2.69 Understand how the kidney carries out its roles of excretion and osmoregulation

Nephrons in the kidneys (contains millions of nephrons) are in charge of excretion and osmoregulation.

Excretion
Excretion is filtering the bad stuff from the body and keeping the good bits we need. The kidneys excrete water, salts and urea from the body.
The liver converts amino acids (which contain nitrogen, which is toxic to the body) into urea. The kidneys filter urea from the blood stream and combine it with water to create urine which then moves into the bladder via the urethra.

Osmoregulation
Osmoregulation is the balancing of water levels in the body (remember OSMO-sis? Therefore water-regulation)

The kidneys react to ADH hormone released by the pituitary gland. 

Basically, in the nephron, there is a collecting duct. If more ADH is released, the walls of the duct will become more permeable and more water will be re-absorbed. If less, duct will become more impermeable.