The kidneys are organs about the size of your fist that sit not far from the spinal column above the lower back. Unlike the appendix, as one example, they perform a variety of vital functions. Dual kidney failure means death. Luckily, the body provides two and one can do the job by itself. But, what is that job?
The kidneys are often said to be the body’s filtering system, and that is true. They filter what would be toxic substances if they built up inside the body. There is more detail to the story, though.
Even ordinary and essential chemical elements, like calcium, potassium, and others would be harmful if they existed in too-high levels in the blood, organs, and bones. Even ordinary body fluid in the form of water can be lethal if we retain too much. The kidneys are part of a regulatory system that keeps all these to the appropriate amounts.
One way the kidneys perform that function is by balancing what are called electrolytes. These are essentially dissolved salts – sodium and chloride, magnesium and sulfates, and others. Salts are crucial to a wide variety of cellular and muscle processes.
The heart, for example, pumps needed blood that bathes the tissues in oxygen and nutrients. That muscle action would be impossible without something called the sodium-potassium pump. In part, they supply the ‘electrical’ stimulus that keeps that critical muscle working in the proper rhythm. The kidneys play an indispensable role in regulating the levels of sodium and potassium.
They also play a role in balancing the level of acidity in the blood. People read the word ‘acid’ and automatically assume it is harmful. Battery acid burns your fingers. Stomach acid produces an upset tummy. But acid, which at bottom is just hydrogen ions, is essential when it exists at the right concentration. Similarly, the body needs certain alkali (hydroxyl ions or oxygen-hydrogen molecules) to perform other functions. The kidneys keep both from getting too far from equilibrium.
Even pH-neutral water has to be kept at the right amount. It is necessary for all cellular reactions, body temperature control, and other life-sustaining activities. But too much can actually cause cells to expand to dangerous proportions and drive electrolyte concentrations too low. The kidneys work in conjunction with the bladder and other systems to keep fluid levels within healthy bounds.
As part of this activity, the kidneys perform another vital function: they help regulate blood pressure by releasing a hormone called renin. They are part of a coordinated effort among many systems. By adjusting how much sodium is excreted, the kidneys are central to that effort. Another aspect of that system is the level of extracellular fluid volume, influenced by the kidneys’ actions.
The net result of all the kidneys’ behavior is to move urine containing waste products to the bladder through two small tubes called ureters. From there, the bladder takes over to eliminate the fluid waste.