The Kidneys and How They Work

What are the kidneys?

The kidneys play key roles in body function, not only by filtering the blood and getting rid of waste products, but also by balancing levels of electrolyte levels in the body, controlling blood pressure, and stimulating the production of red blood cells.

The kidneys are located in the abdomen toward the back, normally one on each side of the spine. They get their blood supply through the renal arteries directly from the aorta and send blood back to the heart via the renal veins to the vena cava. (The term "renal" is derived from the Latin name for kidney.)

The kidneys have the ability to monitor the amount of body fluid, the concentrations of electrolytes like sodium and potassium, and the acid-base balance of the body. They filter waste products of body metabolism, like urea from protein metabolism and uric acid from DNA breakdown. Two waste products in the blood can be measured: blood urea nitrogen (BUN) and creatinine (Cr).

When blood flows to the kidney, sensors within the kidney decide how much water to excrete as urine, along with what concentration of electrolytes. For example, if a person is dehydrated from exercise or from an illness, the kidneys will hold onto as much water as possible and the urine becomes very concentrated. When adequate water is present in the body, the urine is much more dilute, and the urine becomes clear. This system is controlled by renin, a hormone produced in the kidney that is part of the fluid and blood pressure regulation systems of the body.

Kidneys are also the source of erythropoietin in the body, a hormone that stimulates the bone marrow to make red blood cells. Special cells in the kidney monitor the oxygen concentration in blood. If oxygen levels fall, erythropoietin levels rise and the body starts to manufacture more red blood cells.

After the kidneys filter blood, the urine is excreted through the ureter, a thin tube that connects it to the bladder. It is then stored in the bladder awaiting urination, when the bladder sends the urine out of the body through the urethra.

What causes kidney failure?

Kidney failure can occur from an acute situation or from chronic problems.

In acute renal failure, kidney function is lost rapidly and can occur from a variety of insults to the body. The list of causes is often categorized based on where the injury has occurred.

Prerenal causes (pre=before + renal=kidney) causes are due to decreased blood supply to the kidney. Examples of prerenal causes of kidney failure are:

• hypovolemia (low blood volume) due to blood loss;
  
• dehydration from loss of body fluid (for example, vomiting, diarrhea, sweating, fever);
  
• poor intake of fluids;
  
• medication, for example, diuretics ("water pills") may cause excessive water loss; and
  
• abnormal blood flow to and from the kidney due to obstruction of the renal artery or vein.

Renal causes of kidney failure (damage directly to the kidney itself) include:

• Sepsis: The body's immune system is overwhelmed from infection and causes inflammation and shutdown of the kidneys. This usually does not occur with urinary tract infections.
  
• Medications: Some medications are toxic to the kidney, includingnonsteroidal anti-inflammatory drugs like ibuprofen and naproxen. Others potentially toxic medications include antibiotics like aminoglycosides [gentamicin (Garamycin), tobramycin], lithium(Eskalith, Lithobid), iodine-containing medications such as those injected for radiology dye studies.
  
• Rhabdomyolysis: This is a situation in which there is significant muscle breakdown in the body, and the damaged muscle fibers clog the filtering system of the kidneys. this can occur because of trauma, crush injuries, and burns. Some medications used to treat high cholesterolcan cause rhabdomyolysis.
  
• Multiple myeloma
  
• Acute glomerulonephritis or inflammation of the glomeruli, the filtering system of the kidneys. Many diseases can cause this inflammation including systemic lupus erythematosus, Wegener's granulomatosis, and Goodpasture syndrome.

Post renal causes of kidney failure (post=after + renal= kidney) are due to factors that affect outflow of the urine:

• Obstruction of the bladder or the ureters can cause back pressure because the kidneys continue to produce urine, but the obstruction acts like a dam, and urine backs up into the kidneys. When the pressure increases high enough, the kidneys are damaged and shut down.
  
• Prostatic hypertrophy or prostate cancer may block the urethra and prevents the bladder from emptying.
  
• Tumors in the abdomen that surround and obstruct the ureters.
  
• Kidney stones. Usually, kidney stones affect only one kidney and do not cause kidney failure. However, if there is only one kidney present, a kidney stone may cause the lone kidney to fail.

Chronic renal failure develops over months and years. The most common causes of chronic renal failure are related to:

• poorly controlled diabetes,
  
• poorly controlled high blood pressure, and
  
• chronic glomerulonephritis.

Less common causes of chronic renal failure include:

• polycystic kidney disease,
  
• reflux nephropathy,
  
• kidney stones, and
  
• prostate disease.

What are the symptoms of kidney failure?

• In the beginning, kidney failure may be asymptomatic (not producing any symptoms). As kidney function decreases, the symptoms are related to the inability to regulate water and electrolyte balances, to clear waste products from the body, and to promote red blood cell production. Lethargy,weakness, shortness of breath, and generalized swelling may occur. Unrecognized or untreated, life-threatening circumstances can develop.
  
• Metabolic acidosis, or increased acidity of the body due to the inability to manufacture bicarbonate, will alter enzyme and oxygen metabolism, causing organ failure.
  
• Inability to excrete potassium and rising potassium levels in the serum (hyperkalemia) is associated with fatal heart rhythm disturbances (arrhythmias) including ventricular tachycardia and ventricular fibrillation.
  
• Rising urea levels in the blood (uremia) can affect the function of a variety of organs ranging from the brain (encephalopathy) with alteration of thinking, to inflammation of the heart lining (pericarditis), to decreased muscle function because of low calcium levels (hypocalcemia).
  
• Generalized weakness may be due to anemia, a decreased red blood cell count, because lower levels of erythropoietin produced by failing kidneys do not adequately stimulate the bone marrow. A decrease in red cells equals a decrease in oxygen-carrying capacity of the blood, resulting in decreased oxygen delivery to cells for them to do work; therefore, the body tires quickly. As well, with less oxygen, cells more readily use anaerobic metabolism (an=without + aerobic=oxygen) leading to increased amounts of acid production that cannot be addressed by the already failing kidneys.
  
• As waste products build in the blood, loss of appetite, lethargy, and fatigue become apparent. This will progress to the point where mental function will decrease and coma may occur.
  
• Because the kidneys cannot address the rising acid load in the body, breathing becomes more rapid as the lungs try to buffer the acidity by blowing off carbon dioxide. Blood pressure may rise because of the excess fluid, and this fluid can be deposited in the lungs, causingcongestive heart failure.

How is kidney failure diagnosed?

Diagnosis of kidney failure is confirmed by blood tests measuring the buildup of waste products in the blood. BUN, creatinine, and GFR are routine blood tests used to measure the buildup of waste products in the blood. BUN and creatinine become elevated, and the glomerular filtration rate (GFR) decreases. This is the rate with which blood is filtered through the kidneys and can be calculated based upon the creatinine level, age, race, and gender.

Urine tests may be done to measure the amount of protein, detect the presence of abnormal cells, or measure the concentration of electrolytes. Protein in the urine is not normal and can be a clue that damage to the kidneys has occurred. Abnormal aggregations of red and white blood cells called casts can be seen in the urine with kidney disease. Comparing the concentrations of electrolytes in the blood and urine can help decide whether the kidneys are able to appropriately monitor and filter blood.

Other tests are used to diagnose the type of kidney failure. Abdominal ultrasound can assess the size of the kidneys and may identify whether any obstruction exists. Biopsy of the kidney uses a thin needle that is placed through the skin into the kidney itself to get bits of tissue to examine under the microscope.

What is the treatment for kidney failure?

Prevention is always the goal with kidney failure. Chronic diseases such ashypertension and diabetes are devastating because of the damage that they can do to kidneys and other organs. Lifelong diligence is important in keeping blood sugar and blood pressure within normal limits. Specific treatments are dependent upon the underlying diseases.

Once kidney failure is present, the goal is to prevent further deterioration of renal function. If ignored, the kidneys will progress to complete failure, but if underlying illnesses are addressed and treated aggressively, kidney function can be preserved, though not always improved.

Diet

Diet is an important consideration for those with impaired kidney function. Consultation with a dietician may be helpful to understand what foods may or may not be appropriate.

Since the kidneys cannot easily remove excess water, salt, or potassium, these may need to be consumed in limited quantities. Foods high in potassium include bananas, apricots, and salt substitutes.

Phosphorus is a forgotten chemical that is associated with calcium metabolism and may be elevated in the body in kidney failure. Too much phosphorus can leech calcium from the bones and cause osteoporosis and fractures. Foods with high phosphorus content include milk, cheese, nuts, and cola drinks.

Medications

Medications may be used to help control some of the issues associated with kidney failure.

• Phosphorus-lowering medications (calcium carbonate [Caltrate], calcitriol[Rocaltrol], sevelamer [Renagel])
  
• Red blood cell production stimulation (erythropoietin, darbepoetin[Aranesp])
  
• Red blood cell production (iron supplements)
  
• Blood pressure medications
  
• Vitamins

Once the kidneys fail completely, the treatment options are limited to dialysis or kidney replacement by transplantation.

Dialysis

Dialysis cleanses the body of waste products in the body by use of filter systems. There are two types of dialysis; 1) hemodialysis, and 2) peritoneal dialysis.

Hemodialysis

Hemodialysis uses a machine filter called a dialyzer or artificial kidney to remove excess water and salt, to balance the other electrolytes in the body, and to remove waste products of metabolism. Blood is removed from the body and flows through tubing into the machine, where it passes next to a filter membrane. A specialized chemical solution (dialysate) flows on the other side of the membrane. The dialysate is formulated to draw impurities from the blood through the filter membrane. Blood and dialysate never touch in the artificial kidney machine.

For this type of dialysis, access to the blood vessels needs to be surgically created so that large amounts of blood can flow into the machine and back to the body. Surgeons can build a fistula, a connection between a large artery and vein in the body, usually in the arm, that causes a large amount of blood flow into the vein. This makes the vein larger and its walls thicker so that it can tolerate repeated needle sticks to attach tubing from the body to the machine. Since it takes many weeks for a fistula to mature enough to be used, significant planning is required if hemodialysis is to be considered as an option.

If the kidney failure happens acutely and there is no time to build a fistula, special catheters may be inserted into the larger blood vessels of the arm, leg, or chest. These catheters may be left in place for up to three weeks. In some diseases, the need for dialysis will be temporary, but if the expectation is that dialysis will continue for a prolonged period of time, these catheters act as a bridge until a fistula can be planned, placed, and matured.

Dialysis treatments normally occur three times a week and last a few hours at a time. Most commonly, patients travel to an outpatient center to have dialysis, but home dialysis therapy is becoming an option for some.

Peritoneal dialysis

Peritoneal dialysis uses the lining of the abdominal cavity as the dialysis filter to rid the body of waste and to balance electrolyte levels. A catheter is placed in the abdominal cavity through the abdominal wall by a surgeon and is expected to remain there for the long-term. The dialysis solution is then dripped in through the catheter and left in the abdominal cavity for a few hours and then is drained out. In that time, waste products leech from the blood normally flowing through the lining of the abdomen (peritoneum).

There are benefits and complications for each type of dialysis. Not every patient can choose which type he or she would prefer. The treatment decision depends on the patient's illness and their past medical history along with other issues. Usually, the nephrologist (kidney specialist) will have a long discussion with the patient and family to decide what will be the best option available.

Dialysis is life saving. Without it, patients whose kidneys no longer function would die relatively quickly due to electrolyte abnormalities and the buildup of toxins in the blood stream. Patients may live many years with dialysis but other underlying and associated illnesses often are the cause of death.

Kidney transplantation

If kidney failure occurs and is non-reversible, kidney transplantation is an alternative option to dialysis. If the patient is an appropriate candidate, the health care practitioner will contact an organ transplant center to arrange evaluation to see if the patient is suitable for this treatment. If so, the search for a donor begins. Sometimes, family members have compatible tissue types and, if they are willing, may donate a kidney. Otherwise, the patient will be placed on the organ transplant list that is maintained by the United Network of Organ Sharing.

Not all hospitals are capable of performing kidney transplants. The patient may have to travel to undergo their operation. The most successful programs are those that do many transplants every year.

While kidney transplants have become more routine, they still carry some risk. The patient will need to take anti-rejection medications that reduce the ability of the immune system to fight infection. The body can try to reject the kidney or the transplanted kidney may fail to work. As with any operation, there is a risk of bleeding and infection.

Kidney transplants may provide better quality of life than dialysis. After one year, 95% of transplanted kidneys are still functioning and after five years the number is 80%. It seems that the longer a patient is on dialysis, the shorter the life of the transplanted kidney.

If the transplanted kidney fails, the alternative is another kidney transplant or a return to dialysis.

What is the prognosis for someone with kidney failure?

The outlook for kidney failure depends upon the underlying condition that caused it. Kidney function may return to normal, especially if it is due to an acute obstruction and that obstruction is relieved. Other causes of decreased kidney function leading to kidney failure are due to underlying disease and occur slowly over time.

Prevention is the best chance to maintain kidney function, and controlling high blood pressure and diabetes over a lifetime can decrease the potential for progressive kidney damage. Chronic kidney failure may be managed by a primary health care practitioner or a nephrologist to help monitor electrolyte and waste product levels in the bloodstream. Major abnormalities can be life-threatening, and treatment options may be limited to dialysis or transplant.

Kidney Failure At A Glance

• Kidneys are the organs that help filter waste products from the blood. They are also involved in regulating blood pressure, electrolyte balance, and red blood cell production in the body.
  
• There are numerous causes of kidney failure, and treatment of the underlying disease may be the first step in correcting the kidney abnormality.
  
• Some causes of kidney failure are treatable and the kidney function may return to normal. Unfortunately, kidney failure may be progressive in other situations and may be irreversible.
  
• Symptoms of kidney failure are due to the build-up of waste products in the body that may cause weakness, shortness of breath, lethargy, andconfusion. Inability to remove potassium from the bloodstream may lead to abnormal heart rhythms and sudden death. Initially, there may be no symptoms of kidney failure.
  
• The diagnosis of kidney failure usually is made by blood tests measuring BUN, creatinine, and glomerular filtration rate (GFR).
  
• Treatment of the underlying cause of kidney failure may return kidney function to normal. Lifelong efforts to control blood pressure and diabetes may be the best way to prevent chronic kidney disease and its progression to kidney failure. Usually, kidney function gradually decreases over time.
  
• If the kidneys fail completely, the only treatment options available may be dialysis or transplant.

REFERENCE: eMedicine.com. Renal Failure, Acute.
<http://emedicine.medscape.com/article/777845-overview>

Source : http://www.medicinenet.com/kidney_failure/article.htm#tocc