Whether lounging on the beach or casting a line into a shimmering lake, the question may have crossed your mind – do fishes have blood? As a ubiquitous symbol of the watery world, fishes capture our imagination. Yet their inner workings remain a mystery below the surface.
If you’re short on time, here’s a quick answer to your question: Yes, fishes do have blood and circulatory systems to move nutrients, gases, and waste around their bodies.
In this comprehensive guide, we’ll dive deep into the circulatory systems of fish to uncover how they pump blood to survive. You’ll learn about the basic components of a fish’s circulatory system, how they breathe underwater, the differences between warm and cold blooded species, and the specialized adaptations that allow fish to thrive in aquatic environments.
An Overview of Fish Circulatory Systems
The Heart
Like all vertebrates, fish have a closed circulatory system with a heart that pumps blood throughout the body. The fish heart has four chambers – two atria and two ventricles. The atria receive blood, while the muscular ventricles pump blood to the gills and body.
Unique to fish is the single circulatory pattern, where blood passes through the heart once during each circuit around the body.
Blood Vessels
There are two main types of blood vessels: arteries and veins. Arteries carry oxygen-rich blood from the heart to the tissues of the body. The main artery is the dorsal aorta, which runs along the spine and sends out smaller arteries to various parts of the body.
Veins return oxygen-depleted blood to the heart. The oxygen is then replenished as the blood travels through the gills.
Gills for Gas Exchange
A vital component of the fish circulatory system are the gills, which enable gas exchange between the blood and water. Located on each side of the head, the gills are composed of bony arches with a dense network of blood vessels.
As oxygen-poor blood travels through these vessels, it absorbs oxygen from the water and releases waste carbon dioxide before circulating back to the body.
The structure of the fish circulatory system is designed for aquatic life. The heart efficiently moves blood to exchange gases at the vast surface area in the gills. This allows fish to thrive underwater and power sustained aerobic activity like migration across oceans.
According to scientists, the anatomy of the fish heart is homologous to land animals. This evidence supports evolutionary connections between fish and other vertebrates descended from common ancestors.
Blood Composition and Cell Types
The blood of fish serves many of the same essential functions as in humans and other vertebrates. However, there are some key differences in its composition and cellular components that are uniquely adapted to life underwater.
Plasma
Fish plasma makes up between 32-92% of total blood volume depending on species. It is composed of water, proteins, salts, nutrients, hormones, antibodies, waste products, and dissolved gases. An important job of the plasma is to maintain homeostasis of blood pH, nutrients, and salt concentrations despite changes in the external aquatic environment.
One fascinating aspect is that the plasma of marine fish has similar osmolarity to seawater, allowing ions and water to freely flow between the fish’s tissues and environment. By contrast, the plasma of freshwater fish has much lower salt levels than their habitat, requiring specialized kidneys and gills to prevent dangerous ion loss.
Red Blood Cells
The distinctive red blood cells (erythrocytes) of fish are nucleated unlike mammalian RBCs. Their unique oval or elliptical shape allows the cells to efficiently stack and flow through narrow capillary beds in the gills where gas exchange occurs.
Fish RBCs also have a larger size and increased hemoglobin levels compared to humans, which improves oxygen carrying capacity. This adaptation helps counterbalance the low oxygen solubility in water and maintain sufficient delivery to tissues when activity levels rise.
White Blood Cells
While not as numerous as RBCs, fish do have various white blood cells that provide immune defenses. These include granulocytes that handle inflammation/infection, agranulocytes functioning in antibody production, and macrophages that clean up cellular debris.
Research shows that specific WBC percentages and absolute counts can indicate stressed or unhealthy states in fish. As such, hematological data serves as a useful biomarker in aquaculture for monitoring stocking densities, water quality, nutritional status, and disease outbreaks.
Circulatory Differences Between Warm and Cold Blooded Fish
Warm Blooded Fish Circulation
Warm blooded fish like tuna, sharks, and some species of mackerel maintain a higher body temperature than the surrounding water. This allows them to be more active predators in colder ocean environments. Their circulatory system is efficient at retaining body heat.
These fish have a closed circulatory system with a two-chambered heart. The heart pumps blood to the gills for oxygenation first. The oxygenated blood then circulates throughout the body before returning to the heart.
Key adaptations that help retain heat include:
Maintaining a warm body temperature enables faster swimming speeds, expanded habitats and feeding at higher latitudes.
Cold Blooded Fish Circulation
The vast majority of fish species are cold blooded ectotherms. This means they take on the same temperature as their aquatic environment. Without the need to maintain a warmer internal body temperature, their circulatory system is relatively simple.
Like warm blooded fish, cold blooded fish have a closed circulatory system. But their two-chambered heart only needs to pump blood to the gills for oxygenation and then throughout the body.
Key attributes of their circulatory system include:
Being ectothermic provides advantages like lower food requirements and expanding into frigid habitats. But it also means slower reflexes and vulnerability when temperatures drop.
Unique Adaptations for Underwater Circulation
Countercurrent Gas Exchange
Fish have evolved a special mechanism called countercurrent exchange to maximize oxygen uptake from water. As blood flows through the gills in one direction, water flows in the opposite direction. This countercurrent flow allows for efficient diffusion of oxygen into the bloodstream and carbon dioxide out of the bloodstream.
The constant concentration gradient ensures optimal gas exchange.
Some fish species, like tuna, have specialized blood vessels called reticulo-myocardial artery and vein that further aid in countercurrent exchange. The artery carries warm, oxygen-depleted blood from the heart to the gills while the vein transports cool, oxygen-rich blood back to the heart.
Their close proximity allows heat exchange and minimizes cardiac work.
Accessory Breathing Organs
Certain fish have developed accessory breathing organs that allow them to obtain oxygen from air in addition to their gills. These adaptations help fish survive in habitats with low dissolved oxygen levels.
Lungfish have a well-developed lung used to breathe air when pools and lakes start drying up. Some species can even burrow into mud and secretes mucus to form an enclosed chamber where they can respire during drought.
Many fish also have a highly vascularized swim bladder that supplements gill breathing. Species like garfish and bichirs can actively fill their swim bladders with air at the water surface.
High Blood Pressure
To overcome the issue of blood pressure dropping due to gravity, fish blood pressure is generally very high compared to land animals. This allows blood to sufficiently perfuse all organs despite changes in vertical height.
According to an authoritative study published on ScienceDirect website in 2022, blood pressures range from 15 mmHg in hagfish species to as high as 80 mmHg in salmonids. Such high pressures are made possible by the low resistance offered through the gill vasculature.
| Animal | Mean Arterial Blood Pressure (mmHg) |
| Humans | 70-110 |
| Dogs | 80-120 |
| Salmon | 40-80 |
| Goldfish | 20-50 |
| Hagfish | 15-20 |
Clearly, fish blood pressure can be up to double or even five times higher than human blood pressure! Truly a marvelous adaptation to life underwater.
Circulatory Disorders and Diseases in Fish
Anemia
Anemia, a reduction in red blood cells or hemoglobin, is one of the most common circulatory disorders seen in fish. It can be caused by blood loss, inflammation, parasites, or nutritional deficiencies. Symptoms include pale gills, lethargy, and poor growth.
Mild cases may resolve with improved husbandry but severe anemia can be fatal if the underlying cause is not addressed.
Leukemia
Leukemia, or cancer of the blood cells, is rare but can occur in fish. Lymphocytic leukemia, affecting white blood cells, is most commonly seen. Signs can include anemia, swollen abdomen, protruding eyes, and abnormal blood cell counts. Sadly treatment options are limited for fish leukemia.
Supportive care like nutritional support may help slow disease progression.
Septicemia
Septicemia refers to a systemic bacterial infection in the bloodstream. In fish it usually starts with an initial local infection that spreads to the rest of the body via blood vessels. Symptoms can include hemorrhages, dropsy, reddening of fins or body, and abnormal behavior.
Septicemia carries a grave prognosis but if treated immediately with appropriate antibiotics, some fish can recover.
While our understanding of circulatory diseases in fish is still evolving, recognizing common signs of disorders can help aquarists seek timely veterinary care. With improved diagnostics and treatments, we can better manage these conditions to promote fish health and welfare.
Conclusion
As denizens of the watery realm, fishes have evolved complex circulatory systems tailored to their aquatic lifestyles. A central heart pumps blood rich in oxygen, nutrients, and immune cells through arteries, capillaries, and veins to sustain organ function.
Gills extract oxygen from water so it can diffuse into the bloodstream. And a variety of adaptations allow fishes to thrive in habitats from frigid polar seas to tropical coral reefs. The next time you marvel at the diversity of fishes, remember the beating heart and rushing blood that animates their underwater lives.
