Tadpole circulatory system. Deoxygenated blood passes near the gills and becomes reoxygenated. The blood flows through the organism until it has become depleted of oxygen once more. The blood travels in a loop that always passes through the heart once at some point.

1 - The internal gills / point where the blood is reoxygenated

2 - Point where the blood is depleted of oxygen

3 - Two chambered heart

Red - Oxygenated blood

Blue - Oxygen depleted blood

The circulatory system for most fish and juvenile amphibians consists of a two chambered heart and blood vessels. The heart pumps deoxygenated blood in the blood vessels to the gills, which can be either internal or external, where it becomes stocked with oxygen. The oxygenated blood then travels through the blood vessels to all the parts of the body. When the blood gives all its oxygen to the parts, it goes back to the gills to be reoxygenated. Water, containing oxygen, flows over the gills. Since the gills in a tadpole are internal, the tadpole needs to gulp down water through its mouth. The system operates in a loop, so oxygen is always being delivered. There are thousands of blood vessels from which blood can travel. In one circuit, blood only passes through the heart once.

File:Two_chamber_heart.svg

https://commons.wikimedia.org/wiki/Tadpole#mediaviewer/File:Haswell%27s_Frog_-_Paracrinia_haswelli_tadpole.jpg

http://www.biozoomer.com/2014/02/frog-heart-structure-fish-heart.html