Most men tend to consider the third phase of the sexual response cycle, namely orgasm, to be the same as ejaculation. This is not the case, orgasm & ejaculation are in fact separate events, although they seem to occur simultaneously. It is important to note that either of these events can occur in the absence of the other.
Excitement through Plateau
Sexual desire is a complex interaction among cognitive processes, physiological mechanisms (hormones), physical well-being, and positive mood all affecting the drive toward sexual fantasy or behavior.
If a man encounters something that excites him sexually, messages are transmitted to the portion of the brain dedicated to sexual response. Many men believe that sexual arousal is always accompanied by an erection, but this is not necessarily the case at the first stage.
A number of other things happen during the early stages of sexual arousal. The brain is flooded with natural chemicals that act similarly to drugs such as cocaine. These natural chemicals, called endorphins, make the man say to himself that whatever is causing the sexual arousal is very enjoyable and should be continued. If the sexual response is the result of observing another person, the man may make an effort to meet the other person involved.
Most men will produce pre-ejaculate when anything considered to be sexually exciting occurs, perhaps even reading a sexually explicit story or watching a sexually explicit movie. There is no limit to the length of time that a man can be aroused, and throughout arousal the man can continue to produce drops of pre-ejaculate.
Erection Normally, excitement is followed by erection. An erection of the penis occurs as the spongy tissues of the penis are engorged with blood. There is one large primary artery responsible for blood flow into the penis, but several veins that drain the penis of blood. When an erection is not happening, the inflow of blood and the outflow is maintained in balance and the penis remains flaccid. During erection, blood flows into the penis and holes in the spongy tissue in the penis fill with blood. At the same time, flaps in the veins leading out of the penis enlarge, cutting off the drainage. As a result, the penis fills with blood. As more and more blood flows in than out, the penis enlarges and becomes harder. Finally, veins in the penis are compressed from the increasing pressure from the erection itself. Heart rate and blood pressure also increase, the pressure of blood into the penis increases, keeping the penis hard.