OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

Which animal is an example of one that does not possess a double circulatory system?

1. Amphibian

2. Mammal

3. Bird

4. Fish

The correct answer is: Fish

The example of an animal that does not possess a double circulatory system is indeed a fish. Fish have a single circulatory system where blood flows in one continuous loop through the heart, gills, and the rest of the body before returning to the heart. In this system, blood is oxygenated in the gills and then distributed directly to the tissues, without the separation of pulmonary and systemic circulation seen in animals with a double circulatory system. In contrast, mammals, birds, and amphibians all have a double circulatory system. This system allows for a more efficient separation of oxygenated and deoxygenated blood, which enables these animals to deliver oxygen more effectively to their tissues. In mammals and birds, for instance, the heart has two separate pumps that handle pulmonary (lungs) and systemic (the rest of the body) circulation, promoting high metabolic rates that are necessary for their active lifestyles. Amphibians, while having a simpler heart structure, still convey blood through both lungs and the body, creating two distinct pathways for oxygen exchange, albeit with some mixing of blood in the heart. Thus, fish are uniquely positioned with their single circulatory system, making them the appropriate answer to the question.