What does hemoglobin pick up once it has given up all its oxygen molecules?

• A. Nitrogen
• B. Glucose
• C. Calcium ions
• D. Carbon dioxide

Answer: (D)

Verified step by step guidance

1. Understand the role of hemoglobin in the body: Hemoglobin is a protein found in red blood cells that is responsible for transporting oxygen from the lungs to the tissues and returning carbon dioxide from the tissues back to the lungs.
2. Recognize the process of gas exchange: In the lungs, hemoglobin binds to oxygen molecules. As blood circulates through the body, hemoglobin releases oxygen to the tissues.
3. Identify what happens after oxygen release: Once hemoglobin has released its oxygen molecules, it is able to bind to carbon dioxide, which is a waste product of cellular metabolism.
4. Consider the transport of carbon dioxide: Hemoglobin carries carbon dioxide back to the lungs, where it is expelled from the body during exhalation.
5. Conclude the process: The primary function of hemoglobin after releasing oxygen is to pick up carbon dioxide, not nitrogen, glucose, or calcium ions.