How The Female Reproductive System Works
In dogs usually twice yearly and in cats starting each spring, the pituitary gland at the base of the brain produces follicle stimulating hormone (FSH) which travels via the blood stream to the ovaries where it triggers estrogen production and eggs to develop. In dogs, estrogen and another pituitary hormone, luteinising hormone (LH) soon trigger the eggs to be released from the ovaries and enter the Fallopian tubes, on their way to the womb (uterus). In cats, eggs aren’t released until after the cat successfully mates.
Fertilisation by sperm takes place in the Fallopian tubes. In dogs, when eggs are released, the tiny vacant cavities left on the surface of the ovaries produce progesterone, the hormone of pregnancy.
Progesterone continues to be produced for two months, regardless of whether or not a dog has been successfully mated. Cats produce progesterone only after conception occurs.
Fetuses grow in the two horns of the uterus. As birth nears, a change in the balance between estrogen and progesterone develops. Finally, a pituitary hormone, oxytoxin, stimulates contractions. The young and their afterbirths (placentas) pass through the now opened cervix, through the vagina and out the vulva.
How The Male Reproductive System Works
Year round, the pituitary gland produces LH (leutenising hormone) that triggers testosterone and sperm production in the testes. Sperm are manufactured in the testes. The sugar-dense liquid made in the prostate gland helps passage through the female reproductive tract.
Sex hormone in males is uniformly produced throughout the year, although as with us, there is some evidence that male dogs and certainly cats also experience a little hormonal "spring fever" as daylight hours increase. If the testes are removed, testosterone level drops immediately, within eight hours. Sperm already in the system may remain active and viable for another 36 hours.