Male dogs differ from humans in that they do not have seminal vesicles in which to store sperm. Sperm flows directly into the urethra from the spermatic ducts and does not mix with prostatic fluid first. The first part of the male’s ejaculate is clear. The second part is cloudy and contains the sperm. The third part is prostate fluid that washes out the urethra and neutralizes the acidity of the vagina.
After the penis enters the bitch’s vagina, a bulb of erectile tissue at the base of the penis, called the bulbus glandis, enlarges and is held inside the vagina by powerful vulvar constrictor muscles. This tight union between the two dogs is called the tie. For dogs to mate, the bulbus glandis must become enlarged after intromission.
The exact function of the tie is unknown. Its purpose may be to hold the penis in place while sperm flows up from the testicles. To be effective, a tie must last for at least two to three minutes. Many ties last 30 to 40 minutes. Contrary to popular belief, the length of the tie has no effect on the likelihood of pregnancy or the number of puppies conceived.
If the bulbus glandis swells up before intromission (an external tie), the penis cannot fully penetrate and will be withdrawn. Some inexperienced males have premature swelling of the bulbus glandis. These dogs should be led away from the female until the bulb returns to its normal size.