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By: Jai Radhakrishnan, MD, MRCP (UK)
By: Leonard Stern, MD
More than twenty million Americans suffer from some form of kidney and
urinary tract disease, and of these millions, about ten percent—two hundred
thousand people—are suffering from chronic kidney failure. These sufferers
require the use of an artificial kidney machine to stay alive.
But what exactly do the kidneys do? Below, Dr. Jai Radhakrishnan and Dr. Leonard
Stern give us a short course on the kidney, and explain why this little organ
plays such a big role in good health.
Q: Can you give us a basic introduction to the kidneys?
JAI RADHAKRISHNAN, MD: Every person has two kidneys. They're in the middle of
the abdomen in the back, and they're the size of a fist. They drain into two
tubes called the ureters, which lead into the bladder.
Q: What are the basic functions that the kidneys performs?
JAI RADHAKRISHNAN, MD: The most obvious function of the kidney is to excrete
wastes that we produce from our diet and from our metabolism. The kidney also
controls the composition of the body fluids, and produces a number of hormones
that deal with body function.
Q: How much blood filters through the kidneys each day?
JAI RADHAKRISHNAN, MD: It's an unimaginable amount. An average human filters
about two hundred quarts of blood through the kidneys each day, of which all but
two quarts are reabsorbed, which is the urine.
Q: And the human body has about five or six quarts of fluid to begin with?
JAI RADHAKRISHNAN, MD: Correct. The kidneys are involved with minute-to-minute
regulation of body fluids, so if a large quantity of fluid is not processed, you
cannot keep up with the changes that normally occur in the body. It's very
important that a large quantity of fluid be processed, and that's the only way
the kidney can control the minute-to-minute regulation.
LEONARD STERN, MD: Another way of thinking about it is, every time the heart
beats, twenty percent of the blood flow goes directly to the kidney, so it
receives more blood flow than any other organ in the body.
Q: In addition to filtering waste from the body, the kidneys also regulate
electrolytes in the body system. How does that work?
JAI RADHAKRISHNAN, MD: Suppose you drink a bottle of orange juice. The body sees
a sudden excess of an electrolyte called potassium, so the kidney has sensors
which look at the body fluid going through the kidney as plasma, and it detects
that the plasma's potassium has gone up, and it immediately increases the
excretion of potassium so everything returns to normal. And that's just one
electrolyte that the kidney controls. There are at least a hundred elements and
electrolytes that are controlled by the kidney.
Q: The kidney controls the production of red blood cells. Can you explain how
that works?
JAI RADHAKRISHNAN, MD: The kidney controls the number of red cells in the body.
It has sensors that look at the oxygen content of the blood flowing through it,
and if it detects that the oxygen concentration is low, it makes a hormone that
increases the bone marrow's production capacity to make more red cells.
LEONARD STERN, MD: The hormone level is critically important in people with
normal kidney function, and people who have failing kidneys don't make this
hormone, so one of the features of chronic kidney disease is the anemia that
develops. Patients feel horribly fatigued and are unable to make red blood
cells. The pharmaceutical industry has cloned the genes to make this hormone,
and this medication replaces the natural hormone.
Q: It's hard to underestimate the impact that kidney health has on the function
of the entire body.
LEONARD STERN, MD: The kidney is the main organ responsible for regulating the
body's internal equilibrium—or the balance of all of our chemicals-and whatever
we eat has to be modified by our metabolic process, and the waste products are
excreted in the kidney. Without the function of the kidney, we couldn't regulate
the levels of electrolytes and hormones and fluid balance. We would be like fish
out of water.