4. THE CIRCULATORY SYSTEM
163
a deeper lying red portion of the myotome, correlated with an ability
to sustain high cruising speeds and muscle temperatures that are above
ambient.
Nothing is known about the regulation of blood flow to the red and
white muscle fibers during exercise. Contractions of skeletal muscle probably create a uis a tergo force driving blood back to the heart. Satchel1
(1965) demonstrated that an isolated trunk preparation of the Port Jackson shark (Heterodontus portusjacksoni) was able to pump and increase
the flow of a perfusate during flection of the trunk. Arteries in the postpelvic region of the trunk arising from the dorsal aorta have valves which
prevent the reflux of blood into the dorsal aorta during muscle contraction. Veins entering the caudal vein in the postpelvic region have
valves which prevent reflux of blood into the segmental vessels. The
caudal vein is encased in bone, and the contracting muscles squeeze
all vessels in that region except the caudal vein propelling blood from
the arterial to venous side of the circulation. Flow through the trunk
preparation was somewhat higher after electrical stimulation than before, indicating that some other factors also play a role in increasing
muscle blood flow during exercise.
2. TEMPERATURE REGULATION
The muscles of fish are cooled by the blood, loss of heat through
the general body surface is insignificant (Carey and Teal, 1969a).
Metabolic heat, which warms the blood, is quickly lost at the gill surface,
where blood comes into close proximity to the water. Water has a high
specific heat, and thermal diffusion is ten times more rapid than gaseous
diffusion; thus, thermal equilibrium between blood and water must
occur very rapidly in the gills. Tuna and lamnid sharks, however, can
maintain muscle temperatures well above ambient. This is particularly
true of the bluefin tuna, Thunnus thynnus, which can control its muscle
temperature so that the warmest part of the muscle mass varies only
6°C over a 20°C range of water temperatures (Carey and Teal, 1969a).
The red muscle, used in sustained swimming, is the warmest part of the
whole muscle mass (Fig. 10). This ability to thermoregulate is related
to the presence of a rete mirabile formed by arteries and veins supplying
the red muscles (Carey and Teal, 1966) and to the encapsulation of
the red muscle by the white muscle. The retia of bluefin tuna are formed
from arteries and veins arising from a pair of cutaneous vessels. The
segmental arteries arising from the dorsal aorta are of secondary importance in the bluefin tuna, unlike skipjack and yellowfin tuna, which
have central as well as lateral retia. The retia act as countercurrent heat
163
a deeper lying red portion of the myotome, correlated with an ability
to sustain high cruising speeds and muscle temperatures that are above
ambient.
Nothing is known about the regulation of blood flow to the red and
white muscle fibers during exercise. Contractions of skeletal muscle probably create a uis a tergo force driving blood back to the heart. Satchel1
(1965) demonstrated that an isolated trunk preparation of the Port Jackson shark (Heterodontus portusjacksoni) was able to pump and increase
the flow of a perfusate during flection of the trunk. Arteries in the postpelvic region of the trunk arising from the dorsal aorta have valves which
prevent the reflux of blood into the dorsal aorta during muscle contraction. Veins entering the caudal vein in the postpelvic region have
valves which prevent reflux of blood into the segmental vessels. The
caudal vein is encased in bone, and the contracting muscles squeeze
all vessels in that region except the caudal vein propelling blood from
the arterial to venous side of the circulation. Flow through the trunk
preparation was somewhat higher after electrical stimulation than before, indicating that some other factors also play a role in increasing
muscle blood flow during exercise.
2. TEMPERATURE REGULATION
The muscles of fish are cooled by the blood, loss of heat through
the general body surface is insignificant (Carey and Teal, 1969a).
Metabolic heat, which warms the blood, is quickly lost at the gill surface,
where blood comes into close proximity to the water. Water has a high
specific heat, and thermal diffusion is ten times more rapid than gaseous
diffusion; thus, thermal equilibrium between blood and water must
occur very rapidly in the gills. Tuna and lamnid sharks, however, can
maintain muscle temperatures well above ambient. This is particularly
true of the bluefin tuna, Thunnus thynnus, which can control its muscle
temperature so that the warmest part of the muscle mass varies only
6°C over a 20°C range of water temperatures (Carey and Teal, 1969a).
The red muscle, used in sustained swimming, is the warmest part of the
whole muscle mass (Fig. 10). This ability to thermoregulate is related
to the presence of a rete mirabile formed by arteries and veins supplying
the red muscles (Carey and Teal, 1966) and to the encapsulation of
the red muscle by the white muscle. The retia of bluefin tuna are formed
from arteries and veins arising from a pair of cutaneous vessels. The
segmental arteries arising from the dorsal aorta are of secondary importance in the bluefin tuna, unlike skipjack and yellowfin tuna, which
have central as well as lateral retia. The retia act as countercurrent heat