Waste Disposal

After the nutrients are absorbed, waste enters the large intestine, or colon. Benign bacteria here break down the waste and neutralize any hostile bacteria in it.

Water is absorbed trough the colon wall, and mucus is secreted to lubricate the dry waste. When waste accumulates in the rectum, nerves signal that discharge is needed. 

The blood carries waste from the liver to the kidneys, where tubules, called nephrons, filter it out and excrete it in urine, of which cats daily produce 2 fl oz (60 ml).

Nephrons die with age and disease, but only when 75 percent are lost does a cat obviously drink and urinate more. Failing kidneys have difficulty clear urea and creatinine, by-products of protein  and muscle exertion.

The blood levels of these chemicals indicate the state of the kidneys, and treatment includes reducing both protein in the diet and the amount of exercise.

The kidneys also regulate the blood pressure, maintain the chemical balance of the blood, activate vitamin D, and produce erythropoietin, a hormone that stimulates the production of red blood cells.