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Where in the tubule does resorption of bicarbonate happen?





Collecting duct


How is bicarbonate resorption important in controlling acid base balance in the body?

Bicarbonate resorption determines the acidity or alkalinity of the blood


Bicarbonate is freely filtered by the glomerulus


Needs to be reabsorbed


Is bicarbonate resorption an active process?

Yes - uses a lot of energy


What is the normal concentration of bicarbonate in the urine?

40 mmol/L


What causes proximal renal tubular acidosis?

Disease in PCT


Means there is innapropriate reabsorption of bicarbonate ions


Bicarbonate will be present in the blood at low concentrations


What causes acute respiratory acidosis?

Decreased CNS stimulation - sedative drugs

Neuromuscular ventilatory failure


Structural/ mechanical ventilatory failure


Airway obstruction - asthma and inhaled foreign object


What causes chronic respiratory acidosis?

Decreased CNS stimulation - cerebral disease


Neuromuscular ventilatory failure - ALS, DMD


Structural/ mechanical ventilatory failure - obesity, thoracic deformities


Chronic airway obstruction - COPD


What causes acute respiratory alkalosis?

Increased CNS stimulation - anxiety or drugs


Subarachnoid haemorrhage


Mechanical over-ventialtion of an intubated patient


What causes chronic respiratory alkalosis?

Increased CNS stimulation - stroke


Increased hypoxic drive - high altitude


How does the PCT reabsorb bicarbonate?

Apical surface - NHE transporter (sodium hydrogen exchanger)

Sodium enters and hydrogen leaves the cell

Hydrogen is pumped out and combines with bicarbonate ions at the brush border

Carbon dioxide is formed from the carbonic acid produced


Carbon dioxide formed enters the cell


Carbon dioxide combines with the water to form protons and bicarbonate ions


Protons are transported out via NHE


Bicarbonate ions are reabsorbed into the basolateral membrane


What catalyses the breakdown of carbonic acid into carbon dioxide and water?

Carbonic anhydrase


What powers the sodium hydrogen exchanger (NHE)?

Sodium potassium ATPase


Sodium enters down its concentration gradient


How does the blood maintain its acid base balance?

Via the intercalated cells


What are the two cells that form the collecting duct called?

The principal cell

The intercalated cell


What is the role of the principal cell in the collecting ducts?

To regulate salt and water concentration


What is the role of intercalated cells?

To maintain acid base balance


How does the alpha intercalated cell work?

Carbon dioxide inside the cell is hydrated by carbonic anhydrase to form protons and bicarbonate ions


Protons are pumped out in an energy-dependent manner by ATPase into the urine


Bicarbonate ions are pumped into the blood via anion exchanger 1


What does the anion exchanger 1 do?

Exchanges bicarbonate inside the cell for chlorine inside the blood


What is distal renal tubular acidosis caused by?

Pathology to any of the two transporters in the alpha intercalated cells


Draw process bicarbonate reabsorption in PCT 


Draw process of bicarbonate reabsorption in the collecting duct