Understand hemolymph through definition. Explore its functions and importance, the method of transport in invertebrates, and why it's necessary for their survival.
Updated: 03/03/2022
What is Hemolymph?
What is Hemolymph?
Circulatory systems function to transport molecules and nutrients around an organism's body and work to remove waste. When examining circulatory systems in animals, circulatory systems are generally divided into two categories, open and closed. Open circulatory systems are generally found in invertebrate species, lacking a backbone, and are simpler in structure while closed circulatory systems are found in vertebrate species, having a backbone, and are more complex.
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Open circulatory systems, unlike closed circulatory systems, lack structured blood vessels (veins and arteries) and blood, and instead possess a fluid called hemolymph that flows freely throughout the body and is stored in a central body cavity called a hemocoel. What is hemolymph? Hemolymph definition is a complex circulatory fluid found in the open circulatory systems of invertebrates that serves similar roles to lymph fluid and blood in vertebrate species. Although often compared to each other, blood and hemolymph differ in both their function and composition. Hemolymph's makeup is dissimilar to blood as it does not contain any red blood cells (erythrocytes) and is composed mostly of water.
Hemolymph Composition
Hemolymph and blood are often compared due to their circulatory behavior in organisms, however, the composition of these two fluids is very different. Like blood being primarily composed of plasma, hemolymph is primarily composed of water. Hemolymph is also composed of salts, carbohydrates, lipids, and large amounts of free amino acids and proteins.
Unlike blood, hemolymph does not contain red blood cells or hemoglobin which results in hemolymph not possessing a red color but rather a clear or slightly yellow or green pigment. Instead, the hemolymph of many organisms contains hemocyanin, a free-floating protein that binds to and transports oxygen to surrounding cells and tissues. Hemolymph can also contain specialized hemocyte cells that help protect the organism from invading pathogens.
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Hemolymph Functions
Hemolymph tissue is a complex fluid that is responsible for transporting nutrients and molecules around the body and removing waste. Vertebrates possess many specialized fluids, such as blood and lymph, to divide up these roles, however, in invertebrates it is up to hemolymph to perform all of these vital functions.
In humans, blood's function is to transport oxygen from the lungs to other tissues around the body and then remove the waste product carbon dioxide from cells and transport it back to the lungs for respiration. Hemolymph performs a similar function of moving oxygen around the body and removing waste. However, because hemolymph does not travel through structured vessels, it is up to the hemocoel to move oxygen freely around the body. This open system is less efficient than a closed system because there is less force pushing the oxygen around the body.
In blood, hemoglobin molecules are responsible for binding to oxygen molecules to transport them to different tissues around the body. Instead of hemoglobin, some invertebrates with open circulatory systems have hemocyanin, a similarly functioning molecule. Hemocyanin is a copper-containing protein found free-floating in hemolymph that binds to oxygen and moves it to cells and tissues throughout the body. This is in contrast to hemoglobin, which is an iron-containing protein that is bound to red blood cells in vertebrates. Hemocyanin is colorless but turns blue once bound to oxygen molecules.
It is important to note that not all invertebrates have hemocyanin; hemocyanin is primarily found in arthropods and mollusks but is absent in many insect species. Many insects have a tracheal system that delivers oxygen from the air directly to their organs and therefore do not need a specialized protein to transport oxygen. Other invertebrates may possess hemoglobin instead of hemocyanin.
Lymph fluid in vertebrates functions as part of the lymphatic and immune system and works to balance fluid levels throughout the body and transport white blood cells around the body. In invertebrates, hemolymph functions as water and chemical storage and increases desiccation resistance in some species. Hemolymph tissue also contains hemocyte cells that are vital to the immune response of invertebrates. These hemocyte cells protect the body by phagocytizing, or engulfing invading bacteria or viruses and removing them from the body.
So how exactly does the hemolymph transport all of these materials around the body? Even though species with open circulatory systems lack structured vessels, many species still possess hearts that work to pump hemolymph throughout different branches of the hemocoel. The hemocoel extends throughout most of an organism's body and in the extended branches molecules and nutrients are exchanged between the hemolymph to neighboring tissues. Honey bees have a heart with a single aorta containing many small holes in it that runs down the length of the insect. This aorta pumps hemolymph out of the heart and into the hemocoel chambers causing the hemolymph to flow through the body, coming in contact with various organs and exchanging nutrients before flowing back to the heart and starting the cycle again.
Importance of Hemolymph
Hemolymph is vital for the functioning of organisms with open circulatory systems where hemolymph functions as both the circulatory fluid and lymph fluid, working to transport and exchange molecules and nutrients throughout a body's cells and remove waste. Open circulatory systems are generally found in the invertebrate phyla of Arthropoda (insects, crab, lobster, etc.) and Mollusca (snails, clams, etc). However, due to the lack of structured vessels, open circulatory systems are less efficient in moving the hemolymph fluid around the body and this is a major reason why these systems are not found in giant organisms. Larger mollusks, such as octopus and squid, developed closed circulatory systems in order to transport fluids throughout their larger bodies.
Open circulatory systems and hemolymph do have some advantages, such as requiring less energy to move throughout the body; this makes it optimum for animals with slow metabolisms and smaller sizes. This system is also ideal for insects that already have tracheal systems to deliver and transport oxygen and do not need specialized vessels.
Lesson Summary
Hemolymph tissue is the circulatory fluid found in invertebrate species with open circulatory systems and is often compared to blood of vertebrate species. Many species in phylum Arthropoda and Mollusca have hemolymph and open circulatory systems. Unlike the closed circulatory system of vertebrate species, open circulatory systems do not contain structured vessels to transport fluids around the body. Instead, the hemolymph flows freely around the body's organs and is contained within the hemocoel. Hemolymph serves to transport oxygen and other molecules around the body and remove waste.
Water makes up the bulk of hemolymph, but hemolymph also contains proteins, carbohydrates, lipids, and other cells. Hemocytes are specialized immune cells that are found in hemolymph. These cells engulf invading pathogens and protect the organism. In some organisms, the hemolymph also contains a protein called hemocyanin. Hemocyanin is similar to hemoglobin in that it binds to oxygen and moves it around the body. However, unlike hemoglobin which is bound to red blood cells, hemocyanin is free-floating. In organisms with open circulatory systems, hemolymph acts as both their circulatory system fluid and their lymphatic system fluid. Although less efficient at transporting molecules than closed systems, open circulatory systems require less energy and are ideal in insect species that do not need oxygen transport due to their complex tracheal systems.
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Frequently Asked Questions
What animals have hemolymph?
Hemolymph is found in some invertebrates, mostly from the arthropod and mollusk phylum. These include insects, snails, lobsters and clams. Not all arthropods and mollusks, such as octopi, have open circulatory systems with hemolymph but instead have closed circulatory systems with blood.
What is hemolymph and its function?
Hemolymph is a fluid tissue found in organisms with open circulatory systems. Hemolymph contains proteins, carbohydrates, lipids, and ions but is mostly made of water. Hemolymph functions to transport these molecules and nutrients around the body as well as to remove waste.
What is the difference between blood and hemolymph?
Blood is used in closed circulatory systems of vertebrate species while hemolymph is found in open circulatory systems of invertebrates. Hemolymph and blood vary greatly in their composition and function. While blood's main purpose is to transport oxygen around the body, hemolymph combines the functions of both blood and lymph fluid, transporting not only oxygen but other molecules and nutrients as well.
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