Final answer:
The primary mechanism that prevents organisms from entering the bladder is the flushing action of urine, together with the contraction and expansion of urinary system walls and the presence of normal microbiota.
Step-by-step explanation:
The main mechanism that helps keep organisms out of the bladder is the flushing action of urine. The normal flow of urine through the ureters and out of the bladder typically prevents microbial colonization. Additionally, the urethra acts as a conduit for eliminating urine, which helps to clear potential pathogens.
The walls of the urinary system structures, such as the bladder, are able to expand and contract, aiding this process. Hormonal factors also play a role in maintaining sterility within the urinary tract.
During urination, the bladder contracts to expel urine, and the sphincters relax to allow the passage of urine. These actions help to keep the urinary tract free from infection. Moreover, the urinary system is designed with specific anatomical features such as transitional epithelium lining the bladder, which allows stretching and provides a barrier to pathogens.
The presence of normal microbiota in the urogenital system offers an additional layer of defense against infection. However, during medical procedures like surgery, there is a risk of introducing bacteria into the urinary tract. For instance, catheters or surgical instruments, if not sterilized properly, can carry bacteria into the bladder or urethra, leading to possible infection.