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Different substances in the bodies of human beings as well as other animals have to be transported to different places. It requires a transportation system. For example, oxygen must be transported from the respiratory surface to the tissues, and carbon dioxide from the tissues to the respiratory surface, from where the nutrient is absorbed to where it is stored.

In small animals such as monocotyledons and cylinders, such transport is carried out by simple diffusion. That is, the materials moves from a higher concentration to a lower concentration. As the animals get bigger their energy needs increase. Also the distance required to transport the material in such animals increases. For this reason the transport caused by simple diffusion is not sufficient. Therefore a circulatory system is required.

The circulatory system that develops in this way is called the blood circulatory system. The circulatory systems located in different animals vary from animal to animal. It also changes its design. A circulatory system is a system in which blood which is the fluid medium and joins a group of blood vessels. The vessels here are the arteries and veins. The heart pumps blood continuously through these vessels.

Types of blood circulatory systems

• Open blood circulatory system
• Closed blood circulatory system

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The circulatory system in all vertebrates is a closed blood circulatory system. The main feature that clearly distinguishes a closed circulatory system from an open circulatory system is that the blood that flows from the heart through the arteries travels through the capillaries and back to the heart through the veins. An open blood circulatory system is a veinless circulatory system with a body cavity filled with blood. This blood-filled cavity is called the blood vessel. Animals such as arthropods and mollusks have this circulatory system.

The circulatory system in vertebrates is also classified as follows according to its circulation.
• Single circulation
• Double circulation

Single circulation is the flow of blood through the heart once it is fully circulating through the body. This is the kind of circulation that takes place in fish. Dual circulation is the flow of blood twice through the heart, once it is fully circulating through the body. This dual circulation occurs in mammals such as humans.

Double circulation is also again classified as complete double circulation and incomplete double circulation. In animals where the heart is completely divided into left and right halves, complete dual circulation occurs. Thus in animals such as amphibians the heart not divides into left and right halves. That is, they do not have two koshika, the left koshika and the right koshika. There is only one koshika. Therefore, incomplete double circulation occurs in those animals. We all know that the human heart is divided into four parts. Namely, left karnika, right karnika, left koshika, and right koshika. Therefore, there is a complete dual circulation in humans.

The End.

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References :-
Source
(Active animal book)