As you have already learned that in all leaving beings the nutrients and gases are transported to and from all parts of the body. This is essential to carry on various life processes. In case of unicellular and small multicellular organisms transport takes place by diffusion. However, in large multicellular organisms, as the distances between different body parts have increased, they need an elaborate and efficient system for transportation of materials. In large animals, such a system is called circulatory system in which a fluid circulates in all parts of the body. In many invertebrates this fluid is the haemolymph, where as in all vertebrates and in some higher invertebrates this fluid is the blood.
The circulatory system in animals is of two main types (A) open circulatory system and (B) closed circulatory system.
(A) Open Circulatory system
Many invertebrates e.g. arthropods have open circulatory system [fig12.10 (a)].In this system the blood is pumped from the heart into the blood vessel. The blood vessels in turn, empty themselves into open spaces called sinuses. In the sinuses, the blood is in direct contact with the tissues, and after exchange of materials with the tissues it re-enters the heart for circulation again.
(B) Closed Circulatory System
Closed circulatory system is more elaborate, complicated and efficient as compared to the open circulatory system. The closed circulatory system consists of a muscular, and contractile pumping organ, the heart with its incoming (veins) and outgoing (arteries) blood vessels. The blood remains confined in the blood vessels while circulating in the whole body e.g. earthworm, man etc.
In closed circulatory system, the heart pumps blood into the blood vessels (arteries) which take away the blood from the heart to the tissue. In the tissues, the arteries divide and subdivide into very fine branches, called the capillaries. The walls of capillaries are just one cells thick, and in them the blood is in close contact with the tissue cells. Exchange of materials with tissues is carried out here.
The capillaries join and form bigger blood vessels called venules. These venules in turn join to from the veins, which ultimately transport blood back to the heart.
12.6.1 General Plan of Circulatory System of Vertebrates
For instance in fishes the circulation is of single type. In it, only the deoxygenated blood circulates through the heart. The deoxygenated or the venous blood from all tissues of the body enters the sinus venous, from where it passes into the single auricle or atrium. From the atrium it goes in the ventricle.
From the ventricle, the blood is pumped into the gills for oxygenation. The oxygenated blood from the gills is directly distributed to all parts of the body. As the blood circulates once through the heart, therefore, this type of circulation id called single circuit circulation.
In land vertebrates, with the introduction of lung respiration, double-circuit circulation evolved.
The evolution of double circuit circulation led to the division of atrium and ventricle, each into two chambers. The atrium divided into right and left atria (plural of atrium) and likewise the ventricle also divided into two chambers. In amphibians, the ventricle is not divided and in most reptiles, the division of the ventricle is incomplete. In some reptiles and in all birds and mammals, the division of the ventricle is complete. So in these animals, oxygenated and deoxygenated bloods are completely separated from each other and there is no mixing of these two types of blood. This increases the efficiency of the circulatory System in vertebrates is highly developed and among them mammals have the most efficient circulatory system.
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