Right atriumThe right atrium: it receives oxygen-poor blood coming back from the body.
Left atriumThe left atrium: it receives oxygen-rich blood coming back from the lungs.
VentricleThe ventricle: the single lower chamber. Both atria empty into it, and it pumps blood out to the lungs and the body.
AortaThe aorta, carrying oxygen-rich blood out to the body.
The heart: 6 2
Spiral valveIt sits in the vessel leaving the ventricle. It helps send oxygen-poor blood to the lungs and oxygen-rich blood to the body.
Pulmonary arteryThe pulmonary artery, carrying oxygen-poor blood to the lungs and skin.
The two circuits: 2 2
Pulmocutaneous circuitThe loop from the heart to the lungs and skin and back. The blood picks up oxygen in the lungs and skin.
Systemic circuitThe loop from the heart round the body and back. It delivers oxygen.
How this list was compiled: Read 2026-09-27. (1) OpenStax, Biology 2e, section 40.1, https://openstax.org/books/biology-2e/pages/40-1-overview-of-the-circulatory-system : amphibians have a three-chambered heart with two atria and one ventricle; two circuits, one through the lungs and skin (pulmocutaneous) and one to the body (systemic); the atria receive blood from the two circuits and some mixing happens in the ventricle, reduced by a ridge that sends oxygen-rich blood to the body and oxygen-poor blood to the pulmocutaneous circuit; double circulation; fish have one atrium and one ventricle. (2) Wikipedia, Frog, https://en.wikipedia.org/wiki/Frog : frogs have three-chambered hearts; oxygenated blood from the lungs and deoxygenated blood from the tissues enter through separate atria, pass into a common ventricle and are pumped via a spiral valve to the aorta (oxygenated) and the pulmonary artery (deoxygenated); the dissection key names the right atrium, left atrium, ventricle and aorta. (3) Wikipedia, Amphibian, https://en.wikipedia.org/wiki/Amphibian : two atria and one ventricle; deoxygenated blood pumped through the pulmonary artery to the lungs. Licence: own work
The drawing: Drawn for this site on 2026-10-04 by script. The heart's outline (a broad cone of a ventricle, two atria wider than its base, the outflow vessel rising obliquely from the frog's right and forking to both sides) is laid out after fig. 77, "The Frog's Heart", and the spongy inside of the ventricle after fig. 78, "Section of Heart", of St. George Mivart, The Common Frog (1881), public domain, as scanned by the Biodiversity Heritage Library on Wikimedia Commons (https://commons.wikimedia.org/wiki/File:The_common_frog_(Page_133,_Fig._77)_BHL7743542.jpg and https://commons.wikimedia.org/wiki/File:The_common_frog_(Page_134,_Fig._78)_BHL7743543.jpg). The lungs, the skin, the frog and the arrows are our own. Own work, nothing traced. Licence: own work
How it works
This frog heart diagram shows the three-chambered heart cut open, seen from the front, then the two loops the blood travels. Start with the 4 main parts.
Read the full guide
Three chambers
A frog's heart has two atria on top and one ventricle below. The right atrium takes oxygen-poor blood from the body; the left atrium takes oxygen-rich blood from the lungs. Both empty into the single ventricle, where some mixing happens.
Two circuits
The ventricle pumps into a vessel with a spiral valve, which sends oxygen-poor blood out through the pulmonary artery and oxygen-rich blood through the aorta. One loop goes to the lungs and skin, the other round the body.
Seen from the front, the frog's right atrium is on your left, just as when you face a person.
Questions and answers
How many chambers does a frog heart have?
Three: two atria and one ventricle. A human heart has four, and a fish heart has two.
Why does a frog's heart need a spiral valve?
Because both atria empty into one ventricle. The spiral valve, with a ridge in the ventricle, helps keep oxygen-poor blood heading to the lungs and oxygen-rich blood heading to the body.
What is the pulmocutaneous circuit?
The loop that takes blood from the heart to the lungs and the skin, where it picks up oxygen, and back again. Frogs also breathe through their skin.