Final answer:
Open circulation is when blood flows directly from respiratory organs to body tissues without returning to the heart, while closed circulation is when blood flows from respiratory organs to the heart before being sent to body tissues. Both systems have advantages and disadvantage.
Step-by-step explanation:
The system of blood flow directly from respiratory organs to body tissues without first returning to the heart is called 'open circulation'. In this system, blood flows from the respiratory organs (such as lungs or gills) directly to the tissues, and the return of blood to the heart occurs through another set of vessels. This type of circulation is found in some invertebrate organisms, such as insects and mollusks.
In contrast, the system in which blood flows from respiratory organs to the heart before being sent to body tissues is called 'closed circulation'. This is the type of circulation found in vertebrates, including humans.
In closed circulation, blood is pumped from the heart to the respiratory organs for gas exchange, and then it returns to the heart before being distributed to the body tissues.
Both open and closed circulatory systems have advantages and disadvantages. Open circulation allows for a more direct and efficient delivery of oxygen and nutrients to the tissues. It also allows for individual control of blood flow to different parts of the body. On the other hand, closed circulation provides better regulation of blood pressure and allows for more efficient exchange of gases and nutrients between the blood and the tissues.