158
D. J. RANDALL
placed valves is another powerful mechanism for increasing venous
return during exercise.
Hagfish possess several accessory hearts, namely, the portal, cardinal,
and caudal hearts (Chapman et al., 1963). The action of these hearts
plus the pumping action of contractions of the gill musculature (Johansen, 1960) aid the circulation of the blood and regulate venous return
to the systemic (branchial) heart.
The effectiveness of skeletal muscle contractions in increasing venous
return is dependent on the presence of valves directing blood flow toward
the heart. Valves have been demonstrated in the segmental arteries and
veins in the caudal regions of the Port Jackson shark, Heterodontus portus
jucksoni. These valves are at the entrance and exit of the myotomal muscles and are so arranged that the contracting muscles pump blood from
the arterial to the venous side of the circulation, thus increasing venous
return to the heart (Satchell, 1965). Venous valves have been found
in telcosts (Dornesco and Santa, 1963). Blood flow in the dorsal aorta
of some teleosts may be augmented by the presence of a stiff ligament
(de Kock and Symmons, 1959) extending into and partially dividing
the lumen of the dorsal aorta. The ligament extends from the basioccipital to the caudal region and undulations of the body, prcssing the walls
of the dorsal aorta against this stiff ligamcnt, may propel blood in a
caudal direction.
Mobilization of blood from storage organs or venous reservoirs may
aid venous rcturn and increase cardiac output. Excrcise in trout is accompanied by a decrease in splenic volume (Stevens, 1968). Hepatic
vein sphincters in elasmobranchs regulate the volume and transit time for
blood in the liver (Johansen and Hanson, 1967), and their relaxation
would permit the mobilization of blood stored in the liver at times
of increased demand on cardiac output. The hepatic vein sphincters relax
in response to the topical application of adrenaline and contract in response to acetylcholine. Lowering the dorsal fin in the ratfish decreases
the amount of blood in the dorsal fin sinus and may be used to increase
venous return in this fish ( Hanson, 1967).
IV. GENERAL PROPERTIES OF THE
CARDIOVASCULAR SYSTEM
A. Cardiac Output
Three methods have been used to detcrrnine cardiac output in fish.
These are the application of the Fick principle (Mott, 1957; Holeton
D. J. RANDALL
placed valves is another powerful mechanism for increasing venous
return during exercise.
Hagfish possess several accessory hearts, namely, the portal, cardinal,
and caudal hearts (Chapman et al., 1963). The action of these hearts
plus the pumping action of contractions of the gill musculature (Johansen, 1960) aid the circulation of the blood and regulate venous return
to the systemic (branchial) heart.
The effectiveness of skeletal muscle contractions in increasing venous
return is dependent on the presence of valves directing blood flow toward
the heart. Valves have been demonstrated in the segmental arteries and
veins in the caudal regions of the Port Jackson shark, Heterodontus portus
jucksoni. These valves are at the entrance and exit of the myotomal muscles and are so arranged that the contracting muscles pump blood from
the arterial to the venous side of the circulation, thus increasing venous
return to the heart (Satchell, 1965). Venous valves have been found
in telcosts (Dornesco and Santa, 1963). Blood flow in the dorsal aorta
of some teleosts may be augmented by the presence of a stiff ligament
(de Kock and Symmons, 1959) extending into and partially dividing
the lumen of the dorsal aorta. The ligament extends from the basioccipital to the caudal region and undulations of the body, prcssing the walls
of the dorsal aorta against this stiff ligamcnt, may propel blood in a
caudal direction.
Mobilization of blood from storage organs or venous reservoirs may
aid venous rcturn and increase cardiac output. Excrcise in trout is accompanied by a decrease in splenic volume (Stevens, 1968). Hepatic
vein sphincters in elasmobranchs regulate the volume and transit time for
blood in the liver (Johansen and Hanson, 1967), and their relaxation
would permit the mobilization of blood stored in the liver at times
of increased demand on cardiac output. The hepatic vein sphincters relax
in response to the topical application of adrenaline and contract in response to acetylcholine. Lowering the dorsal fin in the ratfish decreases
the amount of blood in the dorsal fin sinus and may be used to increase
venous return in this fish ( Hanson, 1967).
IV. GENERAL PROPERTIES OF THE
CARDIOVASCULAR SYSTEM
A. Cardiac Output
Three methods have been used to detcrrnine cardiac output in fish.
These are the application of the Fick principle (Mott, 1957; Holeton