Which situation most directly leads to respiratory acidosis?

• A. Hypoventilation causing retention of carbon dioxide (increased arterial $P_{aC{O}_2}$)
• B. Hyperventilation causing excessive carbon dioxide loss (decreased arterial $P_{aC{O}_2}$)
• C. Loss of gastric acid due to prolonged vomiting leading to increased blood pH
• D. Increased renal excretion of hydrogen ions leading to decreased blood acidity

Answer: (A)

Verified step by step guidance

1. Understand that respiratory acidosis occurs when there is an accumulation of carbon dioxide (CO\_2) in the blood, which leads to an increase in hydrogen ion concentration and a decrease in blood pH.
2. Recognize that CO\_2 is normally expelled from the body through ventilation in the lungs; therefore, any condition that reduces ventilation (hypoventilation) will cause CO\_2 to build up in the arterial blood.
3. Identify that hypoventilation leads to increased arterial partial pressure of CO\_2, denoted as \(P\_{a}CO\_2\), which directly causes respiratory acidosis.
4. Contrast this with hyperventilation, which causes excessive loss of CO\_2, leading to respiratory alkalosis (not acidosis), and other conditions like loss of gastric acid or increased renal excretion of hydrogen ions, which affect metabolic acid-base balance rather than respiratory.
5. Conclude that the situation most directly leading to respiratory acidosis is hypoventilation causing retention of CO\_2 and increased arterial \(P\_{a}CO\_2\).