The idea that a trout possesses two hearts is a common misunderstanding rooted in the unique structure of the fish circulatory system. Unlike mammals and birds, fish rely on a single circulatory pathway adapted for aquatic life. A trout has only one heart, but its function and associated vessels are distinctly different from typical vertebrate circulation. Examining the physical components of this closed, single-circuit system clarifies this design.
The Anatomy of a Fish Heart
The heart of a bony fish, such as a trout, is a four-part organ arranged in a linear series, unlike the four parallel chambers found in a human heart. This structure processes only deoxygenated blood, classifying it as a “venous heart.”
Deoxygenated blood returning from the body first collects in the thin-walled sinus venosus, which acts as a reservoir. From there, blood flows into the atrium, a muscular chamber that pushes blood into the primary pumping chamber.
The ventricle is the most powerful component, a thick-walled, muscular structure whose strong contraction propels the blood through the entire circuit. Following the ventricle is the bulbus arteriosus, an elastic, non-muscular chamber in teleost fish like trout. This final chamber expands with each ventricular pulse and then slowly recoils, smoothing the pressure of the blood flowing into the ventral aorta.
The Single-Loop Circulatory Pathway
The defining characteristic of the trout’s circulatory system is its single-loop pathway, where blood passes through the heart only once during a complete circuit. The ventricle ejects deoxygenated blood into the ventral aorta, which carries it immediately to the gills. Within the capillary networks of the gill filaments, gas exchange occurs, transforming the low-oxygen blood into oxygenated blood.
This oxygenated blood leaves the gills and flows directly to the rest of the body via the dorsal aorta, without returning to the heart for a second pump. The blood must pass through two successive capillary beds—the gills and then the systemic tissues—before completing the loop. A consequence of this arrangement is a substantial pressure drop as blood is forced through the restrictive capillaries of the gills. For example, blood pressure can drop by approximately 25% between the ventral aorta leaving the heart and the dorsal aorta supplying the body tissues.
Addressing the ‘Two Hearts’ Misconception
The confusion about a second heart often stems from specialized structures that assist the single heart with circulation. The low blood pressure resulting after blood leaves the gills makes it difficult for blood to return from the extremities, especially the tail region. To compensate, many fish have auxiliary mechanisms to promote venous return.
Some fish, like the hagfish, possess accessory hearts with cardiac muscle outside the main heart, such as a portal heart. While trout lack a second primary heart, they and other fish utilize accessory venous pumps, sometimes called caudal hearts, in the tail region. These are specialized, valved sacs compressed by surrounding skeletal muscles to squeeze blood back toward the main heart. These auxiliary venous pumps maintain flow in the low-pressure system and are often mistakenly identified as a second heart.
Efficiency of the Aquatic Circulatory System
The single-loop, low-pressure system is an adaptation suited to the aquatic environment and fish physiology. The pressure drop is less concerning because water supports the fish body, meaning the heart does not need to pump blood against gravity. This low-pressure circulation is sufficient to meet the metabolic demands of fish, which are generally lower than those of warm-blooded mammals.
The system’s efficiency is also tied to the fish’s ectothermic nature, as their body temperature and metabolic rate fluctuate with water temperature. Since oxygen delivery needs are lower and fluctuate with environmental conditions, the single-circuit design provides a simple, energy-efficient solution. Although the system delivers oxygen at a slower rate than the double-loop circulation of mammals, it is an optimized arrangement that conserves energy and supports the fish’s activity in its habitat.