The reproductive and urinary systems are often considered together, and they are referred to as the genitourinary system because of their close proximity to one another. Also, both systems are commonly affected by the same pathogens. The genitourinary system is one of the portals of entry for pathogens, meaning the place where they get into the body.
The urinary system consists of the kidneys, the ureters, the bladder, and the urethra (figure 25.1). The kidneys act as a specialized filtering system to clean the blood of many waste materials, selectively reabsorbing substances that can be reused.
Right kidney Left kidney
Right kidney Left kidney
Figure 25.1 Anatomy of the Urinary System Urine flows from the kidneys, down the ureters, and into the bladder, which empties through the urethra. Sphincter muscles help to prevent microorganisms from ascending.
Waste materials are excreted in the urine, which is usually acidic because of excretion of excess hydrogen ions from foods and metabolism. Consistently alkaline urine suggests infection with a urease-producing bacterium that converts the urea in urine to ammonia. Antimicrobial medications are commonly excreted in the urine and reach concentrations higher than those in the bloodstream. Each kidney is drained by a ureter, which connects it with the urinary bladder. The bladder acts as a holding tank. Once filled, it empties through the urethra. Infections of the urinary tract occur far more frequently in women than in men, because the female urethra is short (about 4 cm or 1.5 inches, compared with 20 cm or 8 inches in the male) and is adjacent to openings of the genital and intestinal tracts. Special groups of muscles near the urethra keep the system closed most of the time and help prevent infection. The downward flow of urine also helps clean the system by flushing out microorganisms before they have a chance to multiply and cause infection.
The urinary tract is protected from infection by a number of mechanisms besides its anatomy. Normal urine contains antimicrobial substances such as organic acids and small quantities of antibodies. During urinary tract infections, larger quantities of specific antibodies can be found in the urine. Antibody-forming lymphoid cells in the infected kidneys or bladder form protective antibodies locally at the site where they are needed. In addition, during infection an inflammatory response occurs in which phagocytes enter the bladder and are of the utmost importance in engulfing and destroying the invading microorganisms. ■ phagocytes, p. 384
The anatomy of the female and male genital systems is shown in figure 25.2. In women during the child-bearing years, an egg, or ovum, is expelled from one of the two ovaries each month and swept into the adjacent fallopian tube. Fertilization normally takes place in the fallopian tube; ciliated epithelium of the tube then moves the fertilized ovum to the uterus, where it implants itself in the epithelial lining. If fertilization does not occur, the epithelial lining of the uterus sloughs off, producing a menstrual period. Infection of the fallopian tubes can cause scarring and destruction of the ciliated epithelium, so that ova are not moved efficiently to the uterus. Note that the fallopian tubes are open on both ends, providing a pathway into the abdominal cavity where the liver and other structures can be infected. Also notice the uterine cervix, a common site of sexually transmitted infections, and a place where cancer can develop. Except during menstruation, the cervical opening is tiny and filled with mucus. The vagina is a portal of entry for a number of infections that can advance to the uterus and fallopian tubes at the time of menstruation. The vaginal opening is framed on each side by two labia, or lips. These plus the clitoris and entryway of the vagina constitute the female external genitalia, or vulva.
In men, the paired reproductive organs, the testes (testicles), exist outside the abdominal cavity in the scrotum. Sperm from each testis collect in a tightly coiled tubule called the epi-didymis and are conveyed by a long tube called the vas defer-ens that enters the abdomen in the groin to join the prostate gland. The sperm and secretions of the prostate gland compose
the semen. The urinary and reproductive systems join at the prostate gland. The prostate can be infected by urinary or sexually transmitted pathogens. In older men it often enlarges and hinders the flow of urine, thus fostering urinary infections.
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