4. THE CIRCULATORY SYSTEM
137
The total peripheral resistance to blood flow probably decreases during swimming in the trout (Stevens and Randall, 1967a,b) but increases
during hypoxia ( Holeton and Randall, 1967a). The pulmonary circulation in the lungfish has a lower resistance to blood flow than the systemic
circulation ( Johansen and Hanson, 1968). When the lungfish, Protopterus
aethiopicus, breathes, blood flow to the lungs increases, whereas during
motor activity, systemic blood flow increases.
Venous pressures in fish range from slightly below atmospheric to
about 10 mm Hg. Contractions of the heart within a noncompliant pericardium in elasmobranchs and lungfish create subatmospheric pressures
in the venous system near the heart (Satchel1 and Jones, 1967; Johansen
and Hanson, 1968), producing aspiratory forces which increase venous
return to the heart. Thus blood flow in veins near the heart in these
animals is determined by the pumping action of the heart and skeletal
muscles and aspiratory forces resulting from contractions of the heart
within a noncompliant pericardium. The relative importance of these
factors will be determined by the size of the vascular resistance to flow
between the heart and the vein in question and the magnitude of the
subatmospheric pressure created in the venous system by contractions
of the heart. Flow in the vena cava of the lungfish, Protopterus aethiopicus, is largely determined by aspiratory forces when the animal is resting
and by the pumping activity of skeletal muscle during exercise. Flow in
the pulmonary vein of lungfish is largely dependent in the pumping
action of the heart, but aspiratory forces do produce small increases in
blood flow (Johansen and Hanson, 1968).
Pressures in the cardiovascular system of hagfish, are very low and are
maintained by the action of the branchial and several accessory hearts
(Johansen, 1960; Chapman et al., 1963; Jensen, 1965). The hearts work
independently of each other and circulate blood to a specific area of the
body. The branchial heart pumps blood from the body and liver into
the ventral aorta. The portal heart pumps blood from the gut and
anterior cardinal vein to the liver. The action of skeletal muscles on a
cartilaginous plate in the caudal region propels blood forward from large
subcutaneous sinuses. This cartilaginous plate, the associated skeletal
muscles, and a pair of lateral sacs with valves to regulate the direction
of blood flow constitute the caudal heart. The activity of the skeletal
muscles of this heart is initiated by impulses from the central nervous
system. Both the branchial and portal hearts are aneural (Chapman et al.,
1963). Systolic arterial pressures are of the order of only a few mm Hg,
and are determined by the interaction of the activity of the various
hearts. Chapman et al. (1963), working on the Pacific hagfish, Eptatretus
stoutii, noted that the branchial heart often stops and occasionally the
137
The total peripheral resistance to blood flow probably decreases during swimming in the trout (Stevens and Randall, 1967a,b) but increases
during hypoxia ( Holeton and Randall, 1967a). The pulmonary circulation in the lungfish has a lower resistance to blood flow than the systemic
circulation ( Johansen and Hanson, 1968). When the lungfish, Protopterus
aethiopicus, breathes, blood flow to the lungs increases, whereas during
motor activity, systemic blood flow increases.
Venous pressures in fish range from slightly below atmospheric to
about 10 mm Hg. Contractions of the heart within a noncompliant pericardium in elasmobranchs and lungfish create subatmospheric pressures
in the venous system near the heart (Satchel1 and Jones, 1967; Johansen
and Hanson, 1968), producing aspiratory forces which increase venous
return to the heart. Thus blood flow in veins near the heart in these
animals is determined by the pumping action of the heart and skeletal
muscles and aspiratory forces resulting from contractions of the heart
within a noncompliant pericardium. The relative importance of these
factors will be determined by the size of the vascular resistance to flow
between the heart and the vein in question and the magnitude of the
subatmospheric pressure created in the venous system by contractions
of the heart. Flow in the vena cava of the lungfish, Protopterus aethiopicus, is largely determined by aspiratory forces when the animal is resting
and by the pumping activity of skeletal muscle during exercise. Flow in
the pulmonary vein of lungfish is largely dependent in the pumping
action of the heart, but aspiratory forces do produce small increases in
blood flow (Johansen and Hanson, 1968).
Pressures in the cardiovascular system of hagfish, are very low and are
maintained by the action of the branchial and several accessory hearts
(Johansen, 1960; Chapman et al., 1963; Jensen, 1965). The hearts work
independently of each other and circulate blood to a specific area of the
body. The branchial heart pumps blood from the body and liver into
the ventral aorta. The portal heart pumps blood from the gut and
anterior cardinal vein to the liver. The action of skeletal muscles on a
cartilaginous plate in the caudal region propels blood forward from large
subcutaneous sinuses. This cartilaginous plate, the associated skeletal
muscles, and a pair of lateral sacs with valves to regulate the direction
of blood flow constitute the caudal heart. The activity of the skeletal
muscles of this heart is initiated by impulses from the central nervous
system. Both the branchial and portal hearts are aneural (Chapman et al.,
1963). Systolic arterial pressures are of the order of only a few mm Hg,
and are determined by the interaction of the activity of the various
hearts. Chapman et al. (1963), working on the Pacific hagfish, Eptatretus
stoutii, noted that the branchial heart often stops and occasionally the