Download File - Wk 1-2

In this system, carbon dioxide (CO2) and water (H2O) is processed and in the intercalated cell of the distal
tubule the result is the excretion of hydrogen ions (H+) into the tubular fluid and the retention of
bicarbonate (HCO3-).
1. Carbon dioxide (CO2) and water (H2O) are catalyzed by carbonic anhydrase (CA) to form carbonic
acid (H2CO3).
2. Carbonic acid (H2CO3) will freely dissociate into hydrogen ions (H+) and bicarbonate (HCO3-).
3. The hydrogen ions (H+) will be excreted via the potassium (K+) counter-transport pump. At the
same time bicarbonate (HCO3-) will be moved into the peritubular fluid and eventually into the
bloodstream using the same potassium (K+) pump as a co-transport system.
The result of this is the retention of the base bicarbonate (HCO3-) and the secretion of hydrogen ions (H+)
into the filtrate. The net effect of this process will work to increase pH and reduce the acidity of the
blood.
Method 3 – Catabolization of Glutamine (Tubules in general)
The third way the kidneys can buffer acidity is by catabolizing glutamine to form a counter-acting base
as well as a product which can “carry” hydrogen ions out of the body.
1. Glutamine is catabolized in the epithelial cells of the tubules to form bicarbonate (HCO3-) and
ammonium (NH3).
2. Bicarbonate (HCO3-) will pass into the peritubular fluid via co-transport with sodium (Na+) and
counter-transport with chloride (Cl-) ions.
3. Ammonia (NH3) in the epithelial cell will then combine with free hydrogen ions (H+) to form
ammonium (NH4+).
4. Ammonium (NH4+) will then be secreted into the tubular fluid via the sodium (Na+) countertransport system.
The end result of this reaction is the net secretion of hydrogen ions (H+) into the urine and the
conservation of bicarbonate (HCO3-).