A Level Biology Practice Exam

What is the primary structural feature of the atria in the heart?

1. Thick muscular walls

2. Thin-walled chambers

3. Valves that prevent backflow

4. Chambers solely for oxygenated blood

The correct answer is: Thin-walled chambers

The primary structural feature of the atria in the heart is that they have thin-walled chambers. This design is essential because the atria are responsible for receiving blood returning to the heart from the body and the lungs. Since they only need to accommodate blood flow and do not generate significant pressure like the ventricles, which pump blood out of the heart and thus require thicker, more muscular walls, their thin walls allow them to easily expand and fill with blood. This structure enables efficient filling of the atria before blood is pushed into the ventricles, ensuring a continuous circulation of blood. As the atria contract, they push the blood into the ventricles without needing the muscular strength that thicker walls would provide. The thin-walled structure is a practical adaptation for their role in the circulatory system.