Re: What are the advantages of having double circulation than single circulatio

Hi Lim Hong Kai,

You cannot consider the advantages of the double-circulation system without considering the hearts as well. Let's compare the following animals: fish, frog and human

Fish: two-chambered heart with single circulation system
Frog: three-chambered heart with double circulation system
Human: four-chambered heart with double circulation system

Mammals have greater need for oxygen because they are endothermic and require more energy than fish or frogs just to maintain body temperature. Mammals also tend to absorb more oxygen through their main gas-exchange organ, the lung, than through their skin when compared to frogs and fish. Thus, the circulation system of a mammal has to perform to much more demanding requirements than either a fish or a frog.

By beating, the heart creates pressure which forces the blood to circulate. By pumping harder (as opposed to faster), the heart creates more pressure and can pump more blood around the circulation system. Well, if you want to create more pressure, then why not have a heart with two pumps instead of one?

Fish hearts have a pair of chambers connected by a valve which acts as one pump. Human hearts have two pairs of chambers and therefore have two pumps. Frogs have two pumps as well, but one of the chambers is shared by both pumps, which results in the mixing of oxygen-rich blood with oxygen-poor blood and which cannot generate as much pressure. This is fine for frogs because their oxygen requirements are lower.

In summary, the double circulation system of mammals in combination with the double-pump heart has two main advantages:

1) The ability to create more pressure to pump blood round the system.
2) The separation of oxygen-rich and oxygen-poor blood (which does not occur in frogs).

To learn more about the circulation system, read the on-line biology book (circulation).

Richard Kingsley