In this article:
- Cleaning house: Your kidneys’ filtering systems
- Understanding how kidneys affect high blood pressure
- Discovering how high blood pressure impairs renal function
- Treating end-stage renal disease and malignant high blood pressure
Although we discuss how high blood pressure affects the heart, kidneys, and brain in three separate chapters, this doesn’t mean that individuals with high blood pressure develop only one of the diseases — heart disease or kidney disease or a stroke (brain attack).
Uncontrolled high blood pressure triggers their developments all at the same time, so all these complications may progress together. Other factors may prevent a heart attack or brain attack (such as taking aspirin), allowing kidney disease to dominate the picture, or a patient may have only two complications at the same time for some other reason. Still, you can prevent all these complications if you control your blood pressure.
This article discusses the kidney and how high blood pressure damages it. We cover how the damage occurs, how it proceeds, and how it leads to end- stage renal disease if unchecked. If you read this whole article, I expect that you’ll have the same great appreciation for your kidneys that we have for ours. You may even raise a glass of water and toast them!
Don’t accept a diagnosis of high blood pressure until you’ve had multiple high readings, and even then, measure your blood pressure at home with a reliable monitor (see article 2) both to confirm the diagnosis and to follow your body’s response to treatment. After high blood pressure is verified, it’s usually persistent. However, high blood pressure may improve, especially when a person makes lifestyle changes, even though some damage to the kidneys is permanent.
Examining the Role of Your Kidneys
Your body has two kidneys, each weighing about 6 ounces (or less than 1⁄2 of 1 percent of the body’s total weight). Each kidney is about 4 inches high, 2 inches wide, and 1 inch thick. The picture below shows their position in your abdomen.
Blood enters the kidneys through the large renal arteries. The renal cortex is the kidney’s outer shell that contains blood vessels and urine tubes. The renal pyramids, the innermost tissue of the kidneys, contain the urine tubes and the specialized tissue that permits fine-tuning of the various substances that leave the kidneys in the urine. The picture below shows an inside view of the kidney’s major parts.
In the following sections, we describe the kidneys’ various functions including filtering, renin production, and hormone production.
Focusing on the filtering function
Your kidneys are truly amazing. The digestive tract only takes out the waste that goes through the digestive tract, but the kidneys filter the blood, which acts much like a cargo carrier as it picks up waste from all the cells in your body. The blood carries the waste to your body’s waste-disposal headquarters — the kidneys.
Your kidneys filter an enormous amount of blood every minute, about 11⁄2 quarts. During this process, the kidneys filter out blood cells (such as your red and white blood cells) and large chemical compounds for recycling. But everything else, including the toxins (normal waste products), makes the trip. In the next step:
- A little water carries the toxins to the bladder as urine, which then passes out of the body through the urethra.
- The kidneys reabsorb the recyclable materials — 99 percent of the desired water, sodium, and other key body elements — back into the body.
Note: The blood that enters the kidneys in the renal (kidney) arteries makes its way through smaller and smaller arteries that eventually branch into capillaries (the human body’s smallest blood vessels). But just before passing into a capillary, blood goes through the afferent arteriole, the tiny artery that brings blood to the nephron (the kidney’s filtering device; see picture below). Each kidney contains 1 million nephrons. After filtering takes place, the liquid part of the blood (with all its dissolved elements — sodium, potassium, uric acid, and many others) passes through these capillaries into the efferent arteriole.
The capillary network within the nephron is the glomerulus. The Bowman’s capsule, a bulb that surrounds the glomerulus, receives all the filtered water and begins returning most of it to the bloodstream. At the same time, this bulb releases all the unnecessary substances as urine into the collecting tubule, which then empties the urine into the renal pelvis. In the final step, the urine travels through the ureter to the bladder (refer to the top picture to see the renal pelvis and the ureter).
Because this entire process requires pressure to force the liquid part of the blood, the kidney has a renin-angiotensin-aldosterone system, a built-in mecha- nism (see Chapter 4) that maintains the necessary level of blood pressure. But too much pressure for too much time causes damage. And if the damage proceeds, the result is end-stage renal disease where the kidneys can’t filter the blood or eliminate toxins without the help of dialysis or transplantation. (For more on dialysis and kidney transplants, see the “Coping with End-Stage Renal Disease” section later in this article.)
Understanding other kidney functions
In addition to filtering out and passing off your body’s waste, your kidneys affect your blood pressure in other ways, too. This section covers two critical problems: your kidneys’ overproduction of renin (an enzyme) and complications of hormone imbalances.
The juxtaglomerular cells are adjacent to the afferent arterioles — the small blood vessels that carry blood to the kidney’s filtering site. But when the arterioles detect that the blood pressure is insufficient for filtration to take place, the juxtaglomerular cells go into action and release renin, which eventually leads to the production of angiotensin II (for more on this process, see article 4). This development has two effects:
- Blood vessel constriction: The blood pressure immediately rises.
- Aldosterone secretion: Blood pressure rises even higher because this secretion causes salt and water retention.
After the juxtaglomerular cells perceive a satisfactory blood pressure, they stop making renin.
Is the kidney the source of most high blood pressure?
Studies show that high blood pressure damages the kidneys, but what causes the onset of essential (unknown cause) high blood pressure? Many scientists insist that the kidneys are the source of essential high blood pressure, citing a study in Circulation Research (June 1975) involving an experiment on rats with hereditary high blood pressure. The results show:
- If a rat from a strain that’s known to have normal blood pressure throughout life has its kidneys removed and receives a normal kidney from a strain that’s known to develop high blood pressure later in life, the rat develops high blood pressure.
- When a rat that’s prone to high blood pressure in the future has its kidneys removed and receives a normal kidney from a normal rat, high blood pressure doesn’t develop.
Therefore, the study concludes, the glitch must be somewhere in the kidneys of the high blood pressure strain; even when a kidney from the strain of rats with high blood pressure has normal appearance and hasn’t been subjected to high blood pressure as yet, it brings on high blood pressure. The rat study’s main conclusion? Rats with hereditary high blood pressure lose the ability to excrete salt from their body at a young age. This salt retention leads to water retention and a subsequent rise in blood pressure.
Specialized cells within the kidneys produce erythropoietin, a hormone that stimulates the bone marrow to make more red blood cells. Whenever blood is lost or a trip to high altitude demands more oxygen, the need for red blood cells is greater, so the kidneys produce erythropoietin. However, if kidney damage occurs, the cells that make erythropoietin decline, and anemia (a fall in the oxygen-carrying red blood cells) develops.
Another important function of the kidneys is the production of 1,25 dihydroxy vitamin D, a hormone that stimulates the uptake of calcium in the intestine. This hormone is actually the active end-product of vitamin D3 as a result of these steps:
- The process begins in the skin, where vitamin D is turned into vitamin D3 by the ultraviolet rays of the sun.
- This substance then circulates to the liver, where it’s converted to the next stage of active vitamin D3 production, 25 hydroxy vitamin D.
- Finally, it reaches the kidneys, where the most active form is produced.
When kidney function declines, highly active vitamin D3 declines. As a result, calcium isn’t taken up in sufficient quantities, causing osteomalacia, the formation of poorly calcified, weak bones.
Damaging the Kidney
Using renin, aldosterone, heart muscles, and many other tools in the body, the kidneys attempt to regulate the blood pressure at the glomerulus (see the earlier section “Focusing on the filtering function” for more on this capillary network). The pressure needs to be high enough to push out the water and dissolved substances, but not so high that it damages the glomerular cells.
When this regulation fails, kidney damage begins. As kidney tissue is lost, each remaining glomerulus must filter more. As a result, pressure at the glomerulus rises and does even more damage. This increased pressure maintains short-term kidney function but creates more long-term damage.
Plenty of experimental evidence suggests that higher pressure in the glomerulus damages the kidney:
- If 90 percent of an animal’s kidney is removed, the increased pressure damages the remaining glomeruli.
- When animals are fed a protein-restricted diet, which lowers glomerular blood pressure, the damage slows.
- Drugs that lower glomerular pressure in humans, even without lowering pressure in the arm, slow the onset of damage.
- Drugs that lower arm pressure in animals but don’t lower the glomerular pressure don’t protect the kidneys.
Just exactly how increased blood pressure in the glomeruli leads to kidney damage is unclear at this point. Current research suggests that the pressure causes production of certain chemicals that stimulate cell growth in the glomerulus and injure normal cells. The endothelial cells (cells that line the inside of blood vessels) may be the source because they make nitric oxide (an important substance that widens blood vessels) and chemicals that may damage the blood vessels. As more cells grow in the tiny space of the glomerulus, the surface area decreases, thereby limiting the amount of blood that can be filtered.
If kidney damage is suspected, your doctor will do blood tests for the levels of urea nitrogen (BUN) and creatinine as well as urine tests to measure the amount of function that remains in your kidneys. Note: One very important test is measurement of albumin (protein) in the urine. More than 300 milligrams daily suggests a more rapid loss of kidney function. In addition, the doctor will do visual studies of the kidneys to further define the amount of kidney loss and to help diagnose the reason for the kidney failure. These studies include:
- Ultrasound of the kidneys, which shows their size and shape
- Kidney biopsy, which identifies the exact disease that is causing the damage
- Computerized tomography, which gives a more detailed picture of the kidneys than a regular X-ray
- Magnetic Resonance Imaging, which can show the kidneys in cross-section
- Renal arteriogram, which shows the appearance of the kidney arteries and any obstruction within them
- An isotopic scan of the kidneys, which provides info on blood flow to the kidneys and the kidney function
Many researchers contend that essential high blood pressure can be linked to a kidney that appears to be normal but has a diminished ability to rid the body of salt. (See the above section “Is the kidney the source of most high blood pressure?”) This scientific perspective is further supported by the fact that every society that consumes salt in large quantities has a high incidence of high blood pressure. Article 10 describes the close connection between salt and high blood pressure in greater detail.
Life before and after miracle drugs
Before medications for high blood pressure control were available, researchers studied and followed a great number of people with uncontrolled high blood pressure. After 15 to 25 years of no control, about 40 percent of the patients had abnormally high levels of protein (an indication of kidney damage) in their urine. They lived about 5 years longer. Patients that had an elevation in blood urea nitrogen (evidence of kidney function loss) lived about another year. A typical study is in the Journal of Chronic Diseases (January 1955), where 500 patients were followed for an average of 20 years.
Today, an individual with end-stage renal disease has prolonged life expectancy due in part to dialysis and kidney transplantation. Currently, about 10 percent of deaths associated with high blood pressure are due to kidney failure.
Managing Malignant High Blood Pressure
Malignant high blood pressure (often found in smokers and in young African American males) affects about 1 percent of people with high blood pressure. It refers to severely high blood pressure (a diastolic reading often greater than 150 mm Hg) and severe complications including the following:
- Severe eye damage such as
- Papilledema (swelling of the optic disc — where the nerve enters the eye from the brain)
- Exudates (white spots from release of fluids that are opaque in front of the retina)
- Bleeding from capillaries in the eye
- Progressive damage to the brain that may first manifest itself as headaches (The individual may become confused and finally lapse into a coma.)
- Rapid development of partial or total loss of kidney function
- Nausea and vomiting
- Congestive heart failure
Dramatically elevated blood pressure isn’t enough to make a malignant high blood pressure diagnosis, and people with the same high blood pressure reading may not have the condition. Several of the symptoms in the preceding list are necessary to make the diagnosis.
A doctor must treat malignant high blood pressure in a hospital setting because it’s a medical emergency. The patient may not have had a diagnosis of high blood pressure in the past. He may appear confused, have heart failure, and have evidence of no remaining kidney function. (A physical exam should disclose the high blood pressure as well as the poor mental functioning.) When the doctor looks in the patient’s eyes, she sees swelling of the optic disc, bleeding that can be small dots or flame-shaped hemorrhages, and spasm of the arteries within the eye.
The exact cause of malignant high blood pressure is unclear. Some possibilities include the following:
- The direct result of poor blood pressure control. The blood pressure is allowed to rise to this dangerous level.
- The production of chemicals by damaged kidneys. These chemicals further damage the kidney and cause contraction of blood vessels, thus raising the blood pressure.
- Suppression of other chemicals that usually widen blood vessels.
Causes of secondary high blood pressure (see article 4), especially blocked kidney arteries (renal artery stenosis), also trigger malignant high blood pressure in as many as one-third of the cases.
In the days before 1950 or so, when treatment for high blood pressure wasn’t available, the majority of patients with malignant high blood pressure were dead within six months. Now that effective treatment is available for lowering the pressure, the response to treatment depends on the amount of irreversible kidney damage:
- If the kidney damage is minimal, then more than 90 percent of patients are alive after five years.
- Among those with kidney damage, the survival is about 65 percent after five years.
If these patients don’t have a fatal heart attack, the eventual cause of death is end-stage renal disease (which we cover in the next section), particularly if their kidneys show signs of damage.
Understanding a kidney aneurysm
As atherosclerosis (the process by which cholesterol in the arteries leads to narrowing of the arteries and higher blood pressure; see article 4) builds up in the body’s blood vessels, it also affects the kidney arteries. But the wall of the kidney artery is weaker than a normal artery. So, under the influence of higher blood pressure, the wall bulges out until it forms an aneurysm, a thin sac that’s filled with blood and is under high pressure. An aneurysm can rupture, leaking blood into the body. Unless the rupture is closed, the blood loss may lead to shock or death. A ruptured aneurysm must be considered when a patient with high blood pressure suddenly develops very low blood pressure.
Coping with End-Stage Renal Disease
End-stage renal disease (ESRD), also known as chronic renal failure, is loss of at least 90 percent of kidney function. The kidneys can’t perform their primary function — waste removal. Thus, waste and excess fluid (which normally pass through the urethra and out of the body) build up within the body. To continue living, the patient requires a kidney transplant or dialysis.
More than 500,000 people in the United States are being treated for ESRD, and the number rises 10 percent each year. In addition:
- About half of those with ESRD are men.
- About two-thirds are Caucasian and about one-third is African American.
- The largest group is between 45 and 64 years of age.
- Almost one-fifth of those with ESRD die each year.
- The cost of caring for people with ESRD is $30 billion each year.
High blood pressure, diabetes, and many other conditions that destroy the filtering nephrons (which we discuss earlier in this chapter) can lead to ESRD. However, individuals who are losing their kidney function are usually unaware of it until they begin to feel sick. In fact:
- Often, they don’t start to feel sick until 90 percent of their kidneys’ ability to get rid of toxins fails.
- High blood pressure, anemia, and bone disease can occur with about 70 percent of kidney function lost. But the individual may still be unaware of these abnormalities.
Efforts to reduce high blood pressure have greatly reduced the rate of heart disease (see article 5) and brain attacks (see article 7). But this result isn’t the case for ESRD, probably because — throughout the world — high blood pressure isn’t controlled sufficiently or long term. Whether high blood pressure is untreated or inadequately treated, the result is much the same: If the patient doesn’t die of heart disease or a brain attack, ESRD will develop.
Currently the most common reasons for ESRD are diabetes (33 percent) and high blood pressure (25 percent); internal diseases of the kidney account for the rest.
The person with untreated ESRD is very ill. Symptoms affect the entire body and include the following:
- Fatigue and weakness
- Easy bruising
- Itchy skin that appears yellow-brown
- A metallic taste in the mouth
- Breath that smells of urine
- Shortness of breath (even while sitting still and more with minimal activity)
- Nausea and vomiting
- Weight loss
- Frequent urination that interrupts sleep
- Cramping and spasms in the legs (especially at night while trying to sleep)
These signs and symptoms subside after treatment begins. Whether the kidneys fail because of high blood pressure, diabetes, or some other cause, the kidney treatment at this stage is the same: dialysis or kidney transplantation.
The choices for the treatment of ESRD, detailed in the following sections, have their pluses and minuses. Ample financial resources are available for all the choices because the federal government in the US as well as private insurance pay for most of the treatment cost. You needn’t worry about paying for your care in the US or the UK.
If your kidneys are failing and you take drugs for other conditions, the dosage of those drugs must be adjusted if they depend upon the urine for removal from the body. Discuss this with your doctor as you consider treatments for ESRD.
If the kidneys fail, as in the case of ESRD, they can’t filter out and eliminate the body’s waste. As we describe in the above section “Life before and after miracle drugs,” an individual with ESRD and untreated high blood pressure can’t expect to live much longer. In these cases, however, modern medicine’s methods of waste removal, peritoneal dialysis or hemodialysis, can filter waste from the blood and rid the body of its nasty toxins. Of the 500,000 patients with ESRD in the United States, 350,000 are undergoing dialysis.
If you have ESRD, you may need peritoneal dialysis, which takes advantage of the peritoneum’s (the abdominal cavity’s lining) filtering abilities. The peri- toneum prevents the passage of larger elements of the blood, such as blood cells and protein, but allows the liquid part with all its dissolved substances to pass through.
Peritoneal dialysis consists of these steps:
- A surgeon places a permanent catheter into the abdominal cavity.
- A dialysate, a salt-and-sugar solution, is put into the abdominal cavity through the catheter.
- Because of their high concentration, body wastes enter the dialysate solution.
- The dialysate, along with the unwanted wastes, is drained out through the catheter.
Each cycle of putting in and removing dialysate is an exchange. The picture below shows a peritoneal dialysis.
Peritoneal dialysis is usually done at home and is much more successful with several exchanges per day instead of one. There are three types of peritoneal dialysis:
- Continuous ambulatory peritoneal dialysis uses gravity to fill and empty the abdomen. Usually the individual needs three to four daytime exchanges and one during sleep.
- Continuous cycler-assisted peritoneal dialysis uses a machine to fill and remove the dialysate from the abdomen, especially during the night to make the overnight dialysis more efficient. Three to five exchanges take place during sleep. Another, longer exchange takes place during the day using a higher concentration of sugar to promote more waste removal.
- Nocturnal intermittent peritoneal dialysis uses six or more nighttime exchanges with the cycler machine to avoid the daytime exchange. Because this technique isn’t the most efficient, these patients usually have some remaining kidney function.
Testing the dialysate as well as the patient’s blood and urine determines the efficiency of each exchange. Measurement of the waste products in these three fluids determines whether the patient needs more dialysis, more solution, a different amount of sugar in the solution, or all three adjustments.
Home peritoneal dialysis has several pros and cons. The advantages include the following:
- The patient is in control of the treatment, so scheduling around his lifestyle is easier.
- The patient doesn’t have to travel to the hospital or dialysis center.
- Dietary restrictions are minimal with the exception of protein intake.
- By carrying supplies or shipping them ahead to the destination, the patient is able to travel.
- Particularly with nocturnal intermittent peritoneal dialysis, the patient may exercise.
The disadvantages include the following:
- Patients often fail to perform the dialysis, so they get sick.
- The patient must be trained to do the dialysis.
- Use of a cycler machine requires help from a partner.
- The dialysis bags and equipment require extra space in the home.
- Peritonitis, an infection of the peritoneal cavity, is a common complication.
Peritonitis raises body temperature. The abdomen may be tender and swollen, and the patient may be nauseated and vomiting. The dialysate becomes cloudy instead of clear, and bacteria may grow out from it.
If you notice these symptoms, see your doctor right away. Peritonitis is treatable and usually responds well to antibiotics. However, it some- times recurs, and the bacteria infect the catheter. Then a new catheter is needed.
The success of peritoneal dialysis depends on:
- How much dialysate can be placed into the abdominal cavity
- How rapidly the wastes pass into the dialysate
Some people can’t use peritoneal dialysis because their peritoneum doesn’t allow sufficient rapid passage of body fluids. In this case, hemodialysis may be the preferred method of treating ESRD.
Other reasons why some people can’t do peritoneal dialysis include:
- The presence of inflammatory bowel disease
- Inability to do self-care or lack of a caregiver
- Extensive prior abdominal surgery with many scars
During hemodialysis, your blood circulates through a dialyzer in the following steps:
A surgeon performs a minor operation in which an artery and a vein are connected to form an arteriovenous fistula (an abnormal connection) that allows for a large blood flow. The fistula heals in a month or so, and needles can then be placed in the artery (to deliver blood to the machine) and the vein (to return blood to the body).
The blood from the artery enters the dialyzer and passes through filters that mimic the glomerulus (see the earlier section “Focusing onChapter 6: Shielding Your Kidneys the filtering function”). Wastes are removed while normal body compo- nents are retained or returned to the blood.
The blood returns to the body through the needle into the vein.
Picture below shows how hemodialysis works.
Hemodialysis usually lasts two to four hours, three times per week. Just as in peritoneal dialysis, the fluid produced by hemodialysis and the patient’s blood can be tested to evaluate the adequacy of the treatment.
Hemodialysis, like peritoneal dialysis, has its share of advantages and disadvantages. Advantages include:
- It can be done at home, at a dialysis center, or within the hospital.
- Treatment at home allows you to set your own schedule as long as you follow your doctor’s recommendations.
- Treatment at a hospital ensures that professional help is available in case it’s needed. Also, because patients usually receive dialysis as a group, they often enjoy being in the company of other patients with similar problems.
- It takes considerably less time than peritoneal dialysis.
Below: Haemodialysis can be carried out at home
Disadvantages of hemodialysis include:
- Patients must follow a fairly careful diet and avoid fluids (depending on how much urine they’re making), salt, foods that contain phosphorus (like milk, cheese, and chocolate), and foods that contain potassium (like citrus and tomatoes).
- Taking treatment away from the home requires regular trips to and from the hospital.
- The dialyzer and equipment take up plenty of space in the home.
- Patients must be trained to administer and monitor home hemodialysis. For example, they need to know how to clamp off the needles if bleeding occurs and must be vigilant about caring for the fistula to prevent infection.
When possible, transplantation is the best answer to ESRD. You end up with a new, healthy kidney that performs like your good old kidneys. Of the 500,000 patients in the United States with ESRD, 150,000 have had kidney transplantation.
In kidney transplantation, you get a new kidney from a living related donor or from a donor who has recently passed away. If the kidney comes from a relative, you can have the surgery immediately. But for a kidney from a donor who has recently passed away, your name goes on a waiting list.
As shown below, the kidney is placed in your lower abdomen during a surgery that takes about four hours.
The failed kidneys are usually left in unless they’re infected or causing high blood pressure. Arteries in the new kidney are attached to your arteries. As the figure shows, the ureter is trans- planted to your bladder as well. The new kidney may take a few days or weeks to make urine. Hospitalization lasts another one to two weeks, and you can go back to work in four weeks.
Ninety percent of transplanted kidneys (whether from family members or unknown donors) still function after one year and eighty percent function after five years. On an average, kidneys from living donors survive 15 years, and kidneys from donors who have died last 10 years.
Before you can have a transplant, you must meet the following standards:
- You need to be well enough to withstand surgery and take the medications to prevent rejection.
- You are willing to take the medication without fail.
- You don’t have other conditions that prevent successful transplantation (such as a cancer in the last two years, severe heart disease, or severe lung disease).
- You have a good support structure to help you.
Due to health reasons, only half of the patients on dialysis are appropriate for a transplant.
Kidney transplantation is a long-term solution with these significant advantages:
- Patients feel as normal as they did before they became sick.
- No dialysis is necessary as long as the kidney continues to function.
- Patients only need to follow a few dietary restrictions such as limiting foods high in fat, calories, and possibly sodium and potassium.
Below: diagram showing transplanted kidney
Like all the treatments, however, disadvantages exist:
- Transplantation requires major surgery.
- Few kidneys are available, so if the person has to wait for a donor, it may be a long wait, sometimes years. Meanwhile, dialysis is necessary.
- Some transplants fail, so a second transplant is needed.
- Patients must take drugs such as immunosuppressants for a lifetime to prevent the body from rejecting the transplanted kidney. These drugs have side effects such as the promotion of diabetes and the weakening of the immune system (making infection more likely).
- The drugs or rejection of the new kidney may be responsible for high blood pressure (present in up to 90 percent of transplant survivors), which can further damage the new kidney.
As of 2012, more than 93,000 people in the United States were awaiting transplantation. About 20,000 transplants are done each year, so the need for more surgeries is huge. The limiting factor is the supply of kidneys.
If you have healthy kidneys, consider placing a sticker on the back of your driver’s license permitting the use of your kidneys as well as other organs if you die suddenly and unexpectedly. You will be giving the gift of life to two suffering people with your kidneys and the gift of sight to two others with your corneas.
If you are on the list for a transplant, be ready to go at a moment’s notice. Have your bag packed and transportation available, and be prepared to stay in the area of the transplant center for up to four weeks. Find out:
- The center’s experience
- The survival of the center’s transplants
- Other services (support groups and assistance with travel and housing)
You will be amazed at how many centers are available in the US. For instance, California and Texas have 26 centers each that do kidney transplantation, and New York has 14. You will find a wide range of experience among those centers.
Continued in this article: Protect the brain from high blood pressure