Under normal circumstances, an animal has two kidneys. The kidneys‘ main task is to filter superfluous and unwanted substances out of the blood. The kidneys are directly linked to the blood vessels and filter about 25% of the blood that gets pumped through the body by the heart. For instance, in a cat 20 litres of blood get filtered every day. The extracted substances leave the body with the urine that gets produced during the filtering process. A cat with well functioning kidneys produces about 0.1 litre of urine a day. The urine leaves the body through the kidneys, bladder and urethra.
A kidney consist of a million small filters, called nephrons. A nephron looks like a cup with a double wall. This wall contains a large group of blood vessels, which contain small pores. Blood that needs to be filtered is pushed through these pores under high pressure. Blood can flow through the double walled cup and the waste is caught between the double walls. The space in between the double walls is connected to a tube that gathers fluids. The kidney transports the fluids, called urine, in this collection tube to the urethra and into the bladder. The bladder can be compared with a stretchable balloon. Urine that arrives from the kidneys is collected here. The bladder stretches as the urine volume increases, until it is almost filled to its maximum limit. Once that level is reached, the bladder sends out a signal over the nervous system to the brain to announce that it’s about time that the urine should leave the body. Your pet then gets the urge to urinate.
Unwanted substances which can be removed:
See picture below of a kidney on the left and a nephron on the right.