The Structure and Function of the Kidneys

It is hard to understand the signs and symptoms of kidney disease unless we appreciate the kidneys’ role in our body. This article explains what the kidneys do and how they accomplish their function.

Clarifying the Terms: Renal? Nephrology?

Let’s get the grammar straightened out at the outset. You might have heard the terms “renal,” “nephrological,” etc when you hear physicians talking about kidneys. The term “renal” is used interchangeably to refer to anything to do with the kidneys. The word comes from the Latin word for the kidneys, renes.

Similarly, “nephros” is the Greek term for the kidneys, while “logos” refers to study. Hence, Nephrology is the subspecialty of medicine that deals with the management of kidney diseases. And nephrologists are specialist physicians who deal with medical management of kidney disease, kidney transplantation, and hypertension.

What Are the Kidneys?

A pair of bean-shaped organs, the kidneys sit in the flanks, closer to the spine than to your belly. They are located just underneath your diaphragm and rib cage. They normally range in size from 8 to 14 centimeters (or 3-5.5 inches). Each kidney weighs between 120 grams (about ¼ lbs) to 170 grams (0.4 lbs). These numbers vary based on a person’s size and abnormal sized kidneys could be a sign of kidney disease. About 380 gallons (1440 liters) of blood flows through the kidneys every day.

What the Kidneys Do

Your kidneys are your silent workhorses who work 24x7 to clean your blood of impurities and toxins that build up from the body's metabolism. This waste fluid, which we know better as urine, is then excreted. However, the kidneys’ role extends to well beyond just “making urine.” They are your body’s very own laboratories that “test” your blood continuously to make sure every electrolyte’s concentration is within the specific range that is necessary for your body to function.

As an example, let’s consider an electrolyte in your blood, like potassium. Potassium is an electrolyte whose concentration needs to be within a tight range for your heart to generate its normal electric impulses. These impulses cause the heart to beat at a set rhythm or pulse. Both high, or low potassium can interfere with this electricity generation and cause your heart to go into an abnormal rhythm. This abnormal rhythm, called arrhythmia, is life-threatening and could cause a person to drop dead in a matter of seconds. However, this does not happen in normal circumstances because the moment the kidneys detect a rise in the blood’s potassium concentration, they dump the extra potassium into urine, thus keeping the potassium level constant in the blood. If it was not for your kidneys, a typical meal that you eat could turn out to be a life-endangering experience owing to its potassium content.

Another important function that the kidneys’ have is maintaining the blood’s concentration. The kidneys’ accomplish this by conserving/excreting the amount of water in your blood. You might have noticed that if you spend a day playing golf under a hot sun and not drink enough water, your urine will tend to look dark and concentrated.

Conversely, if it’s cold outside, the amount of water lost in sweat is greatly reduced, and your urine looks clear. The volume of urine goes up as well. These changes in your urine’s concentration and volume are regulated by your kidneys. The kidneys’ ability to make these changes is one of the reasons that life was able to adapt from the oceans to land, eons ago. (For anyone who is interested in reading further about the kidneys’ role in evolution, check out a seminal piece of work called From Fish to Philosopher. This was a book written by the eminent kidney physiologist Homer Smith, M.D., who was a Professor of Physiology at the New York University School of Medicine. The ​full text of the book is available).

Here are some other roles that the kidneys serve:

• They produce a hormone that is essential to make red blood cells, called “erythropoietin”
• They make sure that your bones stay healthy by producing a form of vitamin D
• They dump excess acid, that is generated from normal metabolism, out from your system
• Very importantly, they control your blood pressure

As you might imagine, all of these functions can go haywire in kidney disease, hence leading to the usual signs and symptoms that one sees in patients with kidney dysfunction.