Smaill: Acid/Base Physiology 2

Question 1

What is respiratory acidosis?

Answer 1

This is a condition in which the concentration of P_CO2 is increased due to an inability to excrete it as efficiently due to impaired ventilation.

Question 2

What is respiratory alkalosis?

Answer 2

This is a condition in which the concentration of PCO2 is decreased due to an increased ventilation.

Question 3

What causes metabolic acidosis?

Answer 3

Excess nonvolatile acid production due to diabetic ketoacidosis or accumulation of lactic acid in circulatory shock, for instance, failure to excrete sufficient acid in renal failure or loss of bicarbonate (in diarrhoea).

Question 4

What are the causes of metabolic alkalosis?

Answer 4

This condition can occur through the addition of nonvolatile alkali to the body - such as through the ingestion of antacids - or through the loss of nonvolatile acids due to vomiting or nasogastric suction.

Question 5

What causes respiratory acidosis?

Answer 5

This occurs due to inadequate alveolar gas exchange relative to carbon dioxide production.

• Inadequate ventilation - Depression of respiratory centres.
• Impaired Gas Diffusion - COPD or Pulmonary Oedema.

Question 6

What causes respiratory alkalosis?

Answer 6

Due to a result of excess alveolar gas exchange relative to CO₂ production. Hyperventilation may be due to endogenous or drug-induced stimulation of respiratory centres. It may also occur due to anxiety or fear.

Question 7

What is a common example of a mixed disorder?

Answer 7

In heart failure, respiratory acidosis occurs due to impaired gas diffusion and reduced cardiac output leads to reduced systemic oxygen delivery resulting in anaerobic metabolism and lactic acidosis. That is respiratory acidosis in combination with metabolic acidosis.

Question 8

How is respiratory rate regulated?

Answer 8

Arterial P_CO2 and pH are sensed by chemoreceptors in the brain and aortic/carotid bodies. When the P_CO2 increases - or the pH decreases - it stimulates ventilatory drive and vice versa.

• Hypoxia also acts as a potent stimulus for the ventilatory drive - This P_O2 is sensed by peripheral chemoreceptors (aortic and carotid bodies).

Question 9

What is the kidneys role in acid/base regulation?

Answer 9

The kidney is involved in the reabsorption of all HCO₃- filtered by the kidney and in the regeneration of all HCO₃- lost in the buffering of nonvolatile acids. Also, has roles in the removal of fixed acids incorporated into non-bicarbonate buffer systems.

Question 10

Where in the kidney is bicarbonate filtered?

Answer 10

• Bicarbonate is freely filtered by the renal glomeruli.
• 80 - 90% is reabsorbed in the proximal convoluted tubules.
• 10-15% recovered in the Loop of Henle and Distal Tubule.

Question 11

How is bicarbonate filtered?

Answer 11

Filtered bicarbonate is exchanged for hydrogen ions in a process that involves the vector transport of sodium ions from the tubular lumen into the peritubular space.

Question 12

What are the steps involved in the renal excretion of H⁺ and conservation of HCO₃^- ?

Answer 12

1. Bicarbonate combines with secreted hydrogen ions in the renal tubule forming carbonic acid.
2. Carbonic acid then dissociates to form CO₂ and H₂O - This reaction is catalysed by carbon anhydrase present in the brush border of the renal tubular cells.
3. CO₂ crosses the tubular cell border down its concentration gradient.
4. Inside proximal tubule, CO₂ recombines with H₂O to form carbonic acid - Again under the influence of carbon anhydrase.
5. Carbonic Acid disassociates to bicarbonate and hydrogen ions.
6. Bicarbonate transported into the bloodstream with sodium.
7. H⁺ passes back into tubular space in exchange fr sodium.

Question 13

What are the processes involved in the renal excretion of H⁺ and regeneration of HCO₃^-?

Answer 13

Regeneration of the bicarbonate lost as a result of nonvolatile acid production occurs in proximal, distal tubules and collecting ducts and is associated with the active transport of hydrogen ions into the tubular lumen, where they combine with phosphate ions and ammonia.

• HCO₃^- is moved into the peritubular space from the distal nephron in exchange for Cl^- .

Question 14

What are the limitations of HCO₃^- regeneration?

Answer 14

• Depletion of the phosphate ion stores.
• Depletion of the ammonia stores - Ammonia is produced in the kidney from glutamine but this requires several days.

Question 15

What regulates HCO₃^- reabsorption?

Answer 15

• In acidosis, secretion of H⁺ by the nephron is increased and there is near complete reabsorption of HCO₃^- .
• In alkalosis, secretion of H⁺ by the nephron is decreased and there is a reduction in the reabsorption of HCO₃^- .

Question 16

What is bicarbonate reabsorption in the kidney related to?

Answer 16

• Directly related to P_aCO₂
• Inversely related to plasma levels of plasma K+ and Cl-
• Increased by increased plasma levels of adrenal corticosteroids - for instance in Cushings Disease.

Question 17

What happens in response to a sustained respiratory alkalosis?

Answer 17

1. Hyperventilation reduces P_CO2 and raises pH - As bicarbonate reabsorption is directly related to P_CO2 this reabsorption reduces.
2. Reduced renal net acid excretion also acts to restore pH and acts reduce HCO₃^- .

Question 18

What occurs in response to acute metabolic acidosis?

Answer 18

1. Metabolic acidosis causes a fall in HCO₃^- concentrations and therefore a reduction in the pH.
2. This drives an increase in the respiratory drive and alveolar ventilation.
3. This leads to a fall in the P_CO2.
4. This leads to a return in pH towards normal.

Question 19

What are the time periods for each buffer/compensatory action to reach full effect?

Answer 19

• Distribution and Buffering in ECF = 1-2 hours
• Cellular Buffering Processes = 8 hours
• Respiratory Compensation = 14 hours
• Renal Base Excretion = 48 hours - Faster than renal acid excretion as you only need to turn off the reabsorption of bicarbonate.
• Renal Acid Excretion = 72 hours - Limited by the stores of ammonia and phosphate.

Question 20

Match the letter to the number column they relate to…

Answer 20

• A = II
• B = III
• C = I
• D = V
• E = IV

Question 21

What acid-base disturbance do these values correspond to?

Answer 21

Acute Respiratory Acidosis