The below mentioned article provides a short note on the Circulatory Systems of Vertebrates.
The circulatory system is of two types: open of closed. In open circulatory system, blood pumped by the heart passes through large vessels into open spaces or body cavities called sinuses. This type of system is present in Arthropods and Molluscs.
In closed circulatory system the blood pumped by the heart is always circulated through a closed network of blood vessels. This system is more advantageous as the fluid is regulated in better ways. The closed circulatory system is present in Annelids and Chordates.
All vertebrates have a muscular heart. Fishes possess a 2-chambered heart with an arterium and a ventricle. Lung fishes and amphibians have a 3-chambered heart with 2 atria and one ventricle. Reptiles except crocodiles have 3-chambered heart with 2 atria and partially divided single ventricle whereas crocodiles, birds and mammals have a 4-chambered heart with 2 atria and 2 ventricles (Fig. 18.6).
In fishes the heart pumps out deoxygenated blood which is oxygenated by the gills and sent to the body parts from where deoxygenated blood is carried to the heart. It is called single circulation.
In lung fishes, amphibians and reptiles, the left atrium gets oxygenated blood from the ills/lungs/skin/buccopharyngeal cavity and the right atrium receives the deoxygenated blood from other body parts. But both oxygenated and deoxygenated blood gets mixed up in single ventricle which pumps out mixed blood.
This is called incomplete double circulation, in crocodiles, birds and mammals oxygenated and deoxygenated blood received by the left and right atria respectively passes on to the left and right ventricles.
Now oxygenated and deoxygenated blood is not mixed. The ventricles pump it out without any mixing. Thus two separate circulatory pathways are found in these animals. It is called double circulation, which includes systemic circulation and pulmonary circulation.