Higher animals require greater and speedier supply of nutrients and oxygen to their tissues and also a rapid disposal of respiratory and nitrogenous wastes. So they have developed blood as a specialized circulatory fluid and the circulatory system consists of heart, blood and blood vessels for conducting and pumping blood to the tissues. The blood vascular system may be of two types. They are open circulatory system and closed circulatory system.

Open and Closed circulatory system 
Open circulatory system

Open circulation is found in invertebrates like prawn, insects, and molluscs.

1. Blood flows through open spaces called lacunae and sinuses

2. Blood flows at a very slow velocity

3. Body cavity is filled with blood (Haemocoel*)

4. Internal organs are bathed by blood

5. Blood takes long time to complete

6. Supply and elimination of materials are very slow

7. Exchange of materials takes place between blood and sinuses

8. Blood flow cannot be regulated

Closed circulatory system

Invertebrates , the heart and blood vessels form a closed system

1. Blood flows through closed vessels

2. Blood flows at a very high velocity

3. Haemocoel is absent

4. Internal organs are not in direct contact with blood

5. Blood takes short time to complete

6. Supply and elimination of materials are very rapid

7. Exchange of materials between blood and tissues takes place through the capillaries

8. Blood flow can be regulated

* Haemocoel:  Is the body cavity in open circulatory system
Extra points: Closed circulatory system was discovered by British Physician William Harvey in 1616. He is regarded as the ‘Father of Physiology’.
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