A urinary tract infection, or UTI, is an infection within the body’s urinary system. These infections are among the most common bacterial infections, and while they can occur in anyone, anatomical differences make them more frequent in women. In a healthy individual, urine is sterile, so a UTI develops when microorganisms establish themselves along this pathway.
Anatomy of the Urinary System
The urinary system is the body’s filtration and drainage system. It begins with the two kidneys, located just below the rib cage, whose primary role is to filter waste and excess water from the blood using millions of tiny units called nephrons. This filtered waste becomes urine.
From the kidneys, urine travels down two narrow tubes called the ureters. The walls of the ureters constantly tighten and relax, pushing urine downward toward the bladder. This one-way flow is a protective mechanism that helps prevent urine from backing up. Small amounts of urine are emptied from the ureters into the bladder about every 10 to 15 seconds.
The urinary bladder is a hollow, muscular organ in the lower abdomen. Its walls relax and expand to store urine, holding up to two cups. When the bladder is full, nerves send a signal to the brain indicating the need to urinate. The final component is the urethra, a tube that expels urine from the body. Sphincter muscles close tightly around the bladder’s opening to prevent leakage until the brain signals them to relax.
The Pathway of Infection
Most urinary tract infections begin when microorganisms enter the urinary tract through the urethra. This process is known as the ascending route. Bacteria, originating from fecal flora, contaminate the area around the urethral opening and travel upward. More than 85% of UTIs are caused by intestinal bacteria, with Escherichia coli (E. coli) being the most frequent culprit.
Once inside the urethra, the bacteria can multiply and migrate toward the bladder. The ability of these microbes to cause an infection depends on their virulence and the body’s defense mechanisms. Many uropathogenic bacteria have adaptations, such as pili, which are hair-like appendages that allow them to adhere to the urinary tract lining and resist being flushed out.
If bacteria evade the host’s immune system and the flushing action of urination, they establish a colony in the bladder. Here, they can multiply in the nutrient-rich urine. From the bladder, the infection can continue its ascent up the ureters to the kidneys, a less common and more serious development.
Classifying UTIs by Location
Urinary tract infections are categorized based on where the infection takes hold. These classifications are divided into lower and upper UTIs, a distinction that correlates with the infection’s severity.
A lower UTI is the most common form and is confined to the urethra and bladder. An infection limited to the urethra is called urethritis. When the infection involves the bladder, it is known as cystitis, the most prevalent type of UTI. The symptoms associated with lower UTIs, such as a frequent urge to urinate and a burning sensation, are caused by this inflammation.
An upper UTI is a more serious condition where the infection has spread beyond the bladder. This includes pyelonephritis, an infection of one or both kidneys. Pyelonephritis can occur when bacteria from a bladder infection travel up the ureters. An obstruction in the urinary tract that causes urine to flow backward can also lead to a kidney infection.