The kidneys are essentially a filtration system for the body, not unlike those used to keep a swimming pool clean. But where a pool filter removes leaves and algae from water, the kidneys remove toxic waste products (such as urea and creatinine) which accumulate in the blood of mammals as their food is converted into energy.
However, the kidneys are not just simple filters. They're highly complex organs which also regulate blood composition and pressure.
Most notably, they control the amount of electrolytes in the blood (potassium, magnesium and calcium, which regulate heart contractions; sodium, which regulates the amount of water in the blood; phosphorous, a constituent of bones and teeth).
The kidneys also produce a substance called erythropoietin (which stimulates the bone marrow to produce red blood cells).
Finally, they produce an enzyme called renin, which maintains and controls blood pressure.
Any failure of the kidneys has a knock on effect on every other organ in the body, making them every bit as important to your cats wellbeing as, say, their heart or lungs.
However, where heart or lung failure tends to have more immediately noticeable effects, the kidneys have a considerable amount of overcapacity. You probably know that most people can survive with only one kidney. In fact, it is not until 75% of kidney function has been lost through disease that outward clinical symptoms start to be seen.