The circulatory patterns are of two types - open and closed.
a) Open type: In this type, blood flows from the heart into the arteries. The arteries open into large spaces called sinuses. From the sinuses, the blood is carried by the veins to the heart. There are no interconnecting vessels, the capillaries, between the arteries and veins.
As the circulatory fluid comes out of the vessels and freely flows in the body spaces, this type of circulatory system is called open type, it is found in leeches, arthropods, molluscs, echinoderms and ascidians.
b) Closed type: In this type blood flows through blood vessels. The blood flows from the arteries to the veins through small blood vessels called capillaries. Closed type of blood vascular system is found in the annelids, cephalopoda among the no-chordates, cephalochordates and all the vertebrates among chordates.
The Plan of circulatory system in the vertebrates
In the vertebrates the principal differences in the blood-vascular system involve the gradual differentiation of the heart into two separate 'pumps' as ey evolved from the gill breathing aquatic life to the lung breathing, romplete terrestrial life.
• Fishes have a 2-chambered heart with an atrium and a ventricle. It pumps out deoxygenated blood to gills for oxygenation, hence the name 'branchial heart'. Blood passes through the heart only once in a entire complete circuit, hence it called single circulation.
• Amphibians have a 3-chambered heart with two atria and one ventricle. Reptiles have two atria and an incompletely divided-ventricle {except in the crocodiles in which the ventricle is divided into two chambers). The left atrium receives oxygenated blood from the gills/lungs/skin and the right atrium receives deoxygenated blood from the other parts of the body through the venae cavae. However, the two types of blood get mixed up in the single ventricle, which pumps out mixed type of blood. Thus these animals (amphibians and reptiles) show an incomplete double circulation.
• Birds and mammals possess a 4-chambered heart with two atria and two ventricles. In these animals the oxygenated and the deoxygenated types of blood received by the left and right atria, passes on to the left and right ventricles, respectively. The ventricles pump the blood out without any mixing of the oxygenated and deoxygenated types of blood i.e., there are two completely separate circulatory pathways namely systemic and pulmonary circulations. Hence, these animals are said to be showing 'double circulation'
