When I told a group of students that the kidneys of a patient with kidney failure whose condition is stable excrete normal amounts of waste products and other substances like normal kidneys, they responded with a kind of disbelief: why then does the patient develop manifestations of kidney failure and why do we call it kidney failure?!
The answer is this.
If a patient or a person is in a steady state i.e. he or she is the same and his plasma urea and creatinine are the same today, tomorrow or after a week, then the input of any substance (entering or produced in his body) must be equal to the output. In the case of urea and creatinine the production in the body by tissues must be equal to the amount excreted by the kidneys. In other words if the blood urea and creatinine are constant over a period of time (regardless of whether they are in the normal level or elevated) then his kidneys must be excreting all that is produced, otherwise the level must rise. The critical point here is that the failing kidneys excrete normal amounts in spite of their reduced glomerular filtration because the level of the substance in the blood, and consequently in the filtrate, is high; so a reduced volume of glomerular filtrate contains the same amount of urea or creatinine as a larger volume with a lower concentration. If for some reason kidney function deteriorates and filtration drops further then the balance between production and excretion is disturbed. Urea and creatinine start to rise. The patient now is not in a steady state. The level goes on rising resulting in increased excretion until the amount excreted in the reduced volume of filtrate becomes equal to the production. The rise in the serum level stops and the patient reaches a new steady state at a higher level of plasma urea and creatinine. In other words failing kidneys excrete normal amounts of various substances on the expense of a higher plasma concentration of these substances.
To make an analogy, think of a tank with a hole in the bottom and water flowing into it from a tap. The level of water in the tank depends on the balance between the water flowing in and that flowing out. If the hole in the bottom is made smaller the balance is disturbed, the inflow exceeds the outflow causing the level in the tank to rise. The rising level increases the pressure driving the water out from the smaller hole. The water outflow starts increasing. The change goes on until the outflow becomes equal to the inflow. The level of water in the tank then stops rising and stabilizes at a new higher level assuming a new steady state.
The ability of our bodies to keep their composition and various characteristics constant (i.e. in steady states) is remarkable. Think of body temperature, body weight, the number of various blood cells, skin cells, intestinal cells, the amount of water in our bodies, the amount and concentration of various chemical substances in our tissues etc. etc. Everything is kept in a steady state; a dynamic, not static, steady state. The essence of it is to equalize the input and output.