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Kidney anatomy
Kidney anatomy
Kidney - blood and urine flow
Kidney - blood and urine flow
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Renal tubular acidosis - proximal

Definition:
A condition where the body is in an acidic state caused by abnormal reabsorption of bicarbonate in the proximal tubule of the kidney.

Alternative Names:
Proximal renal tubular acidosis; Type II RTA; RTA - proximal; Renal tubular acidosis type II

Causes, incidence, and risk factors:
NORMAL ACID-BASE REGULATION
Acid-base balance is defined by the concentration of hydrogen ions (the pH). The normal pH of the body is very slightly alkaline. Acidic substances in the body include carbon dioxide and molecules containing hydrogen ions. These are buffered (counteracted) by alkaline substances, primarily bicarbonate, with some buffering by phosphate, proteins, hemoglobin, and other substances. Increased carbon dioxide or decreased bicarbonate levels can create an acidic state in the body, called acidosis. Decreased carbon dioxide or increased bicarbonate levels can create an excess alkaline state, called alkalosis. If the defect is caused by changes in carbon dioxide levels, it is a respiratory acidosis or respiratory alkalosis. If the defect is caused by changes in bicarbonate levels, it is a metabolic acidosis or metabolic alkalosis.

The respiratory system regulates the levels of carbon dioxide through changes in the breathing rate. Carbon dioxide is lost with faster breathing and increased when breathing slows. This provides fast but temporary regulation of body pH.

The major, long-term regulation of body pH occurs in the kidneys, which excrete acids, and excrete or create bicarbonate for use in the body.

PROXIMAL RENAL TUBULAR ACIDOSIS
Proximal renal tubular acidosis (Type II RTA) is a disorder caused by a partial defect in the secretion of hydrogen ions in the proximalrenal tubule. This causes a reduction in the reabsorption of bicarbonate from the tubule back into the bloodstream. Renal tubular acidosis is one cause of metabolic acidosis.

Type II RTA is an uncommon but well-known form of renal tubular acidosis. It is less common than classical (Type I) RTA. It most commonly occurs during infancy, and spontaneous resolution may occur. The exact cause of the defect varies, including inherited and acquired conditions.

Renal tubular acidosis causes disorders of the body related to the loss of bicarbonate and the inability to excrete hydrogen. The body attempts to maintain an electro-chemical balance between positively charged and negatively charged molecules. If the excretion of a molecule is abnormal, the excretion of other molecules may become abnormal in an attempt to maintain a balance. Type II RTA causes the loss of bicarbonate and is also associated with the loss of glucose and amino acids in the urine. There is excessive loss of phosphate and calcium in the urine. There is a marked loss of potassium in the urine. The body may also lose fluid because water accompanies the excretion of these molecules.

The acidic condition of the body causes calcium to dissolve from the bones. Calcium accumulates in the bloodstream and excess serum calcium is excreted by the kidneys, causing a loss of total body calcium and resulting in osteomalacia or rickets, impaired growth of children, skeletal deformities, and muscle weakness. Unlike type I RTA, there is no increased incidence of kidney stones and nephrocalcinosis associated with type II RTA.

Review Date: 12/7/2001
Reviewed By: Andrew Koren, M.D., Department of Nephrology, NYU-Mount Sinai Medical Center, New York, NY. Review provided by VeriMed Healthcare Network.

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