Kidney Function, Chronic Kidney Disease, and Its Treatment

This section gives an overview of:

• Kidney structure and function
• Kidney diseases
• Diagnosis and treatment of renal failure
• Renal replacement therapy
• Reimbursement for dialysis treatment

This background can help you to better understand the iron deficiency anemia that frequently accompanies chronic kidney disease. Click on the appropriate link for more information.

Kidney Structure and Function
The kidneys are the main organs of excretion in the body, removing wastes and excess water from the blood that would otherwise harm the body.^1,2 The wastes and excess water removed are excreted as urine.² Two healthy kidneys excrete between 1.5 and 2.5 L of urine daily.³

The kidneys, each about the size of a fist, are located near the center of the back, just below the rib cage.² Each kidney is surrounded by a membrane known as the renal capsule. The kidney is bean-shaped, with 2 main regions: the renal cortex (outer section) and the renal medulla (inner section).¹

Blood enters the kidney through the renal artery and exits through the renal vein.¹ Urine is carried away from each kidney by a ureter, which leads to the urinary bladder.

The filtering of blood by the kidneys takes place in tiny functional units called nephrons.² There are approximately 1 million nephrons in each kidney.¹ In addition to filtration, nephrons perform the functions of reabsorption and secretion.

In the nephron, capillaries (tiny blood vessels) are intertwined with tubules (urine-carrying tubes) that carry away wastes and water.² Blood enters the nephron through the glomerulus, a tuft of capillaries where filtration occurs.¹ During filtration, blood fluid is forced from capillaries into the tubules. As the fluid passes through the tubules, substances needed by the body, including water, sodium, phosphorus, potassium, and glucose, are selectively reabsorbed into the blood by the intertwined capillaries.^1,2 In this way, the kidneys regulate the level of these vital substances in the blood.²

The blood exits the nephron through the renal vein.¹ The fluid remaining in the tubules after reabsorption exits as urine into the collecting duct, which eventually leads to the ureter.

In addition to removing wastes, the kidney releases 3 important substances into the circulation²:

• Erythropoietin, which stimulates erythropoiesis
• Renin, an enzyme that is important in blood pressure regulation
• Calcitriol, an active form of vitamin D, which helps maintain calcium balance

Kidney Diseases
An individual with 2 healthy kidneys has 100% renal function.² A drop in renal function to 20% of normal or lower, due to disease, injury, poisoning, or trauma, may lead to serious health problems and even death.

Kidney function decreases with advancing age. The glomerular filtration rate (GFR), a standard indicator of renal function, declines with age at a rate of about 10 mL/min/1.73 m² for each decade beyond the third decade.⁴

The most common diseases affecting kidney function are diabetes and hypertension. Elevated blood sugar associated with diabetes damages the nephrons (diabetic nephropathy).²

Hypertension can damage small blood vessels in the kidneys, decreasing their ability to filter wastes from the blood² (hypertensive nephropathy).

Glomerulonephritis is injury to the glomeruli that causes protein and blood cells to leak into urine, thereby impairing renal function.⁵

An increased risk for glomerulonephritis is associated with certain disorders, including chronic viral hepatitis, rheumatic fever, and systemic lupus erythematosus. Systemic lupus erythematosus is an autoimmune disease characterized by the production of antibodies that attack parts of cells within one's own body.⁶ This disease can lead to the damage of many tissues and organs, including the kidney.

Polycystic kidney disease is a genetic disorder characterized by the growth of cysts in the kidney. The cysts can replace the kidney mass, decreasing renal function and leading to renal failure.²

Poisoning, trauma, and certain medications can also cause kidney disease.²

Acute renal failure (ARF) is a sudden loss of renal function that may result from accidental injury, excessive blood loss, poisoning, or medications. In some cases, ARF may be reversed. Most kidney diseases destroy nephrons slowly and silently, progressing over a period of years to chronic renal failure.²

End-stage renal disease (ESRD) is total, or nearly total, and permanent renal failure.²

Diagnosis and Treatment of Renal Failure
Early in the course of kidney failure, patients may be asymptomatic. Laboratory tests can reveal excess serum creatinine, excess blood urea nitrogen (BUN), or protein in the urine, all signs of improper kidney function. The creatinine clearance test is a useful measure of kidney function; this value is approximated from a blood sample, using a formula that relates the serum creatinine level to a person's age, weight, and gender. A creatinine clearance determination requires a 24-hour urine collection.⁷

Some early signs of kidney disease are frequent headaches, fatigue, and itching.² Disease progression is associated with increases or decreases in urination frequency, loss of appetite, nausea, and vomiting. Other signs and symptoms include decreased mental concentration, darkening of the skin, and muscle cramps.² Renal failure produces uremia, a buildup of excess water and waste products in the body.^2,3 This may result in swelling of the hands and feet, fatigue, weakness, and high blood pressure. In addition, acid produced during metabolic processes can accumulate in the body, causing metabolic acidosis.² The kidneys fail to excrete phosphate, leading to increased blood phosphate levels that, in turn, cause a decrease in blood calcium levels. Low calcium levels can lead to brittle bones (renal osteodystrophy), and rarely, life-threatening hypocalcemia.

Clinical management of kidney disease includes changes in the diet to limit intake of protein, cholesterol, sodium, phosphate, and potassium; treatment of anemia; and control of diabetes and blood pressure.^2,3

Anemia contributes to the development of cardiac disease, the major cause of death in patients with ESRD. The treatment of anemia can help reduce cardiac complications. Epoetin, now widely used to treat ESRD-related anemia, produces increases in Hb and HCT, with subsequent reductions in anemia symptoms and complications.⁸

Renal Replacement Therapy
When renal failure is severe, and about 90% of renal function is lost, a patient requires a form of renal replacement therapy to survive: a renal transplant or dialysis.⁹ Serum creatinine is used frequently as a monitor of renal function. Table 1 lists the serum creatinine values and clearance rates that correspond to diagnoses of mild, moderate, and severe renal failure.⁴

Table 1: Serum Creatinine Concentrations and Creatine Clearance Rates⁴

A renal transplant is the surgical placement of a kidney from a kidney donor into a patient with kidney failure.³

Dialysis is a process that cleans and filters the blood, removing harmful wastes and excess salt and fluids by passing blood across a semipermeable membrane.^3,10 Wastes from the blood diffuse across the membrane into a cleansing solution (dialysate) and bicarbonate diffuses into the blood to neutralize excess acid. Dialysis can control blood pressure and help maintain a balance of electrolytes, including potassium, sodium, and chloride.

Adapted from National Institute of Diabetes and Digestive and Kidney Diseases.¹⁰

There are 2 types of dialysis: hemodialysis and peritoneal dialysis. The hemodialysis machine is connected to a special filter called a dialyzer.¹⁰ During hemodialysis, blood is carried to the dialyzer, where wastes and excess fluid are removed. The cleansed blood is carried back to the body.

Hemodialysis can be performed at a patient's home or at a dialysis center,¹⁰ a special outpatient care facility where hemodialysis treatment is provided by trained nurses and technicians. Hemodialysis is usually performed 3 times weekly, with each treatment lasting 2 to 4 hours. Patients can read, write, sleep, talk, or watch television during treatment.

Peritoneal dialysis uses the peritoneal membrane, the lining of the abdomen, to remove excess water, wastes, and chemicals from the body.¹⁰ A dialysate passes through the abdomen via a surgically placed catheter. Fluid, wastes, and chemicals pass from capillaries in the peritoneal membrane into the dialysate. After several hours, the waste-carrying dialysate is drained from the abdomen.

There are 3 types of peritoneal dialysis¹⁰:

• Continuous ambulatory peritoneal dialysis (CAPD)
• Continuous cyclic peritoneal dialysis (CCPD)
• Intermittent peritoneal dialysis (IPD)

Continuous ambulatory peritoneal dialysis is the most common type.¹⁰ The dialysate passes from a plastic bag through a catheter and into the abdomen. No machine is necessary. After 4 to 6 hours, the solution is drained back into the bag and replaced with fresh solution. The solution is usually changed 4 times a day.

Adapted from National Institute of Diabetes and Digestive and Kidney Diseases.¹⁰

Continuous cyclic peritoneal dialysis is like CAPD, except that a machine is used to fill and drain the dialysate through the catheter.¹⁰ This procedure is performed nightly, for 10 to 12 hours, during sleep. Continuous cyclic peritoneal dialysis requires the help of a partner.

Intermittent peritoneal dialysis uses a machine, much like CCPD, to add and drain the dialysate.¹⁰ Each session may last up to 24 hours. Intermittent peritoneal dialysis treatments are usually performed in a hospital for a total of 36 to 42 hours weekly.

Hemodialysis and peritoneal dialysis are treatments that take over the function of the failed kidneys but do not cure ESRD.¹⁰ Problems frequently associated with long-term dialysis include bone disease, high blood pressure, nerve damage, and anemia.

Reimbursement for Dialysis Treatment
Treatment for ESRD is expensive, but the federal government helps pay for much of the cost.¹⁰ Private health insurance and state programs often pay part of the cost as well.

Medicare pays for 80% of the cost of dialysis therapy, regardless of a patient's age, provided the patient has worked long enough to be insured under Social Security (or is the child of a parent who has) or is already receiving Social Security benefits. Private insurance often pays for the entire cost of dialysis treatment or the 20% not covered by Medicare.

Medicaid is a state program that helps cover the medical expenses of individuals whose incomes are below a certain level. Medicaid coverage varies by state; in some states, it covers the portion of dialysis treatment not covered by Medicare.

References

1. Alcamo IE. The Princeton Review Anatomy Coloring Workbook. New York, NY: Random House; 1997:248-255.
2. National Institute of Diabetes and Digestive and Kidney Diseases. Your kidneys and how they work. Available at: http://www.niddk.nih.gov/health/kidney/pubs/yourkids/index.htm. Accessed May 10, 2000.
3. Kidney Dialysis Foundation. About kidney disease & dialysis. Available at: http://www.kdf.org/sg/aboutkidney-main.html. Accessed May 15, 2000.
4. Kobrin S, Aradhye S. Preventing progression and complications of renal disease. Hosp Med. 1997;33:11-40.
5. HealthAnswers.com,Inc.Primaryglomerulonephritis.Availableat:http://www.healthanswers.com/centers/…/overview.asp?id=urinary+system&filename=2448.htm. Accessed August 27, 2000.
6. HealthAnswers.com, Inc. Systemic lupus erythematosus. Available at: http://www.healthanswers.com/centers/body/overview…/+nervous+system&filename=1787.htm. Accessed August 27, 2000.
7. Berkow R, Beers MH, Fletcher A, eds. The Merck Manual of Medical Information: Home Edition. Whitehouse Station, NJ: Merck Research Laboratories; 1997.
8. Walker R. General management of end stage renal disease. BMJ. 1997;315:1429-1432.
9. King B. WebMDHealth: Preserving renal function: techniques for averting kidney damage. Available at: http://my.webmd.com/content/dmk/dmk_article_5461603. Accessed August 26, 2000.
10. National Institute of Diabetes and Digestive and Kidney Diseases. End-stage renal disease: choosing a treatment that's right for you. Available at: http://www.niddk.nih.gov/health/kidney/pubs/esrd/esrd.htm. Accessed May 10, 2000.