CDS (I) 2020 General Knowledge · Question 15 of 120

Correct — B, Salamander. Heart chambers track how an animal manages its body temperature. Amphibians and most reptiles have three-chambered hearts — two atria and a single ventricle — and tolerate some mixing of oxygenated and deoxygenated blood, because they do not spend energy keeping their body warm. A salamander is an amphibian, so three chambers is exactly right. Fishes have two chambers, and blood passes through the heart once per circuit. Birds and mammals, which burn energy constantly to hold their temperature steady, need fully separated streams and so have four chambers.

• (a)Scoliodon — Scoliodon is a genus of requiem shark, the Indian dog shark of laboratory dissections — a fish, and fishes have two-chambered hearts.
• (c)Pigeon — A bird. Birds maintain a high, steady body temperature and have four-chambered hearts with no mixing of the two blood streams.
• (d)Human being — A mammal, and mammals are four-chambered for the same reason as birds — the energy cost of staying warm demands fully oxygenated blood reaching the tissues.

Vertebrate hearts come in three grades. Two chambers, one atrium and one ventricle, in fishes: blood goes to the gills, is oxygenated, and travels straight on to the body, passing the heart once per circuit. Three chambers, two atria and one ventricle, in amphibians and most reptiles: the two returning streams meet in the single ventricle and mix to some degree. Four chambers in birds and mammals: the oxygenated and deoxygenated streams are kept wholly apart, and blood passes through the heart twice in each circuit, which is called double circulation.

The four options are one from each major vertebrate group, so the item is a check on whether you can place an unfamiliar name in the right class. Scoliodon is the one that decides the question, because a candidate who does not recognise it as a shark has no way to eliminate it. Salamander is easier — the lizard-like body misleads, but salamanders are amphibians, tailed relatives of frogs and toads rather than reptiles. The organising idea worth carrying away is functional rather than taxonomic: mixing is tolerable when the metabolic demand is low, and animals that regulate their own body temperature cannot afford it.

• Amphibian hearts have three chambers — two atria and one ventricle.
• Fishes have two-chambered hearts, and blood passes through the heart once in each circuit of the body.
• Birds and mammals have four-chambered hearts, so oxygenated and deoxygenated blood never mix.
• Scoliodon is a genus of requiem shark in the family Carcharhinidae, found in Indo-West Pacific waters including the Bay of Bengal.
• Salamanders belong to the order Urodela; their closest relatives are the frogs and toads, not the lizards they resemble.

• Reading a salamander as a lizard and so as a reptile; it is an amphibian.
• Not recognising Scoliodon as a fish, which leaves two plausible answers standing.
• Assuming all reptiles are three-chambered — crocodilians are the exception.

As which animal has n chambers, or in reverse — how many chambers does a named animal's heart have.

• practice — not a real PYQ

  The number of chambers in the heart of a fish is

  • (a)One
  • (b)Two
  • (c)Three
  • (d)Four

  Answer(b) Two — one atrium and one ventricle, with blood passing through the heart once in each circuit.

• practice — not a real PYQ

  Which one of the following animals has a four-chambered heart?

  • (a)Frog
  • (b)Lizard
  • (c)Crocodile
  • (d)Shark

  Answer(c) Crocodile — crocodilians are the reptile exception, with a fully divided four-chambered heart.