Fish Blood: Composition, Origin and Functions

The system through which blood circulates in different organs and parts of the body is called Blood Circulatory System. The presence of well-developed circulatory system can be observed in almost all animals with few exceptions. Fish have a closed type of blood circulatory system. Food, oxygen and waste products are transported from one part of the body to another through the blood flowing in such circulatory system.

The circulatory system is actively involved in controlling the metabolism of food, coordinating the various organs and systems of the body, preserving, repairing, and destroying various pathogens. Although the circulatory system has special features compared to other organs, its structure is equally common. The circulatory system of fish consists of blood, blood vessels (arteries and veins) and the heart.

Permeable membranes exist in most areas of the fish body. For this purpose, water is exchanged through the gills, and in addition to the gases dissolved in the gills, the exchange of some nitrogenous wastes and minerals is carried out. The entrance and return of blood from the body of the fish to the gills, except for the lung fish, is accomplished by a single circulation. In this case, the heart exchanges blood with low concentration oxygen and high concentration carbon dioxide.

The blood volume of higher bony fish (teleost) ranges from 1.5% to 3% of total body weight. In mammals, however, the amount of blood is 6% or more of body weight. Spiny dog fish (Squalus acanthias) has a blood volume of 5% of body weight. Plasma or blood cells of fish are produced in greater quantities in different organs or systems than in mammals.

A notable feature of the circulatory system of fish is that there are a significant number of capillary or sinusoidal systems in the arterial or venous flow of blood. The special system created as a result of such capillary arrangement is called portal system. Such systems are found in the gills, liver (hepatic portal system) and kidneys (renal portal system). There is also another capillary resembling vessels in the rete mirabile in one part of the swimbladder of Physoclystous fish.  The arrangement of the chloride glands in the eyes of the teleost is similar.

Some fast-moving fish such as mackerel sharks (Lamnidae), tuna, mackerel (Scombridae) have other organs such as special capillary blood vessel in muscles. As a result of this system, the exchange of gas between blood and tissues is done more efficiently.


The blood of fish is the same connective tissue as other vertebrates. The liquid part is called plasma and the solid part is called blood cells and other substances which are in the liquid part. It includes the following blood cells: red blood cells (erythrocytes or RBCs), white blood cells (leukocytes or WBCs) and platelet (thrombocytes). Red blood cells are red in color because they carry a type of red pigment called hemoglobin. It plays an important role in the transport of oxygen in the blood.

Not all fish have red blood cells and hemoglobin. Some Antarctic fish (Chaenichthyidae, icefish or white crocodile fish) have colorless blood because they do not have erythrocytes. The blood of the small eel (Leptocephalus larvae) is also colorless. Blood pigment of Lamprey (Petromyzon) is not like the hemoglobin of other vertebrates.


The clear liquid that is obtained by separating blood cells from blood is called plasma. In the broadest sense, if blood is collected in a bottle with anticoagulants, the blood will not clot, and in this case, if the blood is centrifuged, the blood cells will be separated and stored as sediment, then the remaining liquid is called plasma.

If the blood is collected in a bottle without anticoagulant, the blood will clot and in this case, if it is centrifuged then the liquid part is called serum. In fact, serum loses the blood clotting component, called prothrombin and fibrinogen, but plasma carries the proteineous blood clotting component.

Plasma contains various protein components (fibrinogen, globulin, albumin, etc.), dissolved minerals (Na +, K +, Ca ++, Mg ++, Cl, HCO3, PO4, SO4), absorbed component as a result of digestion (glucose, Fatty acids, amino acids), tissue waste products (urea, uric acid, creatine, creatinine, ammonium salts), special secretions (hormones and enzymes), antibodies and dissolved gases (oxygen, carbon nitrogen). The sedimentation co-efficiency of major plasma proteins varies from species to species.

The electrolyte (ion) per liter of blood in cod fish (Gadus callarius) is 180 ml of sodium (Na+), 4.9 ml of potassium (K+), 3.8 ml of magnesium (Mg++), 5.0 ml of calcium (Ca++), 5.3 ml of chloride (Cl), 3.1 ml of phosphate (PO4). Concentrations of sodium and chloride are generally lower in freshwater teleost.

Sharks (Squaliformes) have high concentrations of Mg ++ in their blood. However, its blood is mildly alkaline than the blood of higher bony fish (Actinopterygii). The dissolved material in solution indicates the freezing point which can also be measured by osmotic pressure. As the osmotic pressure of the blood increases, water spreads from the permeable membrane to the low density solution.

In freshwater bony fish, the freezing point of plasma is 0.50C. For some freshwater sharks and other fish (elasmobranks) it is 1.00C. For marine bony fish, the value is 0.6-1.00 C. The maximum value in marine elasmobranch is 2.170C. The freezing point of seawater is 2.080C.

Fish have lower plasma proteins than higher vertebrates. The major plasma proteins in fish are albumin (regulating osmotic pressure), lipoprotein (transporting lipids), globulin (binding to hemi part), ceruloplasmin (binding to copper), fibrinogen (helping to blood clots) and iodi-uroforin ( only found in fish, adding inorganic iodine).

The concentration of plasma protein in fish is 2-6 g / liter. The presence of low levels of fibrinogen and prothrombin-like proteins is not associated with rapid blood clotting. Rainbow trout (Salmo gairdneri) can survive above 00 C. At low temperatures, the blood of this fish clots. Because serum of Antarctic fish contains glycoproteins, they can survive at -1.90C. The ratio of albumin and threonine in this protein is 2: 1. Its molecular weight is 2600-33000.

Thyroid-binding proteins such as T3 and T4 are found in the blood plasma of fish. In cyprinid species, it adds vitalogenin. It also contains a variety of enzymes such as CPK, alkaline phosphatase (Alk Pase), SGOT, SGPT, LDH, lipase and carbonic anhydrase and their co-enzymes.

If the serum of some teleosts, especially Anguilla, some catfish (Siluridae), and the tuna (Thunnis) push into the blood of mammals then it shows poisoning reaction.

Types of Blood Corpuscles 

The types of blood cells are mentioned in the following diagram:

1. Red Blood Corpuscles/Erythrocytes

Most fish have red blood cells with round or rectangular nuclei that are located in the center of the cell and are yellowish red in color. Its numbers vary depending on the species, age, season and environmental influences. Its size is large in Elasmobranch and small in teleost. In estuary species such as Fundulus, it is smaller in size than freshwater species.

Red blood cells of deep sea teleosts are larger in size than common teleosts. In species like Clarias batrachus, Notopterus notopterus, Colisa fasciatus, Tor tor, etc., its structure is usually round but in Labeo rohita and Labeo calbasu it is oval in shape.

Some species of Antarctic fish that live in oxygen-rich areas at low and cool temperatures do not have red blood cells. In addition, leptocephalus larvae of eel fish (Anguilla) and some deep-sea fish do not have red blood cells. Their gas exchange occurs through diffusion. The red blood cells of the fish are oval, small and 6 microns in diameter, but in many fish, especially Wrassus (Crenilabrus), the red blood cells are more than 8 microns in diameter. In Protopterus, it is 36 microns of diameter.

The number of red blood cells per cubic mm of blood in fish is 20,000-3,000,000. Inactive fish have a lower number of red blood cells than active fish.

2. White Blood Corpuscles/Leukocytes

There has been considerable research on the white blood cells of fish, so there is no difference in their classification. The number of per cubic mm in the blood of fish is 20,000-150,000. It may be granulocyte or agranulocyte but the number of granulocytes is higher. Granulocytes can be further divided into eosinophils, basophils and neutrophils based on their staining capacity.

Neutrophils and eosinophils have phagocytic properties. Agranular white blood cells are lymphocytes and monocytes. Monocytes produce antibodies. Basophil granulocytes are found in some species, but no function of it has been reported.

A. Agranulocytes

(a) Lymphocytes: Different types of lymphocytes are found in the blood of fish. Their nuclei are round or oval in shape. Lymphocytes are 80-90% of the total white blood cells. It contains a lot of chromatin. Like mammals, freshwater and saltwater fish also have large and small lymphocytes. Large cells contain large amounts of cytoplasm. They do not have granules in their cytoplasm. The main function of lymphocytes is to increase immunity by making antibodies.

(b) Monocytes: Monocytes account for small quantities of white blood cells. However, some fish do not have monocytes. They are thought to originate from the kidneys and are visible when an unwanted object enters the bloodstream. Its cytoplasm is light blue or purple in color. The nucleus is basically large and has a variety of structures. Its main function is to destroy pathogens in the process of phagocytosis.

B. Granulocytes

(a) Neutrophil: Most of the white blood cells in fish are neutrophils. Neutrophils are 5-9% of the total white blood cells in Solvelinus fontinalis and 25% in brown trout. They are named based on the staining capacity of the cytoplasm. Their nuclei are multi-lobed but some fish have neutrophils with bi-lobe nuclei.

In marginal blood smears, the cytoplasm contains pink, red or purple granules. Their nuclei look like human kidneys. Neutrophils react positively with peroxidase and Sudan Black. Neutrophils are active phagocytes. It protects the tissues from inflammation or injury.

(b) Eosinophils: Eosinophils are usually round and its cytoplasm is granular. In acidic solution, it shows dark pink orange or orange red color. Their nuclei are lobed and show dark orange to purple in color.

(c) Basophils: Basophil cells are oval or round in shape. Its cytoplasm is granular and shows dark blue in alkaline solution. The blood of  Anguilled and Plaice do not contain basophils.

3.Thrombocyte/ Platelets

This is also called platelet. They are round, oval or spindle shaped. In mammals, however, the platelets are disc shaped. Fish blood contains platelets which is about half of the total leukocytes.

Herring fish contain 72.2% platelets of red blood cells and only 0.7% of platelets in teleost. Their cytoplasm is granular, the center is more alkaline and its circumference is dull and homogeneous type. In alkaline solution, their cytoplasm shows pink or red color. They help in blood clotting.

Origin of Blood Corpuscles

The process of forming blood cells and blood plasma is called hemopoiesis. In the early embryonic stage, blood cells are produced from the wall of the blood vessel. Red blood cells and white blood cells originate from lymphoid hemoblasts or hemocytoblasts and enter the bloodstream to mature.

In fish, spleen and lymph nodes are involved in the production of blood cells. In chondrichthyes, red blood cells originate from the granulopoietic tissue, the leidig organs, the epigonal organs, and rarely the kidneys. The leidig organ is made up of white tissues and acts like a bone marrow tissues. Such tissues are found in the esophagus but their main source is the spleen. In all these fish, if the spleen is removed, the leidig organs take part in the production of red blood cells.

In teleost, red blood cells and granulocytes originate from the kidneys (pronephros) and spleen. Their spleen has a red cortex on the outside and a medulla with white pulp on the inside. Red blood cells are produced from cortical region of the spleen while lymphocytes and some granulocytes are produced from the medullary region.

The intestinal spiral valves of chondrichthyes and Dipnoi also produce different types of white blood cells. In higher bony fish (Actinopterygii), red blood cells are destroyed in the spleen. The technique of destroying blood cells in jawless fish (Agnatha), Basking shark and Rays is not known.

Thrombocytes originate from the mesonephric kidney of fish, granulocytes originate from the submucosa, liver, gonads, and mesonephric kidney of the digestive tract.

In sharks, rays, and chimaera (chondrichthyes), white blood cells with connective tissues are seen below the mucous membrane of the esophagous. In sturgeon (Acipenser), paddle fish (Polyodon) and South American lung fish, the reddish-brown lobular spongy tissue around the heart produces lymphocytes and granulocytes.

Skull bones of some sharks (Squaliformes), chimaeras (Chimaeridae), Gar (Lepisosteus) and cranial cartilage of Bowfin (Amia) can produce all types of blood cells.

Functions of Blood

Like other vertebrates, the mixed cellular components of blood plasma is found in fish. It consists of a type of one kind of connective tissue and a non-newtonian fluid. Blood flows throughout the body through the cardiovascular system. It is mainly caused by contraction of the heart muscle. Blood has different functions. The functions of blood are given below:

1. Respiration: Blood plays an important role in transporting dissolved oxygen(DO) from water to the gills (respiratory changes) and carbon dioxide(CO2) from the tissues to the gills.

2. Nutrition: The blood carries various nutrients such as glucose, amino acids, fatty acids, vitamins and electrolytes, and secondary elements from the alimentary canal to the tissues.

3. Execration: The waste products produced from blood metabolism such as urea, uric acid, creatine etc. are carried away from the cells. All fish have trimethyl amine oxide in their blood, but its concentration is highest in marine elasmobranch.

Creatine is a type of amino acid that is produced by the metabolism of glycine, arginine, methionine. The amount of creatine in the blood plasma is 10-60 grams and it is excreted through the kidneys.

4. Homeostasis of Water and Electrolyte Concentration: The exchange of electrolytes and other molecules takes place through the blood. The level of glucose in the blood of fish is considered to be a sensitive physiological indicator in most cases and there is no discrepancy in the level of glucose in the blood of fish.

(5) Hormones: There are different types of controlling agents in the blood such as hormones and cellular or humoral agents (antibodies). All these elements are present in different concentrations in the blood which are regulated by the feedback loop and change the concentration and make the necessary components of different organs through the synthesis of hormones and enzymes.

Fish Blood as a Gas Carrier

Oxygen spreads from one liquid to another very slowly. Red blood cells have appeared in fish and other vertebrates to achieve high efficiency in gas transport. This is why one volume of blood can carry 15-25 times more oxygen than water. 99% red blood cells and 1% plasma contribute to this oxygen transport. The red blood cells of fish and other vertebrates contain a type of pigment called hemoglobin. In its presence, the blood turns red and acquires the ability to transport oxygen. In most vertebrates, the molecular weight of a hemoglobin is about 65,000. The oxygen carrying capacity of hemoglobin of some fish is given in the table below:

Table :The oxygen carrying capacity of hemoglobin of some fish


Oxygen Carrying Capacity (Percentage of Blood Volume)

Number of red blood cells (per cubic mm)

Iron content (mg per 100 cc)

Goosefish(Lophius (Sedentary)




Mackerel-(Scomber) (active)




The table above shows that the maximum oxygen holding capacity of Mackerel is 18% but that of mammals is 20%. Oxygen is taken up, transported and released by red blood cells. So we can call this process carriers, loading and unloading.

Table: Differences between plasma and serum



Plasma is the clear, yellowish portion of blood, lymph, or inter-muscular fluid that holds blood cells.

Serum is the clear, yellowish liquid that is obtained after all the solid and liquid components have been separated from it after blood clotting.

It contains fibrin and prothrombin proteins and other soluble blood clotting components.

It does not contain proteins called fibrin and prothrombin.

It helps in blood clotting.

It does not help in blood clotting.

Table: Differences between different types of blood cells of fish

Red blood cells or Erythrocytes

White Blood Cells or Leukocytes

Platelets or Thrombocytes

Most fish have red blood cells with round or rectangular nuclei that are located in the center of the cell.

Their nuclei are round, oval or different in shape.

It is round, oval or spindle shaped.

It is yellowish red in color.

It is white in color.

It is white in color.

In blood of fish, the number of red blood cells per cubic millimeter is 20,000-30,000,000.

The number of White Blood Cells per cubic millimeter is 20,000-100,000.

The number of platelets per cubic millimeter blood of fish is half of leukocytes.

They carry oxygen and play an important role in respiration.

They help prevent disease.

They help in blood clotting.

It is produced from hemoblasts.

It is also produced from hemoblasts.

It originates from the mesonephric kidney of fish.