LISA CLARK: I'm Lisa Clark. Welcome, and thanks for joining us for this webcast. Did you know that more than 20 million Americans suffer from various forms of kidney and urinary tract disease? Of these, about 10 percent -- 200,000 people -- are suffering from chronic kidney failure. They require the use of an artificial kidney machine to stay alive. In order to get a better idea of how the kidneys work and why they are so vital to our health, we're going to spend the next few minutes with two physicians who specialize in the study and treatment of kidney disease.
To my left is Dr. Leonard Stern. Welcome. Down further, Dr. Jai Radhakrishnan. Both men are Assistant Professors of Clinical Medicine at the College of Physicians and Surgeons of Columbia University, and they practice at the Columbia Presbyterian Medical Center. Thank you both for being here.
If I may, begin with a very basic description of what the kidneys look like, where they are located in the body, Dr. Radhakrishnan?
JAI RADHAKRISHNAN, MD: Every person has two kidneys, and they're about in the middle of the abdomen in the back, and they're the size of the fist. They drain into two tubes called the urethrae, into the bladder.
LISA CLARK: What are the basic functions that the kidney performs in the body?
JAI RADHAKRISHNAN, MD: The kidney is a very important organ, and the most obvious function is that it excretes wastes that we produce from our diet and from our metabolism. What's less obvious is that it's the most important organ that controls the composition of the body fluids. In addition, it does produce a number of hormones that deal with body function.
LISA CLARK: Len, if I may ask you, what's the mechanism by which the kidneys actually remove waste from the bloodstream?
LEONARD STERN, MD: The kidney is a filtering organ, so blood is delivered to a very unique apparatus in the kidney called the glomerulus, and there, because of pressure dynamics, a component of the blood is filtered across the membrane, and the first process of creating the urine develops. In this segment of the kidney, in the glomerulus, the composition of that fluid is very similar to blood. The fluid passes through a series of tubules in a structure called the nephron, and there it's modified, heavily modified. A variety of things are reabsorbed that we need, and a variety of toxins are concentrated until the end of the kidney, the end of that nephron segment, drains into something called a papilla, then into the ureter, and then the bladder, and we have what we term "urine," which is a fluid rich in toxic wastes that is excreted.
LISA CLARK: It's a remarkably complex system, and I guess there are a lot of places where things can get hung up along the way.
LEONARD STERN, MD: There are many segments of the kidney where illness can affect the function. For example, there are inflammatory disorders that affect the glomerulus. There are disorders that affect the tubules and the reabsorptive process. There are specific illness that affect the blood vessels that feed the kidney, and then, of course, there are problems with the drainage system and the ureters -- infection, stones, a variety of ailments. Textbooks of ailments.
LISA CLARK: Jai, in addition to filtering waste from the body, the kidneys also regulate electrolytes in the body system. How does that work?
JAI RADHAKRISHNAN, MD: For example, suppose you drink a bottle of orange juice. The body sees a sudden excess of an element 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 increase the excretion of potassium so everything comes back to normal.
LISA CLARK: Again, that's a very complex system. It's amazing that the kidney is able to do that at a moment's notice.
JAI RADHAKRISHNAN, MD: And that's just one element. There are at least 100 elements that are controlled by the kidney.
LISA CLARK: One of those is the production of red blood cells. I wasn't aware that the kidneys worked in that function, as well.
JAI RADHAKRISHNAN, MD: This is correct, because one assumes that the red blood cells are there and you take it for granted, but what controls the number of red cells? So the kidney has sensors that look at the oxygen content of the blood flowing through the kidney, and if it detects that the oxygen concentration is low, it makes a hormone called erythropoietin, which increases the bone marrow's production capacity to make more red cells. For example, if you lose a lot of blood, there's a drop in the oxygen going to the kidney as a consequence of the depletion of these red cells, and there's an immediate increase in the production of this hormone, erythropoietin.
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, and the patients feel horribly fatigued and they're dysfunctional because they're unable to make these red blood cells. A nice part about our pharmaceutical industry is they have cloned the genes to make this hormone, and this is a medication that we now have available that we can give as a replacement.
LISA CLARK: Now, there is also another hormonal activity that goes on in the kidneys. Tell me a little bit about that.
LEONARD STERN, MD: The kidney makes the active form of vitamin D. Vitamin D exists in a number of different precursor forms ranging from the skin and the liver, and the kidney does the final biochemical modification to make the active form that acts on bones and intestines to absorb calcium and to regulate the control of parathyroid hormone. So in essence, this vitamin D is a regulatory hormone that has a variety of functions in the body. The ones we look at most are bones and calcium, but vitamin D also has some immunoregulatory activity and may be needed as a cancer protective mechanism.
LISA CLARK: Oh, really? That's a very interesting side benefit.
LEONARD STERN, MD: A subject of major research in terms of vitamin D metabolites being used as anticancer therapies.
LISA CLARK: When we think about the amount of blood that must be filtered through the kidneys every day, can you give me an approximation of how much actually goes on?
JAI RADHAKRISHNAN, MD: Yes, it's an unimaginable amount. An average human filters about 200 quarts of blood through the kidneys each day, of which all but 2 quarts are reabsorbed, which is the urine.
LISA CLARK: Now, the human body has, what, about 5 or 6 quarts of fluid to begin with? So 30 round trips?
JAI RADHAKRISHNAN, MD: The reason is because 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 go on 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, 20 percent of the blood flow goes directly to the kidney, so it receives more blood flow than any other organ in the body.
LISA CLARK: I guess that would lead to a reason why high blood pressure, then, is so damaging to the kidneys, because there's already a tremendous amount of force of blood going through the kidney system.
LEONARD STERN, MD: That could be one possibility. High blood pressure can be a cause of kidney disease, or a result from kidney disease, because many disorders of the kidney, as part of their features and their presentation, are related to blood pressure elevation. But the concept that high blood pressure damages blood vessels so that the cardiac output can't get to the kidney and the kidneys lose their ability to act as a regulatory organ, and as the kidney fails, the function of the kidney fails and we retain waste products. The hormone function deteriorates, the vitamin D levels go down, the erythropoietin levels go down, and the patients develop a progressive syndrome which we call uremia.
LISA CLARK: So it's really 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 to regulate the internal homeostasis, so the balance state of all of our chemicals, 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 a fish out of water.
LISA CLARK: Exactly so. Well, I'd like to thank you both for joining us tonight, Len and Jai, for giving us and overview of how the kidneys function in the body. We appreciate you being here, and we appreciate you joining us for our webcast. Thanks. I'm Lisa Clark.
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