In acid-base balance, what is controlled by the respiratory system, and what is regulated by the kidney, and how does the speed of the respiratory system compare to the metabolic system in responding to changes in acid-base balance?

Acid-Base Balance Regulation: Respiratory System and Kidneys

In acid-base balance, the pCO2 is controlled by the respiratory system, and the HCO3- is regulated by the kidney. The respiratory system is faster than the metabolic system to respond to changes in acid-base balance. The correct answer is B.

Physiological Control Mechanisms

Respiratory System Control (pCO2)

• The respiratory system regulates carbon dioxide levels in the blood through changes in ventilation 1
• Carbon dioxide is highly soluble in blood and carried in three forms: bicarbonate (70-85%), dissolved CO2 (5-10%), and bound to hemoglobin (10-20%) 1
• When CO2 levels rise, the respiratory center is stimulated, increasing ventilation to eliminate excess CO2 2
• This respiratory response occurs within seconds to minutes, making it the faster of the two systems 1, 3

Renal System Control (HCO3-)

• The kidneys regulate bicarbonate levels through reabsorption and generation of new bicarbonate 3
• CO2 and water form carbonic acid in the kidneys, which then dissociates into H+ and HCO3- 1
• The kidneys can excrete or retain bicarbonate based on the body's acid-base status 3
• This metabolic compensation occurs more slowly, taking hours to days to fully compensate 3, 4

Speed of Response Comparison

Respiratory Compensation

• Occurs rapidly (within minutes) in response to metabolic acid-base disturbances 2, 3
• Involves increased or decreased ventilation to adjust pCO2 levels 2
• The primary mechanism is through changes in tidal volume rather than respiratory frequency 2
• Can achieve partial compensation quickly but cannot fully correct a metabolic disturbance 3

Metabolic Compensation

• Occurs more slowly (hours to days) in response to respiratory acid-base disturbances 3, 4
• Involves renal adjustments to bicarbonate reabsorption or excretion 3
• The kidneys have a large capacity to excrete or retain bicarbonate but work more gradually 5
• Can achieve more complete compensation for respiratory disturbances given sufficient time 4

Clinical Implications

• In metabolic acidosis, hyperventilation is the expected compensatory mechanism to eliminate excess CO2 and raise blood pH 2
• The absence of expected hyperventilation in a patient with metabolic acidosis may indicate respiratory muscle fatigue or neurological impairment 2
• In respiratory acidosis, renal retention of bicarbonate occurs gradually to compensate 3
• Complete correction of a respiratory pH disorder only occurs with correction of the primary disease process 5, 4

This understanding of acid-base physiology is crucial for interpreting arterial blood gas results and determining whether compensatory mechanisms are functioning appropriately in patients with acid-base disturbances.