The anatomy of the trout often raises questions about its internal systems, particularly its heart. The idea that a fish might possess more than a single heart is understandable, given the unique challenges of aquatic life and the different circulatory designs found across the animal kingdom. A trout’s heart functions reveals a system perfectly adapted to its environment, operating on a fundamentally different principle than that of a mammal.
The Single Heart of Bony Fish
Trout, like all bony fish, possess a single heart, contradicting the belief that they have two. This organ is a two-chambered structure, consisting of one atrium and one ventricle, distinct from the four-chambered hearts of birds and mammals. The trout’s circulatory system is classified as a single-circuit system, meaning blood passes through the heart only once during a complete body cycle.
This single-circuit design differs significantly from the double-circuit system found in humans, where the heart pumps blood twice—once to the lungs for oxygenation and a second time to the rest of the body. In the trout, the single heart must generate enough pressure to push the blood through two capillary beds in succession: first the gills, and then the rest of the body’s tissues.
The fish heart is located far forward in the body, situated beneath the gills within the pericardial cavity. This strategic placement allows for the most direct route to the gills for gas exchange. The single-circuit system works effectively because fish have lower metabolic demands than warm-blooded animals, allowing them to tolerate the drop in blood pressure after the gills.
Blood Flow Through the Trout Heart
The process begins with deoxygenated blood returning from the body’s tissues to the heart, collecting first in the thin-walled receiving sac called the sinus venosus. From there, the blood flows into the atrium. The atrium’s contraction then moves the blood into the powerful, thick-muscled ventricle, which is the primary pumping chamber.
The ventricle generates the force necessary to propel the deoxygenated blood forward into the bulbus arteriosus, an elastic-walled bulb at the exit of the ventricle. This bulb is not a contractile chamber but functions as a pressure reservoir.
The bulbus arteriosus expands as it receives the high-pressure surge from the ventricle. Its elastic recoil helps smooth out the blood pressure pulse, ensuring a continuous, less turbulent flow as the blood travels through the delicate capillaries of the gills. After leaving the bulbus arteriosus, the blood moves to the gills, where oxygen is absorbed from the water.
Once oxygenated in the gills, the blood does not return to the heart. Instead, it continues directly to the dorsal aorta, which distributes the now oxygen-rich blood throughout the entire body to nourish the tissues. After delivering oxygen, the deoxygenated blood completes the circuit by flowing back to the sinus venosus, ready to repeat the single loop.
Why the Misconception About Two Hearts Exists
The confusion about trout having two hearts often stems from anatomical complexity and comparison to other aquatic life. Biologically, the trout heart is more complicated than a simple two-chambered pump suggests, as it is a four-part structure in sequence.
While only the atrium and ventricle are considered the primary pumping chambers, the presence of these four distinct, serially arranged compartments can lead to the idea of multiple “hearts” or pumping stations. The bulbus arteriosus, in particular, plays a specialized role in pressure regulation, which might be mistaken for the function of a second heart.
Another source of the misconception is the existence of multiple hearts in different types of aquatic animals, such as cephalopods. A squid or octopus, for instance, has three separate hearts: one systemic heart to pump blood through the body and two branchial hearts dedicated to pushing blood through the gills. This specialized anatomy in invertebrates is often incorrectly generalized to all aquatic creatures, including fish like the trout.