The kidneys are organs that we don't often think too much about in conjunction with our dog's health.  However, their importance cannot be disputed.  Their primary function is to remove toxic waste from the blood.  This toxic waste is a by-product of metabolism, and is known as urine.
     Not only do these two bean-shaped organs remove toxic waste from the dog's body, but they also produce two key hormones, erythropietin and renin.  Erythropietin is a hormone that stimulates red blood cell production within the bone marrow.  Renin is a hormone involved in controlling blood pressure.  The kidneys also keep the body's water and electrolytes balanced.
     The kidneys are only a part of a rather complex system within the dog's body.  The kidneys are located one on either side of the spine, at approximately the level of the last pair of ribs.  The kidneys are a complex filtration system that is made up of millions of structures called nephrons.  Each of these nephrons act independently to extract wastes and excesses from the bloodstream, and forms urine.  The filtered blood than returns to the bloodstream via capillaries.  The urine is emptied into the collecting tube and then it drains into an area called the renal pelvis.
     From the renal pelvis the ureters transport the urine to the urinary bladder.  The ureters are round tubes that contract in a rhythmic pattern to aid in this transport.  There are two ureters within the body, one for each kidney.
     The urinary bladder is an organ, like an oval bag, that is hollow and has sides that can stretch to collect and store the urine.
     The urethra is the tube that connects the urinary bladder to the exterior of the body.  This tube allows for voiding urine.  At the "bottom" of the urinary bladder is a circular muscle that is controlled in a voluntary manner.  This is where the urethra connects to the urinary bladder.  This muscle is called the internal urethral sphincter.  When it is open, or relaxed, it allows the urine to pass down the urethra for voiding; when it is closed, or constricted, it keeps the urine within the bladder.
     The kidneys, the ureters, the urinary bladder, the urethra, and the internal urethral sphincter are all parts of the urinary system.  The urethra, an organ of the urinary system, is shared by the reproductive system, and these two systems together are called the urogenital system.
     The owner of any dog should be aware of the patterns their dog has concerning urination.  It is not simply a matter of opening the door and letting the dog go outside to "take care of his business".  An owner should be aware of the frequency that the dog needs to go, the general amount of urine that the dog voids each time he/she goes, is the dog having difficulty voiding, is the dog always aware of passing urine.  If there are any changes in the habits of your dog is an indication that you may want to take your dog to the veterinarian for testing.  Many kidney and urinary tract infections are without clinical signs, and can be diagnosed with accuracy through a urine sample or blood test.