Hemolymph, or haemolymph, is a fluid, analogous to the
blood
Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the cir ...
in
vertebrates, that circulates in the interior of the arthropod (invertebrate) body, remaining in direct contact with the animal's tissues. It is composed of a fluid plasma in which hemolymph cells called
hemocytes are suspended. In addition to hemocytes, the plasma also contains many chemicals. It is the major tissue type of the
open circulatory system
The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, t ...
characteristic of
arthropods (e.g.
arachnids,
crustacean
Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapoda, decapods, ostracoda, seed shrimp, branchiopoda, branchiopods, argulidae, fish lice, krill, remipedes, isopoda, isopods, barnacles, copepods, ...
s and
insects). In addition, some non-arthropods such as
molluscs possess a hemolymphatic circulatory system.
Oxygen-transport systems were long thought unnecessary in insects, but ancestral and functional
hemocyanin has been found in the hemolymph.
Insect "blood" generally does not carry
hemoglobin
Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythroc ...
, although hemoglobin may be present in the tracheal system instead and play some role in respiration.
Method of transport
In the
grasshopper, the closed portion of the system consists of tubular hearts and an aorta running along the dorsal side of the insect. The hearts pump hemolymph into the sinuses of the
hemocoel
The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, t ...
where exchanges of materials take place. The volume of hemolymph needed for such a system is kept to a minimum by a reduction in the size of the body cavity. The hemocoel is divided into chambers called sinuses.
Coordinated movements of the body muscles gradually bring the hemolymph back to the dorsal sinus surrounding the hearts. Between contractions, tiny valves in the wall of the hearts open and allow hemolymph to enter. Hemolymph fills all of the interior (the
hemocoel
The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, t ...
) of the animal's body and surrounds all cells. It contains
hemocyanin, a
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pink ...
-based protein that turns blue when oxygenated, instead of the
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
-based
hemoglobin
Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythroc ...
in
red blood cells found in vertebrates, giving hemolymph a blue-green color rather than the red color of vertebrate blood. When not oxygenated, hemolymph quickly loses its color and appears grey.
The hemolymph of lower arthropods, including most
insects, is not used for oxygen transport because these animals
respirate through other means, such as tracheas, but it does contain nutrients such as proteins and sugars. Muscular movements by the animal during
locomotion can facilitate hemolymph movement, but diverting flow from one area to another is limited. When the
heart
The heart is a muscular Organ (biology), organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as ca ...
relaxes, hemolymph is drawn back toward the heart through open-ended pores called ostia.
Note that the term "
ostia" is not specific to insect circulation; it literally means "doors" or "openings", and must be understood in context.
Constituents
Hemolymph can contain nucleating agents that confer extra cellular freezing protection. Such nucleating agents have been found in the hemolymph of insects of several orders, i.e.,
Coleoptera (beetles),
Diptera (flies), and
Hymenoptera.
Inorganic
Hemolymph is composed of
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
,
inorganic salts (mostly
sodium
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
,
chlorine,
potassium
Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmos ...
,
magnesium, and
calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
), and
organic compounds (mostly
carbohydrate
In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may o ...
s,
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s, and
lipid
Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids in ...
s). The primary oxygen transporter molecule is
hemocyanin.
Amino acids
Arthropod hemolymph contains high levels of free amino acids. Most amino acids are present but their relative concentrations vary from species to species. Concentrations of amino acids also vary according to the arthropod stage of development. An example of this is the silkworm and its need for glycine in the production of silk.
Proteins
Proteins present in the hemolymph vary in quantity during the course of development. These proteins are classified by their functions: chroma proteins, protease inhibitors, storage, lipid transport, enzymes, the vitellogenins, and those involved in the immune responses of arthropods. Some hemolymphic proteins incorporate carbohydrates and lipids into the structure.
Other organic constituents
Nitrogen metabolism end products are present in the hemolymph in low concentrations. These include
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous ...
,
allantoin,
uric acid, and
urea. Arthropod
hormones are present, most notably the juvenile hormone.
Trehalose can be present and sometimes in great amounts along with
glucose
Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...
. These sugar levels are maintained by the control of hormones. Other
carbohydrate
In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may o ...
s can be present. These include
inositol,
sugar alcohol, hexosamines,
mannitol,
glycerol
Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...
and those components that are precursors to
chitin.
Free
lipid
Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids in ...
s are present and are used as fuel for flight.
Hemocytes
There are free-floating cells, the
hemocytes, within the hemolymph. They play a role in the arthropod
immune system
The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinte ...
. The immune system resides in the hemolymph.
Comparisons to vertebrates
This
open system might appear to be inefficient compared to the closed circulatory systems of the
vertebrates, but the two systems have very different demands placed on them. In vertebrates, the circulatory system is responsible for transporting oxygen to all the tissues and removing carbon dioxide from them. It is this requirement that establishes the level of performance demanded of the system. The efficiency of the vertebrate system is far greater than is needed for transporting nutrients, hormones, and so on, whereas in insects, exchange of oxygen and carbon dioxide occurs in the
tracheal system. Hemolymph plays no part in the process in most insects. Only in a few insects living in low-oxygen environments are there hemoglobin-like molecules that bind oxygen and transport it to the tissues. Therefore, the demands placed upon the system are much lower. Some arthropods and most molluscs possess the
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pink ...
-containing
hemocyanin, however, for oxygen transport.
Specialist uses
In some species, hemolymph has other uses than just being a blood analogue. As the insect or arachnid grows, the hemolymph works something like a hydraulic system, enabling the insect or arachnid to expand segments before they are
sclerotized. It can also be used hydraulically as a means of assisting movement, such as in
arachnid locomotion. Some species of insect or arachnid are able to
autohaemorrhage
Autohaemorrhaging, or reflex bleeding, is the action of animals deliberately ejecting blood from their bodies. Autohaemorrhaging has been observed as occurring in two variations. In the first form, blood is squirted toward a predator. The blood o ...
when they are attacked by predators.
Queens of the ant genus ''
Leptanilla
''Leptanilla'' is a genus of ant in the subfamily Leptanillinae. Like other genera in this subfamily, the queen is fed by the hemolymph of their own larvae, which have specialized processes for this purpose. ''S. swani'' is the only Australia ...
'' are fed with hemolymph produced by the
larvae.
[Genus ''Leptanilla'']
Australian Ants Online On the other hand, ''
Pemphigus spyrothecae'' utilize hemolymph as an adhesive, allowing the species to stick to predators and subsequently attack the predator; it was found that with larger predators, more aphids were stuck after the predator was defeated.
See also
*
Insect physiology Insect physiology includes the physiology and biochemistry of insect organ systems.
Although diverse, insects are quite similar in overall design, internally and externally. The insect is made up of three main body regions (tagmata), the head, thor ...
*
Respiratory system of insects
References
Sources
* {{cite book , last=Chapman , first=R.F. , title=The Insects: Structure and Function , url=https://archive.org/details/insectsstructure0000chap , url-access=registration , publisher=Cambridge University Press , edition=4th , year=1998 , location=Cambridge , isbn=978-0-521-57890-5
External links
* ''
Boston Globe'
articleon hemolymph
Blue Squid Blood
Blood
Arthropod anatomy
Insect anatomy
Crustacean anatomy
Arachnid anatomy
Body fluids