While the bowels treat and remove solid waste, the kidneys, by filtering waste from the blood, carry out a cleansing function, producing liquid waste. They also help regulate blood pressure (clinically very important in cats) and reclaim essential substances such as minerals. Liquid waste passes in two ureters to the bladder. When the bladder is stretched (or inflamed due to injury or disease), a pet feels the need to empty it, which it does through the urethra. Mineral crystals or stones can develop anywhere in the system. Kidney failure may be brought on by injury, disease or immune disorder but it also occurs simply as a result of advancing years. Lower urinary tract disease, involving the bladder and urethra is quite common and usually controlled by altering the diet and the acidity of the urine.
How The Urinary System Works
Each dog kidney consists of around 400,000 individual units called nephrons, each of which is enveloped in a beautiful arrangement of microscopic blood vessels called a glomerulus. Each cat kidney has around 190,000 filtering nephrons. Within the blood vessels and tubules of each nephron, blood is filtered of its waste products. Technically, a process of absorption and reabsorption takes place, controlled by a hormonal messenger from the pituitary at the base of the brain, called anti-diuretic hormone or ADH. For the system to work efficiently, the brain must produce ADH and the kidneys must respond to its messages. Urine from the nephrons collects in the hollow middle of each kidney, the renal pelvis, then travels down tubes called ureters to the bladder where it’s stored. A complex group of muscle applies pressure to the urethra at the exit from the bladder. When the bladder stretches to a certain size (or is inflamed by disease) messages are sent to the brain triggering a conscious need to empty the bladder. A cat trots off to its litter tray, a well-trained dog asks to go out, the muscles around the urethra relax and the pet urinates in its designated location. Disease in this system can affect the efficiency of kidney filtration, the quantity and quality of urine, the need to urinate and control over when urinating occurs.