Urinary System

Urinary System - Function The urinary system filters blood and adjusts the composition of blood/interstitial fluid by removing excess water, salt, acid, and metabolic waste from the body as urine. This allows the urinary system to control body fluid volume, blood pressure, pH, and electrolyte balance. It is a critical system for maintaining homeostasis.

We have seen how the digestive and respiratory systems remove some wastes from the body—undigested food leaves the digestive tract through the anus and carbon dioxide leaves through the lungs and airways. The urinary system (or excretory system) filters blood to remove excess water, electrolytes and other metabolic wastes and reabsorbs water, electrolytes and other molecules as needed to maintain homeostasis in the body fluid. The resulting excess and wastes are excreted as urine. In this way, the urinary system also works with the respiratory system to maintain pH balance in the body.

Urinary System - Organs The organs of the urinary system include:

• Kidneys • Ureters • Urinary Bladder • Urethra The kidneys, the main organs of the urinary system, are located against the posterior wall of the abdomen. They serve as a filtration and reabsorption system, where soluble substances are filtered and then those that the body needs to keep are reabsorbed. Those that are not reabsorbed (or not reabsorbed fully), such as our metabolic waste products, end up in the urine. Because our bodies are constantly producing wastes, the kidneys continuously work to prevent the buildup of waste products and toxins, filtering about 180 liters of fluid a day. Because the average person has about three liters of plasma (the fluid fraction of blood), this means that our plasma is filtered about 60 times a day! One of the most important of the waste products removed from the blood is urea, the main end product of protein metabolism. Other waste products and some toxins are also removed from the blood by the kidneys.

In addition, ions and water are also filtered by the kidneys, but a large fraction of these are reabsorbed to keep the fluid and electrolyte concentration of the blood and other body fluids within an optimal range for proper cell function. The kidneys also play an important role in the regulation of pH by managing the amount of acid in the urine.

As previously described the kidneys filter the blood to form urine. Urine leaves the kidneys and flows through the ureters to the urinary bladder, where it is stored until it passes out of the body through the urethra . On average, two liters of urine are produced per day, but this can vary greatly depending upon fluid intake, fluid loss through perspiration, and other factors.

Urinary System - Anatomy and Direction The right and left kidneys are located against the posterior wall of the abdominal cavity. The kidneys’ location is also described as retroperitoneal because they are behind the peritoneal cavity that encloses the intestines.