The kidneys are located bilaterally in the posterior part of the abdominal cavity (retroperitoneum) next to the lumbar spine. These bean-shaped organs are securely protected not only by their anatomical position inside the rib cage but also by surrounding structures. A tough fibrous capsule, the renal fascia, covers each kidney providing support, and thick layers of fatty tissue act like a cushion. This anatomical setting is intended to prevent damage by external forces and to preserve the important functions of the kidney for the entire organism.
Healthy kidneys and their functions
The primary role of the kidneys is to filter the blood and to excrete metabolic side products, along with water, as urine.
With about four litres of blood passing through them per minute, the kidneys are among the organs with the highest rate of perfusion. The body's entire blood volume passes through the kidneys every five minutes (about 1,300 millilitres per minute = 1,872 litres a day), ensuring that toxic waste materials do not build up in the body.
However, the tasks of the kidneys comprise much more than just the removal of metabolic waste. Other important functions of the kidneys include maintaining a physiological balance of electrolytes, as well as controlling the pH and the volume of the blood. In addition, the kidneys are important regulators of blood pressure and - releasing a variety of relevant hormones such as erythropoietin and activated vitamin D - they are linked to numerous endocrinological feedback systems.
The adaptive responses to metabolic changes in the organism require a very sensitive system of receptors and consequently a well-organized anatomical structure in order to accomplish this crucial and extensive work. Millions of miniature units in the kidneys, called "glomeruli", therefore act as a filter for the necessary primary blood filtration.
At the end of an extensive and sophisticated filtration pathway, only 1-2% of the primarily filtered urine is finally excreted via the bladder. The remaining 98-99% of primary urine is "recycled" into the system.
In summary, in order to keep a human body functioning, the kidneys perform a number of homeostatic activities that sustain crucial biochemical functions within the organism.