A Level Biology Practice Exam

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What defines a single circulatory system?

Blood flows only through arteries
Blood travels from the heart to the body and back to the heart
Blood circulates through two distinct circuits
Blood flows solely within the heart

The correct answer is: Blood travels from the heart to the body and back to the heart

The definition of a single circulatory system is characterized by blood traveling from the heart to the body and then returning back to the heart. In this type of system, there is only one pathway for blood flow. This means that blood typically passes through the gills (in aquatic organisms) before it goes to the rest of the body, circulating in a single loop. In contrast, the other options do not accurately capture the essence of a single circulatory system. The description of blood flowing only through arteries doesn’t encompass the complete process of circulation, which also involves veins. Similarly, the notion of circulating through two distinct circuits refers to a double circulatory system, which is characteristic of mammals and some other animals where blood flows through both the pulmonary (lungs) and systemic (body) circuits. The idea of blood flowing solely within the heart does not represent circulation but rather describes the internal flow that occurs within the heart chambers, which is not the focus when discussing a circulatory system as a whole.