Retrograde ejaculation occurs when semen enters the bladder instead of emerging through the penis during orgasm. Although you still reach sexual climax, you may ejaculate very little or no semen. This is sometimes called a dry orgasm. Retrograde ejaculation isn't harmful, but it can cause male infertility. Treatment for retrograde ejaculation is generally only needed to restore fertility.
During a male orgasm, a tube called the vas deferens transports sperm to the prostate, where they mix with other fluids to produce liquid semen (ejaculate). The muscle at the opening of the bladder (bladder neck muscle) tightens to prevent ejaculate from entering the bladder as it passes from the prostate into the tube inside the penis (urethra). This is the same muscle that holds urine in your bladder until you urinate. With retrograde ejaculation, the bladder neck muscle doesn't tighten properly. As a result, sperm can enter the bladder instead of being ejected out of your body through the penis.
The presence of semen in the bladder is harmless. It mixes with the urine and leaves the body with normal urination.