Question -
Answer -
An evolutionary change in the pattern of heart among the vertebrates has been observed through careful analysis. Vertebrates possess a muscular heart, it is chambered. They have evolved from having a two-chambered heart(fish) to possessing a four-chambered heart(mammals).
A fish has a two-chambered heart. It pumps deoxygenated blood to the gills where it is oxygenated and sent to the body. The blood that is oxygenated is then carried to the heart.
Three-chambered hearts are found in amphibians – a ventricle and 2 atria(left atrium and right atrium). The left atrium receives oxygenated blood from the respiratory organs while deoxygenated blood is received by the right atrium from the organs of the body. But, both types of blood are eventually mixed in the ventricle, hence the body receives mixed blood.
Half septum in reptiles divides the ventricle partially. But in birds, crocodiles and mammals, the heart is completely segregated into halves hence deoxygenated and oxygenated blood are separated.
A structural modification in the hearts of fish up till mammals is observed checking that oxygen-rich blood is supplied to the body while the four-chambered heart ensures that the blood flow is synchronized. As the structure of heart has evolved, the type of circulation also depends on it, if it is single or double circulation.