There is an interesting connection between the health of your kidneys and the health of your heart. Dr. Matthew Weir, professor of medicine at the University of Maryland School of Medicine and head of nephrology and the University of Maryland Medical Center answers questions about blood vessel disease, blood pressure, risk factors for heart attack and stroke and explains how they are all linked to kidney health.
The main job of the kidneys is primarily to handle purification of the blood. Every time we eat, the breakdown products of the proteins can actually accumulate in the blood. It is the job of the kidney to filter those proteins out and keep the blood clean. The kidneys also handle all of the water in our bodies and they regulate the amount of various electrolytes such as sodium, potassium and chloride. They are also responsible for producing hormones which maintain the body's red blood cell count and vitamin D.
There are a variety of things that can lead to damage to your kidneys. We have ways of non-invasively assessing the amount of filtering capability of our kidneys with very simple blood tests like measuring serum creatinine. As we age, we tend to lose a little bit of kidney function every year. This becomes most apparent after the age of 35 or 40, but it's still not a major concern. A person would have to lose probably 75 or 80 percent of their kidneys' filtering ability before they would become symptomatic.
Even medications can negatively affect the kidneys. Sometimes this is seen with antibiotics, but the most common form of a medicine that can hurt the kidneys is medicine we take for pain such as Ibuprofen and Naproxen. There has been some evidence that acetaminophen can contribute to kidney disease, but this has been attributed to large amounts over a long period of time.
We are very capable of treating kidney disease itself. We have strategies to delay progression of kidney disease, whether it's related to high blood pressure diabetes or even the immunological diseases. We even have excellent strategies for dealing with kidney disease when it's at an end-stage level such as kidney transplantation or dialysis. However, what a lot of people tend to forget is that one of the most important aspects of the kidneys is that they provide a barometer, or a perspective, on the overall health of the blood vessels in the body.
The kidneys are a unique series of structures, but many people are born with more than a million tiny filtering units which are at the exterior of the blood vessels' supply to the kidneys. It is between these tiny filters that the poisonous materials in the bloodstream are actually transferred to the urinary space and then ultimately eliminated. We now realize that if we see graded changes in the filtering of the kidneys, it also appears to be related to damage elsewhere in the body. So the kidneys are not only predictive of kidney disease, as we lose filtering ability, but they're also predictive of your risk for having a stroke or a heart attack.
It's a much more sensitive biomeasure of global blood vessel disease in the body. That's really the fascinating part. It is remarkable when you consider that if you have heart disease, you don't know it until it's pretty far advanced. For example, if you have a heart attack, you suddenly get chest pain and that can kill you at the first presentation. The same thing is true if you have a stroke. If you have a stroke, it's terrible, but maybe you have a transient ischemic attack (TIA) and that could be a clue. The beauty of the kidneys is we have a longitudinal opportunity to measure in a graded way the continuous relationship of kidney function and risk for stroke and heart attack. So in fact, the real value of the kidneys is not only telling us about their health, but the health of the rest of the blood vessels in the body.
There are simple inexpensive, easy to obtain tests such as a measure of serum creatinine which is a natural breakdown product of your muscles and it appears in the urine everyday. Before it can appear in the urine, it appears in the blood and your kidneys' ability to move it from the blood to the urine is a way to measure filtering capability. This simple blood test allows us to estimate the filtering ability of the kidneys. In doing this we can then start to estimate your risk for stroke and heart attack. This can be obtained with a simple urine test to measure the amount of creatinine and the amount of protein in the urine. Based on that ratio, the risk can be estimated.
The appearance of protein in the urine is indicative of blood vessel damage, of those very delicate, distal capillaries. Most people don't have symptoms related to kidney disease until it is very far advanced. Occasionally, people can develop massive losses of protein in their urine because the capillaries become leaky before the filtering capability starts to become impaired. Those patients will present with a lot of swelling in their legs, calves and be puffy all over. But if we're talking about loss of filtering ability of the kidneys, it is such a slow, insidious, gradual process that most people will not notice any change at all. As much as 90 percent of the filtering ability of the kidneys can be gone before there is weakness or fatigue. It's important to be screened, as an adult, on a regular basis and to look at kidney filtering ability with this very simple blood test.
We have some simple strategies for evaluating somebody who presents to us with abnormal filtering ability and that would be manifested by an elevated serum creatinine. We would do an ultrasound to make sure the patient has two kidneys because you can be born with just one. We make sure the tubes draining the kidneys to your bladder are not obstructed and we are able to see if the kidneys have shrunk, scarred down or become firm, which are signs of kidney disease. If we suspect there is an immunological cause, we can send off blood tests to look at parts of our immune system that may be reacting against the kidney and that can be very helpful. The most important process that we focus on in patients with kidney disease is whether or not they have diabetes. Those two issues are by far some of the most important factors.
We have data from clinical trials demonstrating that we can slow the progression of kidney disease with better control of blood pressure and blood glucose. However, the sad part is in clinical practice, so many patients come to us long after they have already lost half of their kidney function. Many people have diabetes for 10 or 15 years and don't realize that they even have it. I make a diagnosis every year of diabetic kidney disease where neither the referring doctor nor the patient knows they had it. That's because blood pressure, cholesterol, glucose, the typical traditional measures of blood vessel risk that we all have, progressively change over time. It's not like you have hypertension now but you didn't have it yesterday. They all change slowly with age. We're all on a slope of change of blood pressure, cholesterol and glucose, which is largely predicated by our parents. These are inherited genes or risk factors.
One of the most important things we need to remember is that primary prevention is better than rushing in and trying to do something after you have already developed these conditions. Realizing that blood pressure, cholesterol and glucose change progressively with age provides us in the medical field with an opportunity to longitudinally monitor patients for risk. Part of the difficulty is that much of the education in the past has suggested that hypertension and diabetes occur at certain point in time and before that you don't have it. There's nothing further from the truth. The most important thing to do in practice is to look at family history.
Graded changes in terms of blood pressure and how your body handles glucose should be monitored by vey simple measures that can be done in your doctor's office. This is important so that one can start to individualize when rises in blood pressure may occur. For example, blood pressure goes up with increasing age. In fact, if you live in the United States and you are in that 55 to 60 age range, which I now call middle youth, you have a near 90 percent likelihood of requiring medication for your blood pressure in your remaining life expectancy. These are simple biomeaures of risk and we can capably treat them both with lifestyle modifications and if need be, medications. It is the same with our glucose levels which tend to go up with increasing age. Again, you can modify that with proper diet, regular exercise and maintaining your weight, but if everybody in your family has this kind of a slope, you can run six miles a day and you're still going to have problems with glucose tolerance as you age.
Since blood pressure goes up with age, in general, and again a little bit differently in all of us, there are definitions that have been used to characterize people. Traditionally, 140 over 90 has been used as being the threshold between hypertension and not having hypertension. We now realize that maybe people with more blood vessel disease risk maybe need a lower level of blood pressure. Many of the guidelines for people with diabetes, kidney disease or heart disease suggest that 130 over 80 is a better number. However, if you have a 25-year-old with a blood pressure of 130 over 80 and you have a 60-year-old with 130 over 80, I'm going to be more likely to want to treat the 25-year-old than I am the 60-year-old just because they have many more years for that slope of change in blood pressure to occur. Again, the strategy being maybe earlier freezing of that blood pressure at 130 rather than letting it rise to higher levels may reduce the likelihood for the younger person to develop heart or kidney disease. Particularly if they have a strong family history of blood vessel disease and heart disease in their family.
Blood pressure is very dynamic. Higher by day, lower by night, higher when you require anger management or when you're exercising. We see many different facets of it and what we're realizing now in clinical practice is that you need to take into account blood pressure numbers from the office, home, stress and everywhere. We have very simple ways of doing that and patients can participate in their own care. The other issue about blood pressure people need to remember is that after the age of 55 years or so, the upper number is much more important in terms of predicting risk for stroke, heart attack and kidney failure. The lower number naturally goes down with increasing age so it loses its validity as a measure of risk. That upper number for those of us in middle youth and above is really the most important number that we monitor and have to treat and follow.
It's complicated because you have to remember the heart beats 60, 70, 80, 90 times a minute and so it creates a pulsatile flow, but your blood vessels actually work in a way to make it more progressive throughout the circulation. It's a steady flow and they have characteristics like elasticity, which helps transmit pulsatile to a steady flow throughout the circulation. We're learning a lot about this now and it's also a way we can start to identify early changes in blood vessels and how they work in streamlining the blood supply to the distal organs. And again, it can be an early measure of risk for developing blood vessel disease.
It's really progressed quite well. We're so good at it now. We really recommend that when people start to develop loss of kidney function, certainly 70 or 80 percent, that we refer them right away for kidney transplantation as opposed to waiting until they go on dialysis. Patients appear to do much better. They have less risk for heart disease and complications post transplantation if we get them in earlier and set up to receive a kidney from a living related or unrelated donor.
The medicines are good, the surgeons are good, but I'll say that even with a kidney transplant, we still appreciate the important opportunity of better control of blood pressure, cholesterol and glucose. These factors still remain important. Heart disease, even after a successful kidney transplant, remains just about the most important cause of long term kidney transplant failure. Kidney disease we can deal with, it's the heart disease and strokes that are the major problem. That's why measuring kidney function is so important.