Question

  The heart and circulatory system in vertebrates has changed significantly compared to the two-chambered heart of our fish ancestors. Describe the major changes in the general structure of the vertebrate heart starting with fish to amphibians to mammals and birds. Be sure to explain the evolutionary significance of these changes. For example, explain how changes in the heart have allowed mammals to grow large and both birds and mammals to be more active.

Answer 1

The main changes occur in the heart structure, specifically in the chamber number and how they work, let us analyze each group of organisms:

- Fish: The heart has only 2 chambers, one ventricle and one atrium, the circulatory system is quite simple because blood is pumped in one single direction, unoxygenated blood reaches the gills, oxygenates and then is pumped into the system to oxygenate the tissues.

- Amphibians and reptiles: In this case the heart has 3 chambers, one ventricle and two atria, this change in the number of chambers occurs because of the presence of lungs. One of the atria recieves deoxygenated blood from the system and the other recieves oxygenated blood from the lungs, they both direct the blood to the ventricle and it actually gets mixed, part of this blood is then pumped towards the lungs and other part is pumped towards the system. This is an unefficient mechanism because mixed blood is sent to both directions, the system requires only oxygenated blood and the lungs require only deoxygenated blood, but both of them recieve both kinds of blood.

The circulatory system is now divided into a system circuit and a pulmonary circuit, different to fish that only have one single circuit.

This mechanism provides then lower oxygen quantities and the animals then cannot have high activities

- Mammals and birds: In this case the heart has 4 chambers separated in pairs of 2, one pair works to recieve deoxygenated blood and pump it into the lungs and the other pair recieves oxygenated blood and pumps it into the system. This is a very efficient mechanism because the two kinds of blood are not mixed, the system recieves only oxygenated blood and the lungs recieve only deoxygenated blood. This ends up completely separating a pulmonary circuit and a system circuit. This efficiency provides enough oxygen to maintain high metabolical activity, that is why mammals and birds have much higher metabolisms compared to amphibians and reptiles.