Fish Cardiovascular System

Fish invented the vertebrate heart. Their system is elegantly simple: blood flows from body to heart to gills to body, in a single loop. The two-chambered heart handles low pressures; blood picks up oxygen at the gills; body tissues extract that oxygen; deoxygenated blood returns for another pass. It's less efficient than the mammalian four-chamber system - but perfectly suited to gill-breathing, cold-blooded life.

Blood flows: sinus venosus → atrium → ventricle → bulbus/conus arteriosus → ventral aorta → afferent branchial arteries → gill lamellae (oxygenation) → efferent branchial arteries → dorsal aorta → systemic circulation → veins → back to sinus venosus.

Single-circuit vs double-circuit circulation, low-pressure gill perfusion, hematocrit variation, hemoglobin oxygen affinity, Bohr effect.

Icefish (Channichthyidae) have no red blood cells at all - the only vertebrates with clear blood. Their enlarged hearts and dilated vessels compensate.

Understanding fish cardiovascular physiology is critical to aquaculture health, anesthesia (MS-222), and stress management.

The bluefin tuna heart beats at rest at 60 bpm but jumps to 200+ bpm during high-speed pursuit - one of the widest cardiac ranges in any vertebrate.