What specific elements of the urinary system participate in each of the three (3) functions you described?

There are many functions that may be associated with the urinary system such as regulation of blood volume, retention of urine, and excretion of waste products.
For blood pressure, the kidneys are the element that is connected to such. With urine retention is mostly the bladder and the ureters that present the urine to flow back. Excretion of waste products in the blood is made by basically all the elements of the Urinary System, firstly by the kidneys that strain the blood separating those waste products, and then by all the other elements that conduct the urine to the exterior of the body - ureters, bladder, and urethra.

That is pretty