Biology STAAR (State of Texas Assessments of Academic Readiness) Practice Test

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Study for the Biology STAAR Test. Improve your skills with flashcards and multiple choice questions, each question is accompanied by hints and explanations. Prepare for success!

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What do red blood cells primarily carry in the circulatory system?

Carbon dioxide
Hemoglobin
Oxygen
Nutrients

The correct answer is: Oxygen

Red blood cells primarily carry oxygen in the circulatory system due to the presence of hemoglobin, a specialized protein found within these cells. Hemoglobin binds to oxygen molecules in the lungs and releases them in tissues throughout the body, making oxygen transport vital for cellular respiration and energy production. While hemoglobin itself is indeed crucial for this process, it is the oxygen that is the primary substance being transported to various organs and tissues. In contrast, carbon dioxide is carried from the tissues back to the lungs for exhalation, but this is not the main role of red blood cells. Nutrients, such as glucose, are transported through the bloodstream by plasma and not specifically by red blood cells. Thus, the primary function of red blood cells revolves around the efficient delivery of oxygen to sustain metabolic processes in the body.