The fish heart is highly adapted to the aquatic environment. Unlike the mammalian heart, which manages two separate circuits, the fish heart operates a single, continuous loop. This fundamental difference in design allows the fish circulatory system to efficiently manage gas exchange through gills rather than lungs. The anatomy reflects a simpler, linear arrangement of compartments focused on propelling blood forward.
The Single-Circuit Design
The circulatory system of most fish is a single circuit, meaning blood passes through the heart only once during a complete cycle. Deoxygenated blood returning from the body tissues enters the heart, which then pumps this blood forward to the gills. The heart itself is purely a venous pump.
The blood pathway starts at the heart, moves to the gills for oxygenation, and then continues directly to the body tissues before returning. This arrangement places two capillary beds—the gills and the systemic body capillaries—in series. A consequence of this design is a substantial drop in blood pressure after the blood moves through the capillaries of the gills.
The heart must generate sufficient pressure to push the blood through the gills and the rest of the body before it can return. The resulting low systemic blood pressure limits the metabolic rate and activity level in many fish species. Blood flow back to the heart is often aided by muscular contractions from the fish’s movement.
Structural Components of the Fish Heart
The typical heart of a bony fish, or teleost, consists of four compartments that work in sequence to ensure unidirectional flow. The first is the sinus venosus, a thin-walled collecting sac. It receives deoxygenated blood from the major veins and acts as a reservoir before the blood moves on to the next chamber.
From the sinus venosus, blood flows into the atrium, a larger, thin-walled chamber that functions as a receiving space. The atrium has a slightly thicker muscular wall and contracts to push the blood into the ventricle. Valves between the sinus venosus and the atrium prevent backward flow during contraction.
The ventricle is the most muscular chamber and the power source of the fish heart. Its thick walls generate the high pressure needed to drive the blood through the restrictive gill capillaries. The ventricle’s shape correlates with the fish’s activity level. Active swimmers like tuna possess a pyramidal ventricle with a dense outer compact muscle layer. Less active species often have a saccular ventricle composed mainly of spongy, internal fibers.
The final chamber is the bulbus arteriosus, an elastic, bulb-shaped structure connecting the ventricle to the ventral aorta leading to the gills. This chamber is not muscular but is rich in elastic fibers, which dampen the strong pressure pulse from the contracting ventricle. By expanding and recoiling, the bulbus arteriosus transforms the highly pulsatile flow into a more continuous stream, protecting the delicate gill capillaries from damage.
Notable Variations in Fish Hearts
While the four-part linear heart is characteristic of most bony fish, anatomical variations exist across different fish groups. Cartilaginous fish, such as sharks and rays, possess a rigid, muscular structure called the conus arteriosus instead of the elastic bulbus arteriosus. The conus arteriosus contains multiple rows of valves that prevent backflow and regulate pressure.
Among primitive jawless fish, the hagfish exhibits a simpler circulatory system. The hagfish heart lacks the typical outflow tract components and has accessory hearts elsewhere in its body to help maintain circulation in its low-pressure system. This primitive arrangement suggests a less centrally controlled pumping mechanism compared to teleosts.
A variation is seen in lungfish, which represent a transitional form shifting toward a double-circuit system. Their hearts show an incomplete division of both the atrium and the ventricle by internal septa. This partial separation allows for some routing of oxygenated blood from their lungs away from the deoxygenated blood.
This feature allows the lungfish heart to efficiently manage blood flow, necessary when they rely on air breathing. The presence of these transitional structures highlights the evolutionary path from the simple single-circuit design to the fully separated double circulation found in later terrestrial vertebrates. The heart structure is linked to a species’ metabolic needs and respiratory strategy.