Reproductive System
The reproductive system is important for preserving the species, but it is not vitally important for the health of an individual animal. Unlike the organs of most other systems, a dog can survive quite well without testicles or a uterus. Along with the obvious purpose of producing puppies, the reproductive system also produces hormones that influence a dog's secondary sex characteristics. For instance, in many breeds the males are larger and have more coat.
Male Reproductive System
The male reproductive system starts internally with the prostate gland, which is located next to the bladder. This gland adds secretions to any ejaculate of semen that help to keep sperm alive. The urethra (part of the excretory system) then carries secretions out to the penis. In dogs, the penis has a bony section as well as vascular areas that swell with erection. The penis lies in a skin sheath called the prepuce.
Dogs normally have two testicles enclosed in a skin pouch outside the body called the scrotum. This is located under the tail. Each testicle stores the sperm it produces. The testicles also produce testosterone, which gives intact male dogs their fuller coats, interest in female dogs, and other characteristic behaviors.
Dogs with a retained testicle (one left inside the body — not dropped out into the scrotum) should be neutered, as the internal testicle may become cancerous. These dogs are called cryptorchids.
Just as in people, the prostate gland can get become enlarged (known as benign hypertrophy) and cause problems by putting pressure on the bladder. It can even develop cancer in rare cases.
Female Reproductive System
Female dogs have two ovaries, which are small organs located internally near the kidneys. This is where eggs are produced, as well as the hormones estrogen and progesterone. The activity of the ovary is controlled by hormones from the pituitary gland. When the ovary has mature eggs, they travel via the fallopian tubes to the uterus. The uterus is where the eggs are fertilized and puppy embryos develop.
The uterus has two long horns to accommodate multiple growing puppies. When puppies are being whelped, they pass along the uterus, through the cervix into the vagina, and out the vulva under your dog's tail.
