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Block diagram showing the simplest type of fish circulatory system. Solid black vessels contain blood of lower oxygen content; white vessels contain blood with higher oxygen content. Arrows indicate direction of blood flow. From Mott (1957).

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Serves all bodily functions (respiration, excretion, osmoregulation, digestion).  System of arteries, veins, and capillaries.  Only a few fish species have been investigated extensively: (Hagfish Dogfish Skate Port  jackson  shark Trout Salmon  Carp Cod  Eel   Lungfishes  (Farrell 1993).).

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The primary circulatory system :  Most fish is a "closed" system T ypically  consists of a single  heart Pump  in line with bronchial (gill) and systemic (body) capillary beds connected by arteries and  veins. Hagfish circulatory systems have accessory inline hearts.  Lungfish also differ by the presence of a pulmonary circulation and partial mixing of oxygenated and deoxygenated blood in the  heart. A variety of circulatory adaptations can be found in fishes having accessory respiratory surfaces in cavities, at the skin, or in the  gut.

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Fishes involves a single pump, single-circuit system: Heart             Gills       body and back         heart.            (As shown in the fig).   H eart is located posterior and ventral to the gills. Located farther anterior in  teleosts than in  chondrichthyans . (exception:   lies in a membranous pericardial cavity,  lined with parietal pericardium.  ( The basic fish heart consists of four chambers in series: venous blood enters  Sinus  venosus   (a thin-walled sac) from the ducts of Cuvier and the hepatic veins; it next flows into Atrium ; then into Ventricle , a thick-walled pump; and finally blood flows out of the heart into   Conus  or  bulbus   arteriosus : B arrel-shaped chamber invested with cardiac muscles ( Chondrichthyes , Dipnoi).           The  muscular  conus   arteriosus   is replaced by the  nonmuscular   bulbus   arteriosus   ( actinopterygian ). 	   B ulbus   arteriosus :              onion-shaped elastic reservoir, passively dilated with blood, exits the ventricle.  Bulbus  dampens pressure oscillations, providing continuous rather than pulsed blood supply to the body.

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Lungfishes: atrium and ventricle are partly divided by a partition partially separating oxygenated and deoxygenated blood,  a step toward development of the two pump four-chambered heart of  tetrapods . Australian,  Neoceratodus :  Division is least complete least dependent on atmospheric air. B.  South American,  Lepidosiren: Most complete M ost dependent on atmospheric air for respiration.  Heart valves prevent back flow of blood maintain pressure in the circulatory system. Valves may be present between each of the sections of the heart.   Sinoauricular  valves:   S eparate the sinus  venosus  and atrium.   Auriculoventricular  or  atrioventricular  valves:   vary in number depending on the group. Chondrichthyes  and most  bony fishes :  T wo rows of valves;  Bowfin:  four rows , North American Paddlefish:  five rows , gars and  Polypterus : six rows.

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valves are  absent in lungfishes , do have valves in the  conus   arteriosus .  Number of  ventriculobulbar  valves is  related to the length of the  conus .  Usually one or, rarely, two in bony fishes, 2-7 in  Chondrichthyes , and up to 74 in eight rows in  gars. Valves outside the heart region  can occur in various parts of the circulatory system of fishes, such as segmental arteries and veins in the caudal regions ( Port Jackson Shark and teleost veins ).  Blood   heart muscle from  anterior  hypobranchial   arteries in  Chondrichthyes ,  Actinopterygii , and  Neoceratodus . Lepidosiren:  coronary supply originates from the  second afferent artery.  Hagfishes  have no special coronary circulation; they also differ in other regards. Nervous  innervation   of all fish hearts, except in the hagfish, is from the  vagus .

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Inactive fishes: very small hearts, less than 1 part per 1000 parts body weight.  More active fishes: relatively large hearts.  e.g. mackerels, tunas:  heart makes up 1.2 parts per 1000 parts body weight, and in flying fishes ( Exocoetidae ): heart constitutes 2.1 parts per 1000 parts body weight.

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Postcardinal vein
Subclavian artery
Dorsal aorta
Efferent branchial
artery
Carotid artery
Anterior
cardinal vein
Afferent
branchial artery
Ventral aorta
Jugular vein
Caudal artery
Caudal vein
Heart
Superior
mesenteric artery
Coeliac artery
Common cardinal vein

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Afferent branchial arteries: Bring oxygen deficient blood  to the  gills from the ventral aorta  (varies).  Hagfishes and lampreys : 7-14  (depending on number of gill pouches).  Most  Chondrichthyes :  4 , but sharks and rays:  Hexanchus , Six-gilled Shark: 5 Seven gilled Shark,   Heptranchias : 6    Lungfishes: 4 -5 afferent branchial arteries Bony fishes:4 (Fig. 4.5).  B.  Efferent branchial arteries:   bring oxygenated blood from the gills to the rest of the body.  Merge to form the  dorsal aorta , the largest and longest artery in a fish’s body.  Efferent branchial arteries number: one per  hemibranch   in  Chondrichthyes    one per  holobranch  in bony fishes . Internal carotid arteries run from the aorta to the brain.   Major veins:  facial, orbital, postorbital, and cerebral join into  paired anterior cardinal veins. Empties  into the common cardinal ( duct of Cuvier ) and then into the heart.  jugular vein:  collects blood from the  lower head  also empties into the common cardinal in  Actinopterygii . Pseudobranch: M ay provide oxygenated blood to the visual system (Many fishes).

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Dorsal aorta:  M ain route of transport of  oxygenated blood. Transports   from the gills to the rest of the body. (Fig. 4.6).  Lies directly ventral to the vertebral column, trunk region. Gives off major vessels and segmental arteries. known as the caudal artery. Enters  the closed  haemal  canal of the caudal vertebrae Subclavian  artery:   goes to the pectoral girdle,  coeliaco -mesenteric artery supplies the viscera, and the iliac or renal artery supplies the kidneys.

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From most of the body is the  postcardinal  vein. Best developed on the right side and empties into the  common cardinal or ducts of Cuvier             sinus  venosus                    heart proper.   Advanced  tunas:  Additional pair of large arteries,  cutaneous  arteries, exit the dorsal aorta posterior to the  coeliaco -mesenteric artery and run laterally between the ribs.  Arteries approach the fish’s skin, divide into two vessels, each runs  posteriorly , sending out arterioles to the underlying  red muscle .  passes through an extensive network of capillaries – the countercurrent heat exchanger that retains metabolic heat in the red muscle – cutaneous  vein returns the unheated blood to the heart.  Phylogenetically : M ost advanced tunas show the greatest development of the subcutaneous circulatory system (Fig. 4.7).

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The primary circulatory system of most fish is a "closed" system that typically consists of a single heart as the pump in line with  bnmchial  (gill) and systemic (body) capillary beds connected by arteries and veins (Fig. 4.9). Recent evidence shows numerous fluid connections with the secondary circulation system. Hagfish circulatory systems have accessory inline hearts. Lungfish also differ by the presence of a pulmonary circulation and partial mixing of oxygenated and  cleoxygenated  blood in the heart (Randall 1970). A variety of circulatory  aclaptations  can be found in fishes having accessory respiratory surfaces in cavities, at the skin, or in the gut(Satchell1976). Given the diversity of circulatory adaptations in fishes, it is instructive to examine aspects of the cardiovascular anatomy to better  unclerstand  circulatory function. Of paramount importance is the heart

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Dorsal
Atrium
Atrium
Conus
arteriosus
Sinus
venosus
Ventricle
Shark
Sinus
venosus
Apex
Ventricle
Bulbus
arteriosus
'frout

Diagrams of the heart (A) a shark ( elasmobranch ) and (B) a trout (teleost). Modified from Randall (1968).

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Swi m bindder
artery
Co m mon
aorta
branchial arteries
Head and
nterior Regions
artery
ills
Ventral aorta
Gut
Liver
Hepatic
Gonad
portal
Vein Caudal
Caudal
Posterior
Ody
portal
System

Diagramatic  representation of the major vascular pathways (arterial, gray; venous, black) in (A) a  teleostean  fish. Arrows represent the direction of blood flow.

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aorta
artery
Head a nd
t Regio
Rena
artery
Caudal
artery
Caudal
Kidne
Body

Diagramatic  representation of the major vascular pathways (arterial, gray; venous, black) in (B) an African  lungfish.Arrows  represent the direction of blood flow.

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Ophthal mic
artery
Efferent
pse udobranch
arte ry
Orbital
artery
Hyoi dean
pseudobranch
Efferent
hyoid e an
artery
Afferent
branchial
artery
Lateral dorsal aorta
Efferent
branchial
arteries
Ventral
Chrsal aorta
Coeliac artery
Mesenteric artery
Sutxlavian artery
Gill filaments
trium
Fourth gill arch
Ducts Of Cuvier
Hepatic veins
Sinus venosus
Ventricle
Bulbus arteriosus

Gills and blood vessels of the head of a cod ( Gadus ). From  Lagler  et al. (1977).

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Derived from the  venous part of the blood vascular system ( similar to other vertebrates). Lymph is collected by  paired and unpaired ducts and sinuses,  empty into the main blood system.   Hagfishes and lampreys: M ore connections to the venous system; have a  hemolymph  system.  At least some species of  lampreys and bony fishes have contractile lymph “hearts ”. Hagfishes and lampreys:  have the greatest volume among fishes. Chondrichthyes :  lymph vessels but do not have  sinuses or contractile lymph  “hearts”.

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The pulsed (systole: emptying; diastole: filling) propulsion of blood through the circulatory system of the most fishes is accomplished by a four-chambered heart (Fig. 4.10).  All four chambers are in line and pump only venous blood  Except for a few air-breathing fishes, all flow of blood is to the gills.  The heart and gills are closely associated. The heart is enclosed in the fluid-filled pericardial cavity,  separatecl   fróm  the peritoneal cavity by the pericardial wall. First chamber of the fish heart: sinus  venosus Act as a manifold; i.e. venous blood from the liver and the ducts of Cuvier are collected in this relatively thin-walled chamber.  From there, the  blood is directed to the atrium , in many species through a  sinoatrial  valve. Sinus  venosus  provides the initial transition from smooth to pulsed flow, the atrium provides the first significant circulatory acceleration of the blood.  Atrium is a relatively larger than sinus  venosus , empties ventrally to the ventricle, via the  atrioventricular  valve, in a biphasic manner.   Biphasic filling : B lood starts to fill the ventricle during ventricular diastole (early phase, constitutes 42-73% of the total flow (e.g. teleosts) and continues to  fill immediately after  atrial  systole ( atrial  phase).

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Liver blood Liver  Ducts of  cuvier Venous blood  Sinus venous (first chamber thin walled) Atrium (through  Sinuatrial  Valve)

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Ventricle: Pyramid-shaped: elasmobranchs Conical, apex pointing  posteriorly :  Teleosts relatively large chamber, heavy walls of cardiac muscle (Fig. 4.10). provide the main propulsive force for circulatory flow. Ventricular walls : may be composed of two layers of muscle:  cortex and spongy myocardium.  Cortex : relatively dense cardiac muscle (compact myocardium), generally receives oxygen and nutrients from the coronary artery. well developed in active  specíes  ( skípjack  tuna rainbow trout) (Cameron 1975).  73 of 93  teleostean  species they  examíned  had no ventricular cortex.( Santer  and Greer Walker (1980) Generally more sluggish species  compared  wíth  the 20 that had ventricular cortexes. All14 sharks and rays  examíned  had a cortex.  As rainbow trout  íncreased   swímming  velocity from 0.15 to 1.0 body length/s in  normoxíc  water,  coronary blood flow increases (~110%) paralleled those of  carcliac  power.  Under

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hypoxia, resting coronary blood flow was 35% higher than that under  normoxia , and it increased further (=150% of resting levels) with a velocity at 1.0 body length/s swimming ( Gamperl  et al.1995).  Spongy myocardium:  (More universal inner layer of the ventricular myocardium),  consists of a spongy mesh, supplied with oxygen and nutrients only by the venous blood.  Indeed, cardiac performance in fishes without a compact myocardium may be limited by the low venous oxygen supply under hypoxic conditions (Farrell1991).  Fourth chamber  (  conus   arteriosus  in elasmobranchs, lampreys, hagfish, and  holosteans , and  bulbus   arteriosus  in teleosts)  does not increase the acceleration of the blood.  Acts as an elastic chamber to dampen the pulses of pressure and intermittent flow from the ventricle into a more continuous flow to the ventral aorta and the gills.

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Bulbus  wall consists only of elastic tissue and layers of smooth muscle;  it features no valves ( Priede  1976).  Conversely, the  conus : Many valves (up to 72 in gars ( Lepisosteus )   cardiac musculature in the walls.  more rigid membrane (pericardium), elasmobranchs is thought to produce a more active "rebound" between contractions of the heart, assists in its filling from the veins.   conal  valves ensure significant reverse flow of blood back into the heart does not occur during this rebound.   conus  is more poorly developed in lampreys and  hagfish,a  single pair of valves.  This separate return emanates from a special pulmonary vascular circuit, which is coupled in parallel with the normal systemic and branchial circulation.  The partially divided ventricle moves the arterial (from the lungs) and the venous (from the body) blood through a  bulbus   cordis , which largely maintains separation of the two flows by spiral ridges that twist throughout its length.  The arterial blood is conveyed to the body, whereas the venous blood passes through the functional gills,  sorne  of it subsequently entering the lung. The extent of the separation of flows in the heart correlates with dependence on air breathing. The Australian lungfish, which cannot withstand lengthy air exposure, displays the least separation. African and South American lungfish, however, which must withstand periodic droughts in their natural habitats, show more complete separation of flows.

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CARDIOVASCULAR CONTROL  Fish control their hearts and  carcliovascular  systems with several  aneural  and neural mechanisms.   Aneural  Cardiovascular Control:  Effected by changes in blood volume, by direct responses of heart muscle to temperature changes, and by secretions from various organs.  Mobilization of blood: Associated with spleen, liver, or a blood sinus in various species. For example, Yamamoto (1988)  clescribed  the contraction of the spleen ( decrease in weight and hemoglobin concentration by 47-85%  ancl  56-93%, respectively) in five species ·of freshwater teleosts after 3 to 5 min of severe exercise.  As explained earlier (see Section 4.1), an autonomic nervous signal triggers this contraction.  The extra erythrocytes  woulcl  increase blood oxygen  ancl  buffering (see Chapter 6) capacities, thereby increasing the fish's ability to be active.  Temperature  acts as another  aneural  regulator of circulation by direct action on the pacemakers in the myocardium (Randall1970).

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Secretions  of hormones that affect either the heart or the relative constriction or dilation of the blood  vessels Catecholamines: Epinephrine , affect both the heart and the resistance to flow in various vascular beds.  Levels  of circulating  epinephrine  and  norepinephrine   rise with exercise in rainbow trout . Nakano and Tomlinson (1967)    Epinephrine stimulates increases in HR at relatively low temperatures ( e.g., 6°C), and  Qsv  increases in warmer water ( e.g., l5°C) ( Bennion  1968).  Studies  using  blocking  agents ( e.g.,  inderal )  have  shown that the heart has receptor  sites  that primarily are epinephrine sensitive.  The  interplay of temperature, myocardial stretching, and epinephrine as controlling mechanisms determines the frequency/stroke volume characteristics under the various conditions .   Other studies have shown that primarily  norepinephrine -sensitive receptors  predominate  in constricting systemic vascular beds, whereas epinephrine-sensitive receptors dilate gill vasculature in rainbow trout (Wood and Shelton 1975).  Removal  and metabolism of circulating  catecholamines  apparently occurs in the gills ( Colletti  and Olson 1988).

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Neural Cardiovascular Control  The  hearts of all fish except hagfish are innervated by a branch of the 10th cranial nerve  ( vagus )  (Randall 1970 ).   Stimulation of the lamprey  vagus  produces an increased HR, whereas  elasmobranch  or  teleostean   vagal  stimulation slows the HR .   Because these  stimuli mimic  the effects of acetylcholine , these fibers are termed cholinergic.  Several factors  may alter the level of  vagal  tone  (level of fiber excitement), although stimuli  threshold levels  vary  considerably  between species.  Light  flashes,  suelden  movements of objects or shadows, touch, or mechanical vibrations usually promote a  bradycardia   in teleosts  and elasmobranchs  by increasing the level of  vagal  tone.  Atropine  injections sufficient to block cholinergic  innervation  (the  vagal  nerve supply to the heart) attest to the neural origin of these responses.  Stevens  ancl   Ranclall  (1967)  found  no  vagal  tone in resting rainbow trout, but they  d id find  that  atropine injections  into a large-scale sucker (  Catostomus   macrocheilus ) increased the resting HR from 38 to 55  bpm (beats per minute). Swimming  in this sucker also  recluced  the inhibitory  vagal  tone, thereby elevating the HR.  Several teleosts  also possess (adrenergic) stimulatory fibers from the  vagus  as an additional neural control  mechanism.  Changes  in coronary blood flow with exercise also appear to be  under adrenergic  (not cholinergic) control in rainbow trout  .

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