Genitourinary System

The genitourinary system is the group of organs that enable humans to sexually reproduce and to eliminate the waste products generated during metabolic reactions. The genitourinary system is broken up into two main parts: urinary tract and reproductive system. The urinary tract consists of the organs that allow humans to eliminate the wastes produced from metabolic reactions and dilute them in water before they are expelled during urination. The reproductive system consists of the sexual organs that differ between males and females. The reproductive system consists of the primary and secondary sex organs. Primary organs include the testes in males, and the ovaries in females. Secondary sex organs include the tubes and other structures that carry sperm and egg from the primary sex organs. These two systems carry out separate functions but are sometimes grouped together because of their close proximity to one another and because some of their organs are connected. For example, the penis is used to excrete urine and during reproduction to deliver semen into the vagina.

The urinary tract, which is part of the renal system, starts with the kidneys, and also contains the ureters, bladder, and urethra. Within the kidneys there are functional units called nephrons that are made up of an intricate network of capillaries and tubules that work together to filter blood. As blood flows through each group of capillaries, called a glomerulus, it is filtered to remove electrolytes and metabolites that can be toxic if they accumulate in the blood. An electrolyte is a chemical that dissociates into positively and negatively charged ions in water. These electrolytes and metabolites are then modified and diluted in water drained from the bloodstream to form urine. Urine becomes concentrated or diluted in response to hormone triggers and this helps regulate blood pressure and volume. Urine then passes out of the kidney down the ureters and into the bladder, where urine accumulates until signals sent to the brain stimulate an urge to empty the bladder. During urination, internal and external urethral sphincter muscles relax to allow urine to exit out of the bladder through the urethra.

The reproductive system is made up of reproductive organs, which differ between the sexes. In females, ovaries, located in the peritoneal cavity or lower abdomen, release fertile ova or oocytes (eggs), into the fallopian, or uterine, tubes. The fallopian tubes empty into the uterus, or womb, which is where embryos implant and fetuses develop. In males, sperm are produced in the epididymis of the testes and pass through the ductus deferens and the urethra and out of the penis. When a released ovum is fertilized by sperm, it forms an embryo that implants into the uterine wall and develops into a fetus during pregnancy. Once the fetus is fully developed, it passes through the cervix and then the vagina. The urinary and reproductive systems both transfer substances from the inside to the outside of the body. The tissues of these systems are pathways to the blood from outside the body. Layers of innate and adaptive immune defense, such as maintaining a specific pH and mucus, are present to prevent pathogens from reaching vulnerable tissues. Urine is maintained at a high pH, is hypertonic, and flushes outward to prevent pathogens from reaching deeper tissues. The epithelial lining of the reproductive and urinary tracts produces immune molecules, such as immunoglobulin A (IgA) antibodies, and a protective polysaccharide lining, which prevents pathogen penetration of the mucosa.

The majority of the genitourinary system is either sterile or colonized by a low diversity of bacteria in both healthy males and females. Regular movement of fluid from the bladder through the urethra effectively flushes most microbes from the system, but not completely remove them. Studies have shown that the bladder is not a completely sterile environment, and actually is host to its own microbiome. Research into the reason for this colonization is undergoing. In men, microbes common to the skin are found in distal portions (closest to the external environment) of the urethra, with there being a difference in the concentrations between circumsized and uncircumsized men. In women, microbes also colonize the distal portion of the urethra. The exterior portions of the genitourinary system are colonized by a diverse array of microbes, just as other areas of the skin.

However, the vagina has a modestly diverse native microbe community. As with the skin and alimentary canal, native vaginal microbes create a homeostatic environment that prevents colonization of pathogens by utilizing nutrients and occupying physical space. Lactobacillus species are by far the most prevalent members of the vaginal environment. Lactobacillus produce lactic acid that reduces vaginal pH, increasing acidity and preventing growth of most other bacterial species. Occasionally other species that also produce lactic acid, for example members of the genera Leptotrichia or Megasphaera, are present and provide functional redundancy. The compound glycogen is used as a substrate to synthesize lactic acid, and glycogen release into the vagina by epithelial cells is regulated by the hormone estrogen. Therefore, conditions such as menstruation and menopause that alter estrogen levels have the capacity to influence vaginal pH and microbial community composition. Lactobacilli also produce a significant amount of hydrogen peroxide and antibacterial peptides, which actively kill many species of anaerobic bacteria, bacteria that do not grow in the presence of oxygen.

As people age, their microbiota tend to become less diverse as a handful of species dominate and exclude the rest, which can lead to decreased protection from pathogens because there is less competition with harmful bacteria. For example, Atopobium vaginea is an anaerobic, gram-positive bacteria that can cause vaginosis when allowed to colonize and reproduce unchecked. Vaginitis is also a symptom of bacterial infection, which is called vaginosis, a condition in which natural members of the vaginal microbial community overgrow, causing a fishy odor, production of vaginal discharge, and sometimes pain during urination. Vaginal discharge is combination of mucus, liquid, bacterial cells, and human cells produced normally by the female reproductive tract, but produced in greater volume during infection. When outside influences disrupt the vaginal microbiota, such as douching—using a stream of water to clean the vagina—or antibiotic treatment for an unrelated illness, other bacterial species can increase in abundance and stimulate an inflammatory response. Symptoms of vaginosis include those associated with vaginitis, as well as a burning sensation during urination, and it is typically treated with an oral or topical application of the antibiotic clindamycin.