In human physiology, what is the primary job of red blood cells (erythrocytes)?

• A. Transport oxygen (and some carbon dioxide) between the lungs and body tissues using hemoglobin
• B. Provide immune defense by engulfing pathogens and presenting antigens
• C. Initiate blood clotting by forming a platelet plug at sites of vessel injury
• D. Regulate blood pressure primarily by secreting hormones that control vessel diameter

Answer: (A)

Verified step by step guidance

1. Identify the main function of red blood cells (erythrocytes) by recalling their role in the circulatory system.
2. Understand that red blood cells contain hemoglobin, a protein specialized in binding oxygen molecules.
3. Recognize that the primary job of erythrocytes is to transport oxygen from the lungs to body tissues and carry some carbon dioxide back to the lungs for exhalation.
4. Differentiate this function from other blood components: white blood cells provide immune defense, platelets are involved in clotting, and certain cells regulate blood pressure through hormone secretion.
5. Conclude that the correct answer focuses on oxygen and carbon dioxide transport via hemoglobin within red blood cells.