Sinus venosusA thin-walled sac that collects the blood coming in from the veins.
AtriumA muscular chamber that takes the blood in. It sends the blood on to the ventricle. A fish has only one.
VentricleThe ventricle: the thick-walled, muscular chamber that pumps the blood out.
Ventral aortaThe ventral aorta, carrying blood from the heart forward to the gills.
GillsThe gills, where the blood picks up oxygen from the water.
The heart: 7 3
VeinsThey bring oxygen-poor blood back from the body into the sinus venosus.
Atrioventricular valveIt stops blood flowing back. It sits between the atrium and the ventricle.
Bulbus arteriosusThe outflow at the front of the heart. It leads into the ventral aorta. Some fish have a conus arteriosus here instead, or both.
The loop: 2 1
Dorsal aortaCarries oxygen-rich blood from the gills along the back to the body.
How this list was compiled: Read 2026-09-27. (1) Wikipedia, Fish anatomy, https://en.wikipedia.org/wiki/Fish_anatomy : the heart has two true chambers, one atrium and one ventricle, and pumps blood through the gills and then round the body in a single loop; its four compartments in sequence are the sinus venosus, a thin-walled sac collecting blood from the hepatic and cardinal veins; the atrium, a muscular chamber sending blood to the ventricle; the ventricle, a thick-walled muscular chamber; and the outflow tract to the ventral aorta, the conus arteriosus, the bulbus arteriosus or both; valves between sinus and atrium (sino-atrial), atrium and ventricle (atrioventricular) and bulbus and ventricle; the ventral aorta takes blood to the gills, and it flows on through the dorsal aorta to the body. (2) OpenStax, Biology 2e, section 40.1, https://openstax.org/books/biology-2e/pages/40-1-overview-of-the-circulatory-system : fish have a single circuit and a two-chambered heart with one atrium and one ventricle; the atrium collects blood from the body and the ventricle pumps it to the gills (gill circulation), then on round the body (systemic circulation). Licence: own work
The drawing: Drawn for this site on 2026-10-04 by script. The cut-open heart (a small thin sinus behind, a large thin-walled atrium on top, a thick spongy ventricle below, a pear-shaped thick-walled bulbus narrowing into the ventral aorta) follows, in the forms of its parts, the bony fish heart of fig. 26, "condition of the Conus", of the article "Ichthyology", Encyclopaedia Britannica, 11th edition, vol. 14 (1911), p. 262, after Boas, Lehrbuch der Zoologie, public domain (https://commons.wikimedia.org/wiki/File:EB1911_Ichthyology_-_condition_of_the_Conus.jpg). The fish's outline and fins are measured from the plate of the yellow perch, Perca flavescens, by Duane Raver for the U.S. Fish and Wildlife Service, public domain (https://commons.wikimedia.org/wiki/File:Yellow_perch_fish_perca_flavescens.jpg). The vessels, the gills, the small heart and the arrows are our own. Own work, nothing traced. Licence: own work
How it works
This fish heart diagram shows the heart cut open, with arrows following the blood through its chambers, then the whole fish and its single loop. Start with the 5 main parts.
Read the full guide
Through the heart
Blood from the veins collects in the thin sinus venosus, then passes to the atrium. A valve lets it into the thick, muscular ventricle, which pumps it out through the bulbus arteriosus into the ventral aorta.
One loop
The ventral aorta carries the blood to the gills, where it picks up oxygen. It flows on through the dorsal aorta to the body, and back through the veins to the heart: one loop, not two.
A fish has two true chambers, one atrium and one ventricle; the sinus venosus and bulbus are extra compartments either side.
Questions and answers
How many chambers does a fish heart have?
Two true chambers: one atrium and one ventricle. With the sinus venosus before them and the bulbus arteriosus after, the blood passes four compartments in a row.
Why does a fish have a single circulation?
Its heart pumps blood to the gills, and from the gills the blood carries straight on round the body before coming back. It passes through the heart once per loop.
What does the bulbus arteriosus do?
It is the outflow at the front of the heart, where blood leaves the ventricle for the ventral aorta. Some fish have a muscular conus arteriosus there instead, or both.