A lymph heart is an organ which pumps

lymph Lymph () is the fluid that flows through the lymphatic system, a system composed of lymph vessels (channels) and intervening lymph nodes whose function, like the venous system, is to return fluid from the tissues to be recirculated. At the ori ...

in lungfishes,

amphibians Amphibians are ectothermic, anamniote, anamniotic, tetrapod, four-limbed vertebrate animals that constitute the class (biology), class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all Tetrapod, tetrapods, but excl ...

reptiles Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...

, and flightless birds back into the circulatory system. In some amphibian species, lymph hearts are in pairs, and may number as many as 200 in one animal the size of a

worm Worms are many different distantly related bilateria, bilateral animals that typically have a long cylindrical tube-like body, no limb (anatomy), limbs, and usually no eyes. Worms vary in size from microscopic to over in length for marine ...

, while newts and

salamander Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All t ...

s have as many as 16 to 23 pairs of lymph hearts. Lymph hearts are thought to have evolved in ''

Rhipidistia Rhipidistia, also known as Dipnotetrapodomorpha, is a clade of lobe-finned fishes which includes the tetrapods and lungfishes. Rhipidistia formerly referred to a subgroup of Sarcopterygii consisting of the Porolepiformes and Osteolepiformes, a de ...

''. Mammals have lost the lymph heart as a centralized organ, instead having the lymph vessel themselves contract to pump lymph.

and other amphibians

The lymphatic system of a frog consists of

lymph vessels The lymphatic vessels (or lymph vessels or lymphatics) are thin-walled vessels (tubes), structured like blood vessels, that carry lymph. As part of the lymphatic system, lymph vessels are complementary to the cardiovascular system. Lymph vessel ...

, lymph heart, lymph spaces and

spleen The spleen (, from Ancient Greek '' σπλήν'', splḗn) is an organ (biology), organ found in almost all vertebrates. Similar in structure to a large lymph node, it acts primarily as a blood filter. The spleen plays important roles in reg ...

Lymphatics and lymph

As lymph is a filtrate of blood, it closely resembles the plasma in its water content. Lymph also contains a small amount of metabolic waste and a much smaller amount of

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

than that of blood. Lymph vessels carry the lymph and, in the frog, open into the four lymph hearts. These lymph hearts are located on the dorsal side of frog's body. The front pair is situated below the shoulder blades. The posterior pair is on either side of a long, rod-like bone called a urostyle, formed by the fusion of the last few vertebrae. The anterior pair opens into the

subclavian vein The subclavian vein is a paired large vein, one on either side of the body, that is responsible for draining blood from the upper extremities, allowing this blood to return to the heart. The left subclavian vein plays a key role in the absorption ...

and the posterior pair into the

femoral vein In the human body, the femoral vein is the vein that accompanies the femoral artery in the femoral sheath. It is a deep vein that begins at the adductor hiatus (an opening in the adductor magnus muscle) as the continuation of the popliteal v ...

. The pair near the third vertebra pumps lymph into the jugular vein. The other pair at the end of the

vertebral column The spinal column, also known as the vertebral column, spine or backbone, is the core part of the axial skeleton in vertebrates. The vertebral column is the defining and eponymous characteristic of the vertebrate. The spinal column is a segmente ...

pump lymph into the iliac vein in the legs. The position of mammalian jugular lymph sacs coincide with that of amphibian anterior lymph hearts.

Mechanism of the lymph hearts

The lymph hearts rhythmically and slowly pump to drive the lymph into the veins. It is possible to see the lymph hearts beat by looking on the dorsal surface on either side of the urostyle. In the toad, the normal lymph heart rate is about 50 beats per minute. Thus the lymph emerging out of blood ultimately merges into the blood. It returns the proteins back to blood. Amphibian lymph hearts are made up from three tissue layers analogous to the three layers of blood vessels. There are the '' tunica interna intima'' made up of endothelial cells, ''

tunica media The tunica media (Neo-Latin "middle coat"), or media for short, is the middle tunica (layer) of an artery or vein. It lies between the internal elastic lamina of the tunica intima on the inside and the tunica externa on the outside. Artery The ...

'' made up of muscle, and the ''

tunica externa The tunica externa (Neo-Latin "outer coat"), also known as the tunica adventitia (Neo-Latin "additional coat"), is the outermost tunica (biology), tunica (layer) of a blood vessel, surrounding the tunica media. It is mainly composed of collagen an ...

'' for anchor to organs. The

MyoD MyoD, also known as myoblast determination protein 1, is a protein in animals that plays a major role in regulating muscle differentiation. MyoD, which was discovered in the laboratory of Harold M. Weintraub, belongs to a family of proteins kn ...

-expressing muscle is developmentally more similar to skeletal muscle than to heart muscle.

In fish

There was great debate as to whether

teleost Teleostei (; Ancient Greek, Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts (), is, by far, the largest group of ray-finned fishes (class Actinopterygii), with 96% of all neontology, extant species of f ...

fish have a lymphatic system. Current evidence shows that they likely have a "primitive" version. They do not seem to have lymph hearts, instead using blood pressure to move lymph. Lungfish, which are more related to

tetrapods A tetrapod (; from Ancient Greek τετρα- ''(tetra-)'' 'four' and πούς ''(poús)'' 'foot') is any four- limbed vertebrate animal of the clade Tetrapoda (). Tetrapods include all extant and extinct amphibians and amniotes, with the lat ...

, use many "micropumps" to move lymph around, reaching densities as high as 100 per cubic millimeter in the fins of South American lungfish.

In reptiles and birds

Lymph heart size and pump rate

These hearts vary in size from microscopic in lungfish to an estimated 20-liter capacity in some of the largest

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...

s. In newts,

frogs A frog is any member of a diverse and largely semiaquatic group of short-bodied, tailless amphibian vertebrates composing the order Anura (coming from the Ancient Greek , literally 'without tail'). Frog species with rough skin texture due to ...

and

turtle Turtles are reptiles of the order (biology), order Testudines, characterized by a special turtle shell, shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Crypt ...

s they pump at rates higher than the blood heart and the volumes pumped are quite remarkable. In toads and frogs, this volume can amount to about 1/50 the output of blood from the heart. In amphibians, lymph hearts lie at vein junctions. Frogs and salamanders have 10 to 20 lymph hearts, while caecilians have more than 100. Conversely, reptiles have a single pair of lymph hearts in the pelvic area. In flightless (

ratite Ratites () are a polyphyletic group consisting of all birds within the infraclass Palaeognathae that lack keels and cannot fly. They are mostly large, long-necked, and long-legged, the exception being the kiwi, which is also the only nocturnal ...

) birds, the lymph heart function is less clear and the two almond-sized hearts located near the

spinal column The spinal column, also known as the vertebral column, spine or backbone, is the core part of the axial skeleton in vertebrates. The vertebral column is the defining and eponymous characteristic of the vertebrate. The spinal column is a segmen ...

close to the hip joint are thought to be involved in inflating and deflating the phallus with lymph, which is of a significant size in both sexes of

emu The emu (; ''Dromaius novaehollandiae'') is a species of flightless bird endemism, endemic to Australia, where it is the Tallest extant birds, tallest native bird. It is the only extant taxon, extant member of the genus ''Dromaius'' and the ...

s and ostriches.

References

{{Reflist Animal physiology