Renal tubular acidosis

With renal tubular acidosis, renal refers to the kidney, tubular refers to the main tube-portion of the nephron, and acidosis refers to having too many protons or increased acidity in blood, so renal tubular acidosis or RTA describes increased acidity in the blood because the renal tubules can’t get rid of protons.

The kidneys contain millions of nephrons, each of which has a renal corpuscle, and a renal tubule that ends in a collecting duct.

The renal corpuscle filters large amounts of solutes that go from the blood into the filtrate and eventually the urine, and the renal tubule and collecting duct are responsible for fine tuning the reabsorption and secretion of solutes to adjust the amount that ultimately gets retained by or removed from the body.

Broadly speaking, renal tubular acidosis can develop in either the proximal convoluted tubule, sometimes called just the proximal tubule, or the distal convoluted tubule, or distal tubule, and the nearby collecting duct.

The proximal tubule is lined by brush border cells which have two surfaces. One is the apical surface that faces the tubular lumen and is lined with microvilli, which are tiny little projections that increase the cell’s surface area to help with solute reabsorption.

The other is the basolateral surface, which faces the peritubular capillaries, which run alongside the nephron.

Now - when a molecule of bicarbonate approaches the apical surface of the brush border cell it binds to hydrogen to form carbonic acid.

At that point, an enzyme called carbonic anhydrase type 4 which lurks in tubule among the microvilli like a shark, swims along and splits the carbonic acid into water and carbon dioxide. The overall equation looks like this:

H+ + HCO3- <-> H2CO3 <-> H2O + CO2

The water and carbon dioxide happily diffuse across the membrane into the cells where carbonic anhydrase type 2 facilitates the reverse reaction - combining them to form carbonic acid, which dissolves into bicarbonate and hydrogen.

A sodium bicarbonate cotransporter on the basolateral surface snatches up the bicarbonate and a nearby sodium, and shuttles both into the blood.

Meanwhile, a sodium-hydrogen exchanger on the apical surface, pulls sodium into the cell, while pushing hydrogen back into the tubule. So at the end of the day, there’s a movement of bicarbonate from the tubule to the blood.