Heart (Two-Chambered)

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The fish heart has only two chambers - one atrium and one ventricle - and pumps deoxygenated blood in a single circuit through the gills.

Region: Trunk Universal in fish Unique to: all

📖 Overview

Unlike the four-chambered mammalian heart, a fish has one atrium and one ventricle. Blood flows from body → sinus venosus → atrium → ventricle → bulbus (or conus) arteriosus → ventral aorta → gills (where it picks up oxygen) → body. This is a single-circuit system - relatively low-pressure but efficient for gill-breathing life.

🐠 Variations Across Species

Lungfish have a partially divided atrium - a transitional state toward four-chambered hearts. Icefish (Channichthyidae) of the Antarctic lack red blood cells entirely and rely on massive hearts and low-viscosity plasma.

✨ Fun Fact

The icefish heart pumps five times the blood volume per minute of a comparably-sized fish - and it does so at 2 degrees C in Antarctic seas.