Which one of the following classes demonstrates double circulation of blood?

Correct — D, Mammals. In double circulation the blood passes through the heart twice in one complete circuit — once through the pulmonary circuit to the lungs and back, once through the systemic circuit to the body and back. Mammals show this in its complete form: a four-chambered heart with a fully divided ventricle keeps oxygenated and deoxygenated blood entirely separate, which is what a warm-blooded animal with a high metabolic rate needs. Of the four options only mammals have that complete separation, and only one option can be marked.

• (a)Invertebrates — Most invertebrates — insects, molluscs other than cephalopods, arthropods generally — have an open circulation in which blood leaves the vessels and bathes the tissues in a haemocoel. There is no second circuit through a respiratory organ and back.
• (b)Fishes — Fishes have a two-chambered heart and single circulation: blood is pumped once to the gills, passes straight on to the body, and returns to the heart. It crosses the heart only once per circuit, which is the definition of single circulation.
• (c)Amphibians — Amphibians do run two circuits, but through a three-chambered heart with a single undivided ventricle, so oxygenated and deoxygenated blood mix — this is described as incomplete double circulation. Against 'Mammals' in the same list, it is the weaker answer, since mammals show the arrangement in full.

Circulation is classified twice over. Open circulation, found in most invertebrates, releases blood into body spaces; closed circulation keeps it inside vessels. Within closed systems, single circulation sends blood through the heart once per circuit — the fish pattern, heart to gills to body to heart. Double circulation sends it through twice, using separate pulmonary and systemic circuits. Amphibians and most reptiles have double circulation with a three-chambered heart and therefore some mixing; crocodilians, birds and mammals have four chambers and complete separation.

The item is worded 'demonstrates double circulation', and both amphibians and mammals do to some degree, so the exam is asking which one demonstrates it properly. The tie-breaker is the ventricle: an undivided ventricle mixes the two streams, a divided one does not. That is why textbooks call the amphibian arrangement incomplete double circulation and reserve the unqualified term for birds and mammals. Note the honest wrinkle here — the wording could be argued over, and a candidate who marked amphibians has reasoned rather than guessed.

• Double circulation means blood passes through the heart twice in one complete circuit.
• Fishes have a two-chambered heart and single circulation.
• Amphibians and most reptiles have a three-chambered heart, so their double circulation is incomplete.
• Birds and mammals have four-chambered hearts and complete double circulation with no mixing.
• Most invertebrates have an open circulation with blood filling a haemocoel rather than staying in vessels.

• Marking amphibians because they do have two circuits — theirs is the incomplete form.
• Assuming a three-chambered heart means single circulation; the chamber count and the circuit count are different questions.
• Forgetting that cephalopods among invertebrates have a closed circulation, so 'invertebrate' is not a single pattern.

A classification item that rewards the exact textbook phrase — complete double circulation is the mammal-and-bird arrangement, not the amphibian one.

• practice — not a real PYQ

  Single circulation, in which blood passes through the heart only once per circuit, is found in

  • (a)amphibians
  • (b)fishes
  • (c)birds
  • (d)mammals

  Answer(b) fishes — the two-chambered heart pumps blood to the gills, from where it goes directly to the body before returning.

• practice — not a real PYQ

  The mixing of oxygenated and deoxygenated blood in an amphibian happens because it has

  • (a)two auricles and one ventricle
  • (b)one auricle and two ventricles
  • (c)two auricles and two ventricles
  • (d)a single chambered heart

  Answer(a) two auricles and one ventricle — the undivided ventricle receives both streams, so the separation is incomplete.