The urinary system consists of two kidneys, two ureters, a bladder, and a urethra. When considering the microbiota of this system, it is convenient to deal with the male and female urinary systems separately for a number of reasons. Firstly, the anatomy of the system in males and females differs significantly. Secondly, in males, the terminal portion of the urinary system, the urethra, also constitutes part of the reproductive system, resulting in important functional differences from the urethra of females. Thirdly, the urethral opening in females is closer to the anus than in males and is also close to the vaginal introitus, and these heavily colonised sites provide important additional sources of potential microbial colonisers. These factors combine to generate significant differences not only in the types of microbes colonising the urinary system of males and females, but also their relative susceptibility to infection.

The urinary system of females

Anatomy and physiology

Whereas the kidneys, ureters, and bladder are normally sterile, the urethra of females is usually colonised by microbes along its whole length. Further discussion of the anatomy of the urinary system is, therefore, confined to the urethra. In females, the urethra is a short tube (approximately 3.8 cm in length) leading from the floor of the bladder to the external environment via the external urethral orifice which lies in front of the vaginal opening (Figure 5.1).