• 21 parts
  • Checked October 2026

Evolution of Heart in Vertebrates Diagram

Four hearts on one chart: a fish, a frog, a lizard and a bird or mammal, every chamber labeled.

Evolution of Heart in Vertebrates Diagram

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
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Write the name of each numbered part.Answer key

  1. Atrium
  2. Ventricle
  3. Gill capillaries
  4. Body capillaries
  5. Single circuit
  6. Right atrium
  7. Left atrium
  8. Ventricle
  9. Lung and skin capillaries
  10. Right atrium
  11. Left atrium
  12. Ventricle
  13. Partial septum
  14. Lung capillaries
  15. Right atrium
  16. Left atrium
  17. Right ventricle
  18. Left ventricle
  19. Complete septum
  20. Systemic circuit
  21. Pulmonary circuit

Drawn for this site on 2026-10-01 from plain geometry. Licence: own work.

Each numbered ring is a button. Use the arrow keys to move between them and Enter to choose. Names are read out in Study, and once a part is answered.

Next step Next: the human heart, labeled

All 21 parts on the diagram

Show the list

Fish 5

  1. Atrium The one chamber that receives blood. Blood from the body collects here, then passes to the ventricle.
  2. Ventricle The one chamber that pumps. Its thick, muscular wall pushes blood to the gills.
  3. Gill capillaries Blood picks up oxygen here. It flows on to the body without going back to the heart.
  4. Body capillaries Blood gives up its oxygen here. Then it returns to the atrium.
  5. Single circuit One loop: heart, gills, body, heart. Blood passes through the heart once on each trip.

Frog 4

  1. Right atrium It receives blood from the body. That blood is low in oxygen.
  2. Left atrium It receives blood from the lungs. That blood is rich in oxygen.
  3. Ventricle Both atria empty into this one ventricle. It pumps to the body and to the lungs and skin together.
  4. Lung and skin capillaries A frog takes in oxygen through its lungs and its skin. Blood from the lungs returns to the left atrium.

Lizard 5

  1. Right atrium Its blood is low in oxygen. The ventricle sends it on to the lungs.
  2. Left atrium It receives blood from the lungs. The ventricle sends it on to the body.
  3. Ventricle One ventricle, partly divided in two. The left part pumps to the body, the right part to the lungs.
  4. Partial septum A wall of muscle that partly divides the ventricle. It leaves a gap near the openings of the arteries.
  5. Lung capillaries Blood picks up oxygen in the lungs. It returns to the left atrium.

Bird or mammal 7

  1. Right atrium It receives blood from the body. That blood passes to the right ventricle.
  2. Left atrium It receives blood from the lungs. That blood passes to the left ventricle.
  3. Right ventricle It pumps blood to the lungs. Its blood is low in oxygen.
  4. Left ventricle It pumps blood to the body. Its blood is rich in oxygen.
  5. Complete septum A full wall between the two ventricles. The two streams of blood are kept apart.
  6. Systemic circuit The loop heart to body and back.
  7. Pulmonary circuit The loop heart to lungs and back.

How this list was compiled: Every name and fact was checked on 2026-10-01 in at least two of three sources, for facts only and with nothing copied: (1) Wikipedia, Heart (the section on other animals) and Circulatory system (closed circulatory system); (2) J. W. Kimball, Biology Pages, Animal Circulatory Systems, https://www.biology-pages.info/A/AnimalHearts.html; (3) lecture notes of a university course in comparative vertebrate anatomy, Circulatory System, read in an archived copy, https://web.archive.org/web/2019/http://people.eku.edu/ritchisong/342notes9.html. A part only one of them names is not a part of this page. Licence: own work

The drawing: Drawn for this site on 2026-10-01 from plain geometry; nothing traced. Licence: own work

How it works

This evolution of heart in vertebrates diagram shows four hearts, each with its vessels to the body and to the gills or lungs. Every heart is seen from the belly side, so the animal's right is on the left.

Read the full guide

Count the chambers

A fish heart has one atrium and one ventricle. A frog has two atria and one ventricle. A lizard has two atria and a ventricle that a wall of muscle, the septum, partly divides. In birds and mammals the septum is complete, so there are two ventricles.

One circuit or two

In a fish the blood runs in a single loop: heart, gills, body, heart. In the other three it runs in two loops that meet in the heart. The pulmonary circuit goes to the lungs and the systemic circuit to the body.

  • The arrowheads on the vessels show which way the blood runs.
  • Find the septum first: none in the frog, part of one in the lizard, a whole one in the bird or mammal.

Questions and answers

How many chambers does a fish heart have?

Two are usually counted: one atrium that receives blood and one ventricle that pumps it. Some books count four, adding the small chambers where blood enters and leaves the heart.

How many chambers does a frog heart have?

Three: a right atrium, a left atrium and one ventricle. Blood from the body and blood from the lungs and skin both pass into the same ventricle.

Do reptiles have a three-chambered heart?

Lizards, snakes and turtles do: two atria and one ventricle that a septum partly divides. Crocodiles and their relatives have a complete septum and four chambers, as birds and mammals do.

What is the difference between single and double circulation?

In single circulation, as in a fish, blood passes through the heart once on each trip around the body. In double circulation it passes through twice: once on the way to the lungs and once on the way to the body.

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