The primary function of the kidney is not to act as a filter, as is commonly thought. The primary function of the kidney is to maintain proper balance of various substances such as water, sodium, potassium, magnesium, glucose, urea, bicarbonate and many other chemicals. The method by which the kidney does this is to work as a filter, but it's not the filtration itself that is the point - the point is the chemical balance. In addition to helping maintain proper chemical balances (known as homeostasis), the kidney also plays a very important role in the production of several key hormones used by your body.
Let's take a look at the issue of filtration and homeostasis first. The specific workings of the kidney as a filter can be quite complex, and mostly serve to create nightmares for medical students worldwide. Terms like "proximal convoluted tubule", "Bowman's Space", "Loop ofHenle ", and "Collecting duct" rapidly make the actual physiology of the kidney quite complex and difficult to explain without the aid of charts and drawings. But as mentioned previously, the mechanism of the filtration is not as important as why the filtration is taking place.
It should be reasonably common knowledge that your body needs correct amounts of various substances in order to function properly. Ions such as sodium, potassium and calcium are used in a variety of biological functions, such as muscle contraction. Urea is a waste product of protein metabolism. Water is used for a myriad of chemical reactions. Other trace elements are equally as vital your your health as well. Too high a concentration of many of these chemicals can be just as dangerous as low concentrations. Proper health is predicated on their being just the right concentrations.
I'll use water as an example of how the kidneys work to maintain proper homeostasis. Your body needs water to survive. More specifically, it needs the proper amount of water. Too little water and you die of dehydration. Too much water and your body overloads, making it impossible for your heart and cardiovascular system to function properly (there are other side effects of water overload as well).
There are various sensors scattered throughout your body that sense how much water you are carrying. Most of these sensors are related to blood pressure. When there is too much, your kidneys initiate a sequence of events that lead to increased urine output and lowering of total body water. Again, the details of this are quite complex and beyond the scope of this article. Similarly, if you have too little water as measured by your blood pressure, your kidneys react by retaining more water and lowering your urine output. People suffering from massive bleeding will have the kidneys try to compensate for the fluid loss by shutting down urine production as blood is mostly water.
Another key element of your body that must be maintained properly is the acidity of your blood. Measured by a value known as pH, it is absolutely vital that your blood be within a very limited range of acidity in order for your metabolism to function properly. In addition, your blood can only carry oxygen efficiently within a narrow pH range. It is your kidneys that do the major work of keeping your blood pH with that range - although your lungs and breathing are involved as well.
The second function of the kidneys is to produce certain hormones. One of those hormones, renin, is directly involved in the mechanism of water retention mentioned above. Also produced by the kidney is a hormone known as erythropoietin. Commonly known as EPO, this hormone is involved in red blood cell production and is one of the chemicals used by athletes in "doping". Injections of extraEPO stimulate your bone marrow to produce more red blood cells and thus allow you to carry more oxygen and run faster or longer.
The kidneys are also involved in the production of the chemically active form of Vitamin D, which is vital to your body in the regulation of calcium levels as well as other functions.
The kidneys are very complex organs with many vital functions to the human body. Although there is a lot of redundant capacity to the kidneys, you cannot live long without the regulation they are able to provide. Many medications that are used to fight blood pressure are directed at kidney function. People with little kidney function must undergo artificial filtration, known as dialysis. While it is true that the kidney is a filter, it's true function is much more complex that just the simple act filtering.