Renal tubular defects: Pathology review

In the Emergency Department, two people came in. One of them is 40 year old Sarah, who came in with rapid, shallow breathing and tachycardia. The other one is 35 year old Alfred, who came in with slow and shallow breathing. An arterial blood gas was taken, along with electrolytes. Results showed that Sarah had low blood pH, bicarbonate and pCO2 levels and her potassium level was also low. Alfred had a high pH, bicarbonate and pCO2 levels, but his potassium level was low. Based on these results, Sarah diagnosed with metabolic acidosis, while Alfred had metabolic alkalosis. Further investigations were done, such as electrolytes and urinalysis. This showed that Sarah had hypophosphatemia and urinalysis showed phosphaturia, aminoaciduria and glucosuria, while Fred had hypokalemia and urinalysis showed hypercalciuria.

Now, the results from the labs of both individuals point towards some kind of a renal tubular defect that’s causing acid-base disorders. Before talking specifics,let’s remember the physiology of the renal tubules. The proximal convoluted tubule or PCT reabsorbs bicarbonate, all glucose, uric acid and amino acids. Apart from this, it also reabsorbs water, potassium, chloride, phosphate and most of the sodium, as well as most of the calcium. PCT also secretes hydrogen and phosphate into the urine. The thin descending loop of Henle reabsorbs water and that’s pretty much it. The thick ascending loop of Henle or TAL on the other hand reabsorbs potassium, chloride and sodium. Now, in order to reabsorb sodium, potassium and chloride, there’s a Na/K/Cl cotransporter or NKCC2, that’s only found in the kidney and its role is to snatch these ions from the urine and reabsorb them. Apart from this, TAL can also reabsorb calcium and most of the magnesium, but doesn’t reabsorb water. Now, the distal convoluted tubule or DCT reabsorbs sodium and chloride, through a sodium-chloride cotransporter, as well as calcium and some magnesium. Again, it doesn’t reabsorb water. Finally, the collecting tubule is regulated by aldosterone and reabsorbs sodium in exchange for potassium and hydrogen. Now, aldosterone acts on mineralocorticoid receptors and in the principal cells of the collecting tubule, leads to potassium secretion, while in the 𝛼-intercalated cells, leads to hydrogen secretion.