The kidneys excrete a hormone called erythropoietin, which helps perform an important regulatory function in the body, according to the Merck Manual. One key purpose of this hormone is the production of red blood cells in bone marrow. It also contributes to bone growth and development.
The kidneys also help a person maintain steady levels of calcium and phosphorus through the conversion of inactive Vitamin D into an active form of Vitamin D. The Merck Manual reports that this inactive form, called calcitriol, performs a similar function as a hormone. It allows the body to absorb calcium and phosphorus from the small intestine