Urinary System Organs and Structures

 The main urinary system organs and other urinary system structures are the kidneys, bladder, ureters, and urethra. These form a sterile environment which is protected against the non-sterile outside world, provide the forces necessary to expel urine through a series of organs, and respond to sensitive involuntary and voluntary nervous stimulation.

Kidneys

Kidneys are located just below the ribs and are close to the middle of the back. They have multiple functions. The kidney creates balance in the amount of water and electrolytes in the body’s circulatory system, removes waste products (including drugs) from the body, controls blood pressure through the release of hormones (RAAS), regulates red blood cell production and subsequent tissue oxygenation levels (erythropoietin), and increases bone mass through calcium and phosphorus regulation.

Homeostasis is an automated mechanism whereby concentrations of chemicals and fluids within the biological systems of an organism are stabilized. Kidneys contribute vastly to homeostasis, maintaining extracellular fluid volumes, acidity and alkalinity (pH), osmotic concentrations (concentration of solute molecules in a solution), and excreting potentially harmful products or excess molecules which can upset natural homeostasis.

Ureters

The urinary system includes two ureters which extend from kidney to bladder. Peristalsis and gravity bring urine along these narrow, hollow, thickly layered tubes. Because the hollow areas are narrow, ureters can become blocked by debris such as salt crystals which have stuck together to create urinary stones. Complete blockage in one tube causes high pressures and urine buildup in its attached kidney and requires emergency surgery before permanent damage is done. Alternatively, reflux or the return of urine back into the ureters can cause infection and dangerously high pressures within the kidneys.

Bladder

The bladder is kept in place by a series of external ligaments. Its main function is as a reservoir for urine. The bladder expands as it fills due to its thick layer of transitional epithelium. The cells of this layer can stretch and change shape, allowing the bladder to hold increasing amounts of urine. Two sphincter muscles stop urine from continually leaking out of the bladder. When this mechanism does not function properly incontinence is the result. The urinary nervous system provides the cues for bladder emptying and depends on signals sent from the stretch receptor cells inside the bladder wall. The image below shows the neural circuitry involved in urination.

Urethra

Urethra lengths differ in male and female urinary systems. The comparatively long male urethra that runs from the bottom of the bladder to the external urethral meatus of the penis is approximately 20 cm in length.

The urethra of the human female is only around 4 cm in length. This short length is the reason why women are more susceptible to urinary tract infections than men. The image below shows urinary catheterization in male and female subjects. Note the significantly longer length of the male catheter compared to that of the female.

The single urethra allows urine to pass outside the body once the brain has released the involuntary internal sphincter and the person in question has voluntarily released the external sphincter. No peristalsis occurs in the urethra – the passage of urine into the outside world is controlled by a combination of the pressure of the urine in the bladder, muscle tone, and gravity.