A male’s penis is a compound texture of muscle, nerves and blood vessels. When a man is sexually excited, the nerves send signals and commands to relax the smooth muscle tissue in the penis. This helps the blood vessels to pump extra blood into the chambers inside the muscle. The penis becomes harder, it fills with blood and the veins expand and prevent the blood coming out of the penis.
Semen is a substance which carries sperm. Semen is produced by seminal vesicles attached to the prostate gland and by the gland. The testicles produce sperm and send it through the tubes to the prostate gland where it is mixed with the semen. The sperm with semen is ejaculated through ejaculatory ducts when the orgasm is reached.
After ejaculation, the nerves cease sending commands, the smooth muscle shrinks, the extra blood leaves the penis and the penis gets its normal size.