The Anatomy and Structure of the Fish Heart

A fish heart works on a simpler plan than ours: one pump that sends blood around a single loop, rather than two pumps working side by side. Most features of the powerful hearts of mammals and birds trace back to simpler versions in the hearts of cold-blooded vertebrates like fish.1PubMed. Evolution and development of the building plan of the vertebrate heart Yet the fish heart is far from fixed, since many fish rebuild it as water warms or cools, oxygen drops, or they swim harder.2PubMed Central. Temperature-induced cardiac remodelling in fish

The Four Chambers and How Blood Flows Through Them

Sharks and rays have a different outflow chamber in that spot instead. It is muscular and lined with valves, where the bony fish’s bulb is stretchy and acts as a pressure reservoir.

If you are cleaning a fish, the heart sits low in the throat, just behind and below the gills. The ventricle is the firm, dark red lump, often triangular or rounded. The atrium lies above and behind it and looks thinner and darker. In front, the pale, whitish bulb leads toward the gills. The junctions between chambers look like narrow necks between the lumps. In an adult these necks are fitted with flap valves that close after each chamber contracts, so blood cannot slip back into the chamber behind.

Blood leaving the heart is low in oxygen and goes straight to the gills to pick some up. From the gills it travels on to the body without returning to the heart first. Because of this single loop, pressure is lower by the time blood reaches the body, and in most fish the whole heart handles only oxygen-poor blood.

The chambers are visible from very early on. Two days after fertilisation, a zebrafish heart is already a tube split into all four segments, each marked by a pinch between them.7The Anatomical Record. Structure and function of the developing zebrafish heart The walls are about one cell thick, except the ventricle, which has two to three layers.8The Anatomical Record. Structure and function of the developing zebrafish heart There are no valves between the segments yet.9The Anatomical Record. Structure and function of the developing zebrafish heart

How the Fish Heart Compares With the Human Heart

Hearts began as a simple single-layered tube in animals like sea squirts and lancelets.10PubMed Central. The vertebrate heart: an evolutionary perspective Jawed fish then evolved a heart with one atrium and one ventricle.11PubMed Central. The vertebrate heart: an evolutionary perspective Amphibians added a second atrium, giving three chambers.12PubMed Central. The vertebrate heart: an evolutionary perspective Crocodiles, birds and mammals evolved four chambers, with two atria and two ventricles.13PubMed Central. The vertebrate heart: an evolutionary perspective

People sometimes count the four fish chambers and call it a four-chambered heart like ours. But the fish chambers are in a row, not side by side, and only the atrium and ventricle are true pumping chambers in the human sense.

Our extra chambers split the work: the right side sends blood only to the lungs, and the left side only to the body. Fish manage with one pump because their gills sit right after the heart. Warm-blooded animals, with their much higher energy use, need the extra pressure.

The two hearts also sit differently. Ours is in the middle of the chest, protected by the ribcage, and blood returns to it from the lungs and the body. A fish heart is a small organ tucked low behind the gills, close to the throat. It receives blood returning from the body, not freshly oxygenated blood from the gills.

Fish pump differently too. They raise their heart’s output mainly by pushing more blood per beat, not by beating faster.14PubMed Central. Cardiac Hypoxia Tolerance in Fish: From Functional Responses to Cell Signals People rely much more on raising their heart rate. Like us, though, many fish such as goldfish have nerves that can both speed up and slow down the heart.15PubMed Central. Autonomic Regulation of the Goldfish Intact Heart

Inside the Ventricle Wall

In salmon and trout, the ventricle has an outer compact layer of muscle cells running around the chamber.16PubMed Central. The intercellular organization of the two muscular systems in the adult salmonid heart, the compact and the spongy myocardium Inside that shell is a spongy layer, a mesh of fine strands stretching across the chamber’s hollow.17PubMed Central. The intercellular organization of the two muscular systems in the adult salmonid heart, the compact and the spongy myocardium Blood flows between the strands, bathing much of the muscle directly. In adult trout the spongy layer makes up more than 80% of the ventricle.18PubMed Central. The Dynamic Nature of Hypertrophic and Fibrotic Remodeling of the Fish Ventricle In sharks, swordfish and tuna the compact layer is built from orderly layers of fibre bundles.19PubMed. Ventricular myocardial architecture in marine fishes

Cut a ventricle open and the compact layer looks like a smooth, solid, darker rim. The middle looks like a sponge or honeycomb. Fast, active swimmers like tuna tend to have a thicker solid rim, while sluggish fish often have hearts that are almost entirely spongy.

A spongy heart suits a fish because the strands give muscle a large surface to touch passing blood, and oxygen can soak in from the blood inside the chamber. A thin solid shell adds strength without needing its own blood supply everywhere.

How the Heart Feeds Itself Oxygen and Sets Its Beat

In many fish the spongy layer takes oxygen directly from the oxygen-poor blood passing through the chamber, with no coronary vessels needed. The oxygen left in that blood is enough when the fish is resting, but it leaves little margin when the heart works harder.

That is where coronary vessels come in. Oxygen-poor blood in the chamber can’t easily reach the outer wall, and in big ventricles the outer layer sits farthest from it. Coronary vessels deliver fresh oxygen from the gills to this outer muscle. Where bony fish have them, they reach into the heart muscle to different depths by species.20Canadian Journal of Zoology. The coronary and luminal circulations of the myocardium of fishes In some freshwater fish they run along the outflow chamber, then over the ventricle surface, and enter only the compact layer.21PubMed. Myocardium Arrangement and Coronary Vessel Distribution in the Ventricle of Three Neotropical Freshwater Teleosts Sharks have coronary vessels throughout the heart muscle.22Canadian Journal of Zoology. The coronary and luminal circulations of the myocardium of fishes A dogfish’s spongy layer therefore seems to depend mainly on coronary vessels, unlike rainbow trout’s.23PubMed. Morphological arrangement of the coronary vasculature in a shark (Squalus sucklei) and a teleost (Oncorhynchus mykiss) Coronary blood matters most during long swims, low-oxygen water and recovery from exercise.24Canadian Journal of Zoology. The coronary and luminal circulations of the myocardium of fishes

The beat itself starts at a ring of pacemaker cells. In zebrafish, the ring sits at the base of the valves where the collecting chamber meets the atrium.25PubMed Central. Identification and Functional Characterization of Cardiac Pacemaker Cells in Zebrafish From that junction the signal spreads to the atrium, which contracts first. Then the ventricle squeezes, pushing blood into the outflow chamber.

Sharks, Sturgeon and the Suction-Powered Heart

In sharks, pressure in the veins near the heart is below that of the surroundings, and the heart’s suction helps draw blood back in.26PubMed. Central venous pressure and mean circulatory filling pressure in the dogfish Squalus acanthias: adrenergic control and role of the pericardium The sac around the heart keeps that pressure negative.27PubMed. Central venous pressure and mean circulatory filling pressure in the dogfish Squalus acanthias: adrenergic control and role of the pericardium A small canal from the heart sac to the belly cavity helps by letting fluid drain out.28PubMed. Function of the pericardium and pericardioperitoneal canal in elasmobranch fishes

Each ventricle squeeze shrinks the heart inside a rigid sac. That lowers the pressure in the sac and pulls the atrium open, so blood is drawn in rather than only pushed, a bit like squeezing and releasing a rubber bulb. This matters because fish have no pump after the gills to push blood back to the heart, so venous blood arrives at low pressure. The sac has to be firm, not stretchy, or it would not hold the lower pressure.

Sturgeon have the same one-way canal and similar sac pressure.29PubMed. Pericardial and pericardioperitoneal canal relationships to cardiac function in the white sturgeon (Acipenser transmontanus) Their heart sac holds less fluid relative to body size, about 2.4 to 2.7 ml per kg versus 3.5 to 5.4 in sharks, so they have less room to boost each beat by releasing sac fluid.30PubMed. Pericardial and pericardioperitoneal canal relationships to cardiac function in the white sturgeon (Acipenser transmontanus)

Lungfish and Other Air-Breathing Fish

In the African lungfish, an incomplete wall splits the ventricle into two chambers.31PubMed. Ventricle and outflow tract of the African lungfish Protopterus dolloi It lets fresh blood from the lung-like organ head mainly to the body, while oxygen-poor blood can be steered toward the gills and lung.

Fish that breathe air often have reworked circulation to route blood to and from the air-breathing organ, whether a lung or a modified gill chamber.32Aqua-BioScience Monographs. Evolution of the Cardiorespiratory System in Air-Breathing Fishes Blood returning from that organ must reach the body, so the plumbing differs from the usual gills-then-body plan. Even so, apart from snakeheads and lungfish, most seem unable to keep oxygen-rich blood separate from oxygen-poor blood inside the heart.33Aqua-BioScience Monographs. Evolution of the Cardiorespiratory System in Air-Breathing Fishes That includes fish a reader may keep, such as bettas and some catfish, which gulp air at the surface and depend on surface access when water oxygen is low.

How Temperature, Low Oxygen and Exercise Reshape the Heart

Trout kept warm and trout kept cold develop distinctly different hearts.34PubMed Central. Cardiac Remodeling in Fish: Strategies to Maintain Heart Function during Temperature Change Cold zebrafish develop smaller ventricles with a thinner compact layer, but returned to 27°C, their hearts grow back to normal.35PubMed. Cardiac remodeling caused by cold acclimation is reversible with rewarming in zebrafish (Danio rerio) Cold slows the chemistry in muscle, so a cold fish heart contracts less forcefully for its size. A larger or thicker heart can make up for this, which is why some fish build one in the cold.

In low oxygen, fish hearts usually slow down, which cuts the heart’s own oxygen needs.36PubMed Central. Shaping the cardiac response to hypoxia: NO and its partners in teleost fish Slower beats give blood longer in the chamber, letting the muscle extract more oxygen.37PubMed Central. Cardiac Hypoxia Tolerance in Fish: From Functional Responses to Cell Signals Zebrafish kept in low oxygen grow a compact layer about 69% thicker.38PubMed Central. Integrated response of the zebrafish (Danio rerio) cardiovascular system to hypoxia acclimation That gives the heart more muscle supplied by coronary vessels rather than by the chamber’s blood. Low-oxygen-tolerant fish like carp keep normal or stronger heart function.39PubMed Central. Shaping the cardiac response to hypoxia: NO and its partners in teleost fish

Exercise remodels the heart too. Four weeks of swim training enlarged zebrafish hearts in a healthy way and raised swimming capacity.40PubMed Central. Cardioprotective responses to aerobic exercise-induced physiological hypertrophy in zebrafish heart Hard-trained salmon gained about 19% more ventricle mass relative to body size than untrained fish.41PubMed Central. Cardiac Molecular-Acclimation Mechanisms in Response to Swimming-Induced Exercise in Atlantic Salmon

For pond and aquarium fish, gradual temperature changes give the heart time to adjust, while sudden swings don’t allow remodelling and stress it. Good aeration spares the heart from low-oxygen strain, and fish kept in gentle current may build stronger hearts.

Fish Hearts That Heal Themselves

Some fish and salamanders can regrow organs, including the heart, as adults.42PubMed. Advances in understanding the mechanism of zebrafish heart regeneration Zebrafish fully regrow their heart within about 2 months after a fifth of the ventricle is removed.43PubMed. Heart regeneration in zebrafish Heart muscle cells multiply at the edge of the wound instead of forming scar.44PubMed. Heart regeneration in zebrafish The wound first fills with a clot and some temporary tissue, which is gradually replaced as new muscle grows in.

Human hearts, by contrast, heal damage with scar tissue. A heart attack leaves permanent scar that weakens pumping, so learning how zebrafish avoid scarring could guide future treatments.