The correct option is 2 only.
Explanation
Vertebrate circulatory systems have evolved to meet varying metabolic demands. The complexity of the heart (number of chambers) and the nature of circulation (single vs. double) are directly linked to an animal's energy requirements and its ability to regulate body temperature.
Statement-wise Analysis
- Statement 1 is Incorrect.
Animals that maintain a constant body temperature (homeotherms, such as birds and mammals) have high metabolic rates and energy requirements. To sustain this, they require a highly efficient supply of oxygen. Consequently, the separation of the right (deoxygenated) and left (oxygenated) sides of the heart is necessary to prevent blood mixing and ensure maximum oxygen delivery to tissues.
- Statement 2 is Correct.
Amphibians typically possess a three-chambered heart consisting of two atria and a single ventricle. In the ventricle, oxygenated blood from the lungs/skin and deoxygenated blood from the body mix to some extent. Since amphibians are cold-blooded (poikilotherms) and have lower metabolic needs compared to birds and mammals, they can tolerate this mixing of blood.
- Statement 3 is Incorrect.
Fish possess a two-chambered heart (one atrium and one ventricle). Deoxygenated blood is pumped from the heart to the gills for oxygenation, after which it flows directly to the rest of the body and then returns to the heart. This is known as single circulation, meaning blood passes through the heart only once during a complete systemic cycle.
Key Takeaway: The evolution of the four-chambered heart in birds and mammals allows for double circulation with no mixing of blood, which is essential for maintaining high metabolic rates and constant body temperature. Fish exhibit single circulation, while amphibians exhibit incomplete double circulation.