What type of heart dose a fish have?

* One atrium: This chamber receives deoxygenated blood from the body.

* One ventricle: This chamber pumps the deoxygenated blood to the gills, where it picks up oxygen.

This is a simpler system than the four-chambered hearts found in mammals and birds. The single ventricle means there's some mixing of oxygenated and deoxygenated blood, leading to less efficient oxygen delivery. This is why fish generally have lower metabolic rates than mammals and birds.