Saturday, January 2, 2010

Renal Tubular Acidosis

Renal tubular acidosis (RTA) is a renal disease which involves an accumulation of acid in the body because the kidneys can not appropriately excrete acids into the urine, causing a person’s blood to remain too acidic. When blood is filtered by the kidney, the filtrate passes through the tubules of the nephron, allowing for exchange of salts, acid equivalents, and other solutes before it drains into the bladder as urine. The metabolic acidosis that results from renal tubular acidosis may be caused either by failure to recover sufficient (alkaline) bicarbonate ions from the filtrate in the proximal tubule, or by insufficient secretion of (acid) hydrogen ions into the latter portions of the nephron (distal tubule).

Although a metabolic acidosis also occurs in those with renal insufficiency, the term RTA is reserved for individuals with poor urinary acidification in otherwise well-functioning kidneys. There are several different types of renal tubular acidosis, which all have different syndromes and different causes.

The process of breaking down food into energy generates acids. The kidneys eliminate these acids from the blood, excreting them in the urine. This function is predominantly performed by the kidney tubules. In renal tubular acidosis, the ability of the kidneys to excrete acids is partially impaired, and acid levels build up in the blood (metabolic acidosis). The balance of electrolytes is also affected. Renal tubular acidosis usually leads to these problems: calcium deposits in the kidneys, which can lead to kidney stones; low or high potassium levels in the blood; painful softening and bending of the bones; dehydration.

Healthy kidneys keep acid-base balance by excreting acids into the urine and returning bicarbonate—an alkaline, or base, substance—to the blood. This reclaimed bicarbonate neutralizes much of the acid that is created when food is broken down in the body. The movement of substances like bicarbonate between the blood and structures in the kidneys is called transport.

Distal renal tubular acidosis (dRTA) is the most commun form of RTA. Distal RTA is characterized by a failure of acid secretion by the alpha intercalated cells of the cortical collecting duct of the distal nephron. This failure of acid secretion may be due to a number of causes, and it leads to an inability to acidify the urine to a pH of less than 5.3.

Renal tubular acidosis treatment consists of a correction of the acidemia with oral sodium bicarbonate or sodium citrate. This will correct the acidemia and reverse bone demineralisation. Hypokalemia and urinary stone formation and nephrocalcinosis can be treated with potassium citrate tablets which not only replace potassium but also inhibit calcium excretion and thus do not exacerbate stone disease as sodium bicarbonate or citrate may do.