Some science behind the scenes

Kidneys

Abdominal cavity showing kidneys (L2 and L3)
Source Wikimedia.org

The kidneys are located at the rear of the abdominal cavity.

Their input is blood received from the paired renal arteries.  Their output is also blood, but drained via the paired renal veins, and urine.

The kidneys thus take blood and filter it. 

All waste products are then excreted via the urinary system.

The kidneys regulate the electrolytes in the blood, and the acid balance of the blood.  As a consequence they also have a role in maintaining blood pressure .  The kidneys excrete a variety of waste products produced by metabolism.

The kidneys are connected to both the sympathetic and parasympathetic nervous systems.