Open and closed systems

An open system involves the blood circulating through a large open space. This system is found in many arthropods, where the heart pushes the blood to the front of the body and it slowly circulates to the back end again. Such open systems do not give rapid flow of materials. Closed systems involve the movement of blood through a series of blood vessels. The heart pushes the blood under pressure through the tubing and exchange occurs in the smallest vessels called capillaries. 

Single Circulations

In fish the blood is pumped from the heart along an artery to the gills where a capillary network absorbs oxygen. The capillaries then join up and lead into the dorsal aorta which gives off branching arteries to supply the various body organs.

Such a circulatory system has some problems with maintaining a high enough pressure to drive the blood through two sets of capillaries. Since the capillaries are very tiny in diameter, just a few thousandths of a millimetre, and they are leaky to allow easy exchange of materials, the pressure drops rapidly as blood is passed through them and the pressure in the main vein returning blood to the heart can be very low and this may reduce blood flow rate. 

Double circulations

In mammals and birds the heart is completely divided into two halves by the interventricular septum.  This effectively means that the blood is pressurised by the heart and passes through only one set of capillaries before returning to the heart to be repressurised before passing through the second set of capillaries. This gives a faster more efficient blood flow without the need to develop tremendously high pressures. An added bonus of this system is that the blood pressure in the side of the heart feeding the lungs can be lower than that to the other body organs, which avoids the problem of excessive fluid loss into the lungs as a result of high hydrostatic pressure.