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Nephrology

The word nephrology is Greek for “the study of the kidney.” The word kidney is synonymous with “renal” as well as “nephro” and the three terms are commonly interchanged. Nephrology is the branch of internal medicine that focuses on the structure and function of the kidneys—both healthy and sick. A nephrologist is a physician who diagnoses and treats kidney diseases.

The Function of the Kidneys

Normally, a person is born with two kidneys. Each kidney is bean-shaped and about the size of an adult's fist—about 4 inches long and 2.5 inches wide. The kidneys are located directly below the rib cage, on either side of the vertebral column. The tasks of the kidneys are numerous. Foremost, these organs are responsible for regulating the composition of the body's blood. This includes keeping the fluid level in the body constant, regulating the acidity level, balancing the electrolyte concentrations, and removing waste products from the blood. The kidneys receive the body's blood from the renal artery. The renal artery delivers the blood to specific parts of the kidney called nephrons. The nephron is made of the glomerulus and convoluted tubule. The glomerulus is composed of the capillaries, which are tiny blood vessels involved in the filtration of blood. The convoluted tubule is a tiny tube that collects the waste products from the filtered blood. The filtered blood, which includes sodium, phosphorous, and potassium, exits the kidney via the renal vein and is returned into circulation. The waste products, consisting of urea, toxins, and surplus drugs, exit the kidney and travel to the bladder via the ureter. These waste products are turned into urine, which is then stored in the bladder until it is released through the urethra. This process is continuous. Without the filtering action of the kidneys a person would die within eight days.

In addition, the kidneys are responsible for producing three important hormones: erythropoietin, renin, and calcitriol. The kidneys produce erythropoietin if low oxygen levels are detected in the blood. Erythropoetin then stimulates the production of red blood cells in the bone marrow. Red blood cells are responsible for carrying oxygen in the blood and, therefore, an increase in erythropoietin will ultimately result in an increase in oxygen. Renin is the hormone produced when the kidneys detect low blood pressure, low volume of blood, or decreased salt levels in the body. Production of renin results in the constriction of blood vessels—which increases blood pressure. Renin also stimulates the adrenal gland to make aldoste-rone—which increases salt concentration in the body. An increase in salt concentration causes more water to be absorbed as well as stimulates the brain to make a person feel thirsty. The final hormone produced by the kidneys is calcitriol. Calcitriol is the active form of vitamin D—which is responsible for calcium reabsorption in the intestinal tract and the bone.

What Medical Tests Detect Kidney Disease?

If kidney disease is suspected, a patient's doctor will perform a blood pressure measurement, check the urine for protein, and take a sample of blood to measure the serum creatinine and blood urea nitrogen (BUN). A blood pressure check is necessary because high blood pressure can cause kidney disease—as well as heart disease and stroke. Proteinuria, protein in the urine, may represent kidney dysfunction because a normal kidney separates protein from waste products. Checking the serum creatinine is an essential part of the exam because it allows for the further calculation of the glomerular filtration rate (GFR). GFR is a calculation in which the result can be used to evaluate kidney function. Lastly, a BUN may be performed. The blood in the body contains protein, which is delivered to cells throughout the body. These cells use the protein as energy and in the process create the byproduct, urea. Urea is then carried to the kidneys where it is filtered out into urine. If a BUN level is high it indicates that the kidneys may not be efficiently filtering out the urea—another sign of kidney disease.

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