This mixture of fluids and sperm is called semen. The semen travels through the penis and out of the body by way of the urethra, the same tube that carries urine. In response to physical or emotional stimulation, more blood than usual flows into the blood vessels of the penis. As a result, the penis becomes larger and harder and stands away from the body. This is an erection.
Ejaculation is a two-part reaction to sexual stimulation.
Then the muscles in the penis contract and squeeze the semen through the urethra and out of the body. After ejaculation, the amount of blood in the penis decreases and it returns to its relaxed state and becomes flaccid. Normal Concerns of Puberty During puberty, boys are very anxious to know that what they are experiencing is normal. They are usually caused by sexual stimulation, but can occur for no apparent reason. If a boy can avoid worrying about the erection, this is the best way to avoid attracting attention, while helping it subside.