Kidneys are those parts of the urinary system that take out waste products from the blood. Water products move out from the kidneys into the rest of the urinary tract. The kidneys and urinary system are basically focused on getting undesirable substances or waste out of the body, with the kidneys filtering the blood, and the rest of the urinary tract storing and transporting urine.
Blood runs through the body of humans and acts as a transport mechanism for all those molecules and cells that need to get from one area to another. It carries oxygen from the lungs, and carbon dioxide to the lungs. Blood also moves energy molecules like glucose around, and picks up waste products from cell metabolism for disposal. The substances that are destined for disposal, such as salts and urea, are removed from the both through the kidneys and urinary system.
The organs that take out undesirable substances from the blood are the kidneys, which have filtration structures in place to help waste products move across from the blood into the kidneys themselves. The cleaned blood moves through the individual kidneys and back into the normal circulation system, prepared for more transport work. Inside the kidneys, the unwanted substances are moved into an area of the organs that makes urine from these substances and water.
Ureters are tubes that bring the urine from the kidneys to the bladder. The bladder is a storage container for urine, so the urine output can be controlled by the person or the animal. For the final exit of the urine to the outside of the body, the liquid passes down another tube called the urethra. In women the urethra comes out an exit point above the vaginal entrance, and for men, the end point of the kidneys and urinary system is out of the end of the penis.
Most of the complex work of the kidneys and urinary system is performed by the kidneys themselves. Normally, humans have two kidneys, but it is possible to survive on one kidney. The rest of the urinary tract is storage and transport, as opposed to filtration of blood and production of urine.
As well as making urine, the kidneys also tweak the water content of urine depending on how much water is already in the body. A hormone called antidiuretic hormone is produced by the pituitary gland in the brain when the body knows it does not have lots of water to spare. Kidneys respond to high levels of this hormone by making the water already in the urine move out of the urine and back into the body. When little or no hormone is present, the kidneys allow the extra water out of the body in dilute urine.