If a molecule of CO2 released into the blood in your left toe is exhaled from your nose, it must pass through all of the following except a. the pulmonary vein. b. the trachea. c. the right atrium. d. the right ventricle.

Verified step by step guidance

1. Identify the path of blood flow and gas exchange in the body: Blood carrying CO2 travels from the body tissues to the heart, then to the lungs for gas exchange, and back to the heart to be pumped to the rest of the body.
2. Understand the role of the heart chambers and major vessels: The right atrium and right ventricle are involved in pumping deoxygenated blood (high in CO2) from the body to the lungs. After gas exchange in the lungs, oxygenated blood (low in CO2) returns to the left side of the heart.
3. Recognize the function of the pulmonary vein: The pulmonary vein carries oxygenated blood from the lungs back to the left side of the heart, specifically to the left atrium. It does not carry high CO2 blood.
4. Analyze the options given in the question: Since the pulmonary vein carries only oxygenated blood from the lungs to the heart, it does not transport CO2-rich blood from the body tissues.
5. Conclude the correct answer: The molecule of CO2 released from your left toe would not pass through the pulmonary vein on its way to be exhaled from your nose. Therefore, the correct answer is 'a. the pulmonary vein.'