Lesson 6 of 12 · 6 min
Circulatory pathways in animals
NCERT §15.3
In the biology lab after Sports Day, Meher's class looks at a cockroach, a frog and a preserved fish. Their hearts are all built differently. Why?
The lesson in notes
In short
Circulation is either open or closed.
Open circulation (arthropods, molluscs): blood leaves the heart through large vessels and pours into open cavities, the sinuses, where it bathes the tissues directly.
Closed circulation (annelids, chordates): blood pumped by the heart always travels through a closed network of vessels. It is considered more advantageous because the flow can be regulated more precisely.
All vertebrates have a muscular, chambered heart. Fishes have 2 chambers (one atrium, one ventricle); amphibians and reptiles other than crocodiles have 3 (two atria, one ventricle); crocodiles, birds and mammals have 4 (two atria, two ventricles).
Single circulation (fishes): the heart pumps deoxygenated blood to the gills, where it is oxygenated and sent to the body; deoxygenated blood returns to the heart.
Incomplete double circulation (amphibians, reptiles): the left atrium receives oxygenated blood from gills, lungs or skin and the right atrium deoxygenated blood from the body, but the two mix in the single ventricle, which pumps out mixed blood.
Double circulation (birds, mammals): oxygenated and deoxygenated blood enter the left and right atria respectively, pass to the ventricles of the same side and are pumped out without mixing, through two separate pathways.