THE USMLE TIMES - #90
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Welcome to issue #90 of The USMLE Times… This is the 90th straight week we’ve posted our USMLE Times Newsletter! Thank you to all of our supporters—new and old!
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Here’s what’s on tap for issue #90 of the USMLE Times:
Question of the Week (RBC Physiology)
This week’s video training (Answer USMLE Qs With Speed)
Links to this recent tips, tricks, & questions
Question deep-dive & breakdown
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Let’s dive in!
A 42-year-old woman presents to the emergency department with a 3-day history of fever, cough, and difficulty breathing. Her vital signs are as follows: Heart rate 110/minute, respiratory rate 30/minute, blood pressure 125/80 mmHg, temperature 101.5°F (38.6°C), and oxygen saturation 94% on room air. Arterial blood gas analysis reveals a PaCO2 of 28 mmHg and PaO2 of 68 mmHg. The patient is diagnosed with pneumonia and admitted for treatment. Which of the following best describes how C02 is eliminated from the body?
A. CO2 diffuses into erythrocytes and directly combines with hemoglobin
B. CO2 diffuses into erythrocytes and reacts with water to form carbonic acid
C. CO2 diffuses into erythrocytes and combines with deoxygenated hemoglobin to form carbaminohemoglobin
D. CO2 diffuses into erythrocytes and is converted to bicarbonate by the action of carbonic anhydrase
E. CO2 diffuses into erythrocytes and combines with oxygenated hemoglobin to form carbaminohemoglobin
The answer & question breakdown is at the bottom of the post.
🔗 LINKS TO RECENT POSTS
Q&A Tuesday (Get The Residency “It Factor”) - READ HERE
USMLE Practice Question #47 - TRY IT HERE
USMLE Practice Question #46 - TRY IT HERE
USMLE Practice Question #45 - TRY IT HERE
USMLE Times Issue #89 - READ HERE
ANSWER + QUESTION BREAKDOWN
The Mental Model for this week’s question:
Step 1. Identify the question’s goal (Understand how C02 is eliminated from the body - this is a straightforward 1st-order question).
Step 2. There is no need to read the vignette; it won’t give you any additional information (if you find yourself wanting to read it, it’s because you’re unsure of RBC physiology, which means you should review this information).
Step 3. Ask yourself how C02 is eliminated from the body based on your knowledge.
Step 4. Look for the answer choice that accurately reflects your answer.
GENERAL ANALYSIS
The question involves understanding the mechanism by which carbon dioxide (CO2) is transported and eliminated from the body, particularly in the context of a patient with pneumonia. The focus is on the physiological process of CO2 elimination, which is crucial for maintaining acid-base balance and proper respiratory function.
ANSWER CHOICES:
ANSWER CHOICE A: CO2 diffuses into erythrocytes and directly combines with hemoglobin
CO2 can bind to hemoglobin, but this is not the primary method of CO2 transport.
ANSWER CHOICE B: CO2 diffuses into erythrocytes and reacts with water to form carbonic acid
This reaction occurs in erythrocytes and is catalyzed by carbonic anhydrase.
ANSWER CHOICE C: CO2 diffuses into erythrocytes and combines with deoxygenated hemoglobin to form carbaminohemoglobin
CO2 can bind to deoxygenated hemoglobin to form carbaminohemoglobin, but this is not the primary method of CO2 transport.
ANSWER CHOICE D: CO2 diffuses into erythrocytes and is converted to bicarbonate by the action of carbonic anhydrase
Inside erythrocytes, CO2 reacts with water to form carbonic acid, which dissociates into bicarbonate and hydrogen ions. This reaction is catalyzed by carbonic anhydrase.
ANSWER CHOICE E: CO2 diffuses into erythrocytes and combines with oxygenated hemoglobin to form carbaminohemoglobin
CO2 binding to hemoglobin occurs more readily with deoxygenated hemoglobin.
THE VERDICT…
The primary mechanism by which carbon dioxide is transported in the blood involves its conversion to bicarbonate. When CO2 diffuses into erythrocytes, it reacts with water to form carbonic acid, a reaction catalyzed by carbonic anhydrase.
Carbonic acid then dissociates into bicarbonate ions (HCO3−) and hydrogen ions (H+). The bicarbonate ions are transported in the plasma and serve as the main form of CO2 transport in the blood.
When blood reaches the lungs, the process reverses, allowing CO2 to be released and exhaled. This mechanism is crucial for maintaining acid-base balance and efficient gas exchange in the body
FINAL ANSWER: D: CO2 diffuses into erythrocytes and is converted to bicarbonate by the action of carbonic anhydrase
That’s it for issue #90 of The USMLE Times!
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