosal spaces to fill rapidly. This rapid filling causes the compression of the venous plexus and the larger veins passing through the tunica albuginea, effectively cutting off the escape route for the blood filling the penis. This process is often referred to as the corporeal veno-occlusive mechanism.

It is the combination of increased inflow and decreased outflow from the corpora cavernosa which causes the penis to harden into erection.

During the excitement period vasocongestion also makes nipples hard, tightens the scrotum up, and swells the testes.

The man's heart rate, muscle tension, and blood pressure rise

Depending on the man, and on his desires at the time, this phases can last from about three minutes to several hours.

Plateau Phase

The erection process described above continues until the penis is fully rigid and the testes are enlarged to approximately 50% again of their resting size

During this period, Cowper's or Bulbourtheral glands produce pre-ejaculatory fluid

Blood pressure, heart rate, respiratory rate, and muscle tension continue to increase. Many men experience a "sexual flush" -  redness of skin mostly appearing on the chest.

Orgasmic Phase

Just before orgasm, rhythmic contractions of the seminal vesicles, vas deferens, prostate and ejaculatory ducts push semen into urethra. This is the time known as the "point of imminence"; a few seconds during which a man knows he will orgasm no matter what happens.

At orgasm the urethra contracts all along itself, forcing the semen out. The pelvic floor and anal sphincter contract at the same time. The first few contractions are strong and less than 1 second apart. Weaker contractions follow for up to several seconds. Most of the sensation of orgasm is felt in the penis itself.

It is interesting to note that physical stimulation of genitals not always needed. Some men can achieve orgasm by thought alone.

Resolution Phase

This phase is basically a return to the relaxed state. Blood drains away from genitals as the corporeal veno-occlusive mechanism gives up it's hold, allowing the penis to soften into its resting state. The testes return to their normal size, the scrotum relaxes, muscle tension decreases, nipples soften and any sex flush that was apparent begins to disappear.

Heart rate, breathing and blood pressure return to normal.

Refractory Phase

This is where male sexual response differs most from female. The refractory period is a time during which a man simply cannot have an orgasm no matter what the stimulus.

In younger men the refractory period is generally a few minutes. In older men, or those with hormone deficiencies, the refractory period can last hours or even days.

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