The connection between the urinary and reproductive systems lies in the early fetal development of these systems and the anatomical locations of the systems. The urinary and reproductive systems in humans are created from the same embryonic layer during fetal development. During the first trimester of maturation of the fetus, these two systems differentiate themselves to become two separate bodily functions. The urinary system develops into a system of bodily functions to remove waste, and the reproductive system develops into a system of bodily functions with the sole purpose to perpetuate the species.
The urinary and reproductive systems develop from the intermediate mesoderm layer. From this layer evolves the kidneys, gonads, Wolffian ducts and Müllerian ducts. The Wolffian ducts atrophy in females but go on to develop the seminal vesicles, vas deferens, epididymis and ejaculatory duct in males. The Müllerian ducts form into the uterus, fallopian tubes and vagina in females. The gonads become the testicles in males and the ovaries in females.
Later in development, after the kidneys have ascended into place, the bladder is formed from parts of the Wolffian ducts and the endodermal cloaca. The urethra is formed fully in females and partially formed in males from the endodermal cloaca. The remainder of the male urethra develops from the external genitalia.
In males, the urinary and reproductive systems slightly overlap. The urethra in males has a dual purpose for both the elimination of urine and the ejaculation of sperm. Urine is formed in the kidneys as a waste material and travels down the ureters to the bladder for storage until elimination from the body. At the point of urination, the urine flows from the bladder through the urethra and exits the body from the penis.
The gonads of males are called the testicles. Sperm develops within the testicles and is stored in the coils of the epididymis. Prior to ejaculation, sperm travels from storage in the epididymis through the vas deferens, which lies just behind the bladder and connects to the urethra. The sperm is then ejaculated from the body via the urethra through the end of the penis.
For females, the urinary and reproductive systems are separated, although both the urethra and the vaginal opening are located within the labia minor of the vagina. As in males, urine is produced in the kidneys and travels to the bladder via the ureters. Urine is emitted from the female through the urethra, which is located in front of the vaginal opening housed within the labia minor.
Eggs are produced in the female gonads, or ovaries. Typically, a single egg will travel from the ovaries into the fallopian tubes and finally to the uterus. If fertilization of the egg does not occur, menses will ensue, allowing for the endometrial layer of the uterus and the unfertilized egg to be emitted through the cervix, into the vagina and out the body through the vaginal opening.