BRIEF REVIEW AND DIFFERENCE BETWEEN OPEN AND CLOSED CIRCULATORY SYSTEM

Open and Closed Circulatory System:

We all know that in all life forms the gases and nutrients are transported to all parts of the body. There is an efficient transport system which carries all the activities of transportation in living beings. In unicellular organisms and small multicellular organisms, diffusion is the key process for transportation. Diffusion is a slow process. As distances are increased, diffusion no longer facilitates the transportation process as many of the cells are not exposed directly to the atmosphere. We need an elaborate and sophisticated transport system. Circulatory system is one of the most sophisticated transport systems in body. In circulatory system, a fluid circulates in body. Haemolymph and blood are the fluids which circulate in invertebrates and vertebrates’ body respectively.

Characteristics of Circulatory System:

A circulatory system is the flow of materials from one part of the body to another part of the body. Following are the characteristics of circulatory system:

A fluid to circulate – blood or haemolymph

A pumping mechanism

Blood vessels in which blood or haemolymph flows

Circulatory system is categorized into two types:

• Open circulatory system
• Closed circulatory system 

Open Circulatory System:

Open circulatory system is considered as primitive circulatory system because it is not capable of upholding blood pressure.  In open circulatory system, Haemolymph or blood doesn’t remain enclosed in the tubes or vessels and comes in direct contact with the body cells or tissues. The pumping machine, the heart pumps the haemolymph into the tubes and then the tubes vacant themselves into sinuses. These sinuses are open spaces and haemolymph directly comes in contact with tissues and transport nutrients. After bathing the cells and tissues, haemolymph again goes through the heart for the next circulation. During the bathing of cells, exchange of nutrients takes place*.

*= this system only transports nutrients. Gases are not transported by this system. Gases are transported by the tracheal system.

Closed Circulatory System:

Closed circulatory system is a sophisticated and elaborate system as compared to open circulatory system. In closed circulatory system, blood is restricted in the blood vessels during circulation. There is a unified system of arteries, veins and capillaries. Unlike open circulatory system, closed circulatory system is capable of transporting gases.

Arteries are the specialized tubes which take the blood away from away from the heart and veins are the dedicated vessels which bring back the blood from all parts of the body to heart. The pumping organ i.e. heart pumps the blood. Arteries take the blood from heart and carry it to tissues. For exchange of materials between blood and tissues, arteries divide and subdivide into very tiny and fine branches called capillaries. These one celled thick capillaries exchange nutrients between blood and tissues. The capillaries join and form bigger blood vessels called venules. These venules then form veins, which in the end bring back blood to heart.

The comparison between closed and open circulatory system is shown in following table:

Open Circulatory System	Closed Circulatory System
1. Blood isn’t restricted to blood vessels. Blood is in direct contact with body tissues.	1. Blood is always restricted in blood vessels i.e. arteries, capillaries and veins.
2. There are no characteristic blood vessels. Haemolymph flows in sinuses of hoemocoel.	2. There is a sophisticated and unified system of arteries, capillaries and veins.
3. When blood is in direct contact with tissues, only then exchange of materials takes place.	3. Through capillaries, nutrients and waste materials are exchanged between tissues and blood by means of tissue fluid.
4. System doesn’t support transport of gases.	4. Not only nutrients are transported, gases are also transported.
5. This system can't maintain blood pressure. There is no respiratory pigment dissolved in blood. It is white.	5. This system can maintain Blood Pressure. Haemoglobin, a respiratory pigment is present in blood.