Acid Base Balance 1&2

Question 1

What is the relationship between [HCO3-] (bicarbonate) and Pco2 which determines the Ph?

Answer 1

1. The Ph is controlled by the volume of bicarbonate and the partial pressure of CO2.
2. The volume of bicarbonate is controlled by the amount of CO2 dissolved in the plasma, which itself depends on the solubility of CO2 and the partial pressure of CO2.

Question 2

What are the mechanisms of the kidney which maintain bicarbonate concentration?

Answer 2

1. The kidney can eliminate H+ to regulate the bicarbonate conc.
2. The kidneys can also reabsorb bicarbonate from the urine (dependent on H+ secretion)
3. The kidneys can generate new bicarbonate (dependent on H+ secretion)

Question 3

What is respiratory acidosis and what are some possible causes?

Answer 3

1. A drop in Ph caused by a respiratory change
2. This can be caused by:
   
   1. Reduced ventilation caused by:
      
      1. Acute – drugs which depress respiratory centres
      2. Chronic – Lung diseases such as bronchitis, emphysema, asthma.
3. The bodies response to respiratory acidosis is to increase bicarbonate concentration.
   
   1. The increased CO2 partial pressure leads to increased secretion of H+ and increased bicarbonate

Question 4

What is respiratory alkalosis and what are the possible causes?

Answer 4

1. A fall in PCO2 results in an increase in Ph.
2. This is caused by increased ventilation and CO2 blow-off
3. Medical causes:
   
   1. Acute – voluntary hyperventilation, aspirin, ascent to high altitude
   2. Chronic – long term resistance at altitude, decreased PO2 stimulates peripheral chemoreceptors to increase ventilation
4. In response to respiratory alkalosis, bicarbonate concentration should be decreased
5. The decrease in bicarbonate is controlled by the fact there are less H+ ions (alkaline conditions) therefore less bicarbonate is reabsorbed

Question 5

What is metabolic acidosis and what are the possible causes?

Answer 5

1. An acidosis of metabolic origin which must be due to a decrease in bicarbonate concentration.
2. Metabolic acidosis is caused by:
   
   1. Increased H+ production
   2. Failure to excrete H+ as seen in renal failure
   3. Loss of bicarbonate in diarrhoea

Question 6

What is metabolic alkalosis and what are the possible causes?

Answer 6

1. An alkalosis of metabolic origin which must be due to an increase in bicarbonate concentration.
2. Metabolic alkalosis is caused by:
   
   1. Over secretion of H+ by the kidneys, caused by aldosterone excess, or excess liquorice ingestion
   2. H+ loss by vomiting out gastric contents.
   3. Administration of too much bicarbonate can cause problems in patients with impaired renal function.

Question 7

What mechanisms are used for the secretion of organic acids such as PAH?

Answer 7

1. PAH (Para-amino-hippuric acid) is freely filtered at the glomerulus
2. The remaining PAH in the plasma is then actively secreted into the tubule so that > 90% plasma is cleared of PAH in one pass of the kidney
3. The clearance of PAH therefore is a measure of all the plasma flowing through the kidneys in any given time (660mls/min).