The closed circulatory system of vertebrates has evolved from a single circuit system in fishes to separate pulmonary and systemic circuits in birds and mammals. A key step was the evolution of a primitive lung in a lineage of air-breathing fishes. Further adaptations increasingly separated the flows of oxygenated and deoxygenated blood in separate circuits, as seen in amphibians and reptiles. Ectothermic reptiles retained the ability to direct blood to either the pulmonary or systemic circuit, but mammals and birds have complete division of those two circuits, allowing the circuits to function at different pressures.

Question 1

Deoxygenated blood returns from the systemic circulation in veins that converge on the sinus venous that leads to the atrium. From the atrium, blood flows into the single, muscular ventricle. When the ventricle contracts, blood under pressure flows into the elastic bulbus arteriosus. The elastic recoil of the bulbus arteriosus maintains a continuous blood flow through the gills where respiratory gas exchange occurs. The oxygenated blood from the gills flows through the systemic circuit.

Question 2

The direction of O₂ diffusion across the gill membranes depends on the gradient between the blood and the water flowing across the gills. By taking air into its lungs, the fish can maintain a higher in its blood than in the surrounding water. As a result, O₂ diffuses from the blood to the water, increasing the in the water surrounding the eggs.

Question 3

Ectothermic reptiles have a left and a right aorta. Except for the crocodilians, the ventricles are not completely divided and thus blood from the right ventricle can flow either into the right aorta or the pulmonary aorta. When the animal is resting or submerged and not breathing, the resistance in the lung circuit is high and blood from both ventricles flows into the aortas. When the animal is breathing, resistance in the lung circuit goes down, and blood from the right ventricle flows into the lung circuit while blood from the left ventricle flows into the aortas. In crocodilians there is complete separation of the ventricles and the right aorta opens into the right ventricle. But, just where the aortas leave the heart there is a connection between them. When the animal is breathing and resistance in the lung circuit is low, back pressure from the left aorta closes the valve between the right ventricle and right aorta so all blood from the right ventricle flows to the lungs. When resistance in the lung circuit is high, then pressure in the right ventricle is high enough to open the right aortic valve and blood from the right ventricle flows in the right aorta to the systemic circuit.

Question 4

At birth, when the newborn starts to breathe, the resistance in the pulmonary circuit falls. If the ductus arteriosus does not close, the higher pressure in the left ventricle will pump blood into the pulmonary circuit through the open (patent) ductus arteriosus. This will cause the pulmonary circuit to become congested and the systemic circuit to be deprived of oxygenated blood.