135
4. THE CIRCULATORY SYSTEM
experimental procedure. However, the type of experimental procedure
used can affect the results obtained. Differences in results reported in
the literature may, therefore, result from the techniques used or reprrsent interspecific or intraspccific variations in function. Thus, in discussing the cardiovascular system of fishes, one must be aware, first
that generalizations rest on studies of only a small number of fish,
and, second, that variations in results may reflect either species differences or differences in the techniques used.
11. BLOOD PRESSURES IN ARTERIES AND VEINS
The branchial or systemic heart in fish is usually the main propulsive
organ for circulating the blood. Contractions of the heart serve to convert chemical energy into mechanical energy in the form of pressure
and flow. The dissipation of energy as blood flows through the vascular
network results in a fall in blood pressure. The hydraulic resistance to
flow is inversely proportional to the fourth power of the radius of a blood
vessel; therefore, the major portion of the drop in pressure occurs across
the smallest blood vessels.
In fish, blood from the heart passes first through the ventral aorta into
the gill circulation and then via the dorsal aorta into the general body
circulation. In the majority of fish, the capillary bed of the gills represents a considerable proportion of the total vascular resistance to flow,
and it is in series with all other capillary networks. In the lungfish, blood
passes from the heart through the branchial vessels into the systemic
and pulmonary circulations. The pulmonary vascular bed is in series
with the gill circulation but in parallel with the systemic circulation
(Johansen and Hanson, 1968). In the lungfish Neoceratodus forsteri
there are capillaries in all gill arches, but in other species of lungfish,
Protopterus aethiopicus and Lepidosiren paradoxa, there are no capillaries
in the first and second gill arches, and branchial vessels connect the
ventral and dorsal aortae directly.
In all fish, except the hagfish, the highest blood pressures have been
recorded from the ventricle during systole; The resistance to flow between ventricle and ventral aorta is low; therefore, when the intervening valves are open, pressures in the ventricle and ventral aorta
do not differ from each other by more than 1 mm Hg. Arterial
pressures reported by earlier workers (see Mott, 1957) are sometimes unreliable because the fish were either restrained, out of water,
ventral side up, anesthetized, or not intact. Johansen (1962) recorded
4. THE CIRCULATORY SYSTEM
experimental procedure. However, the type of experimental procedure
used can affect the results obtained. Differences in results reported in
the literature may, therefore, result from the techniques used or reprrsent interspecific or intraspccific variations in function. Thus, in discussing the cardiovascular system of fishes, one must be aware, first
that generalizations rest on studies of only a small number of fish,
and, second, that variations in results may reflect either species differences or differences in the techniques used.
11. BLOOD PRESSURES IN ARTERIES AND VEINS
The branchial or systemic heart in fish is usually the main propulsive
organ for circulating the blood. Contractions of the heart serve to convert chemical energy into mechanical energy in the form of pressure
and flow. The dissipation of energy as blood flows through the vascular
network results in a fall in blood pressure. The hydraulic resistance to
flow is inversely proportional to the fourth power of the radius of a blood
vessel; therefore, the major portion of the drop in pressure occurs across
the smallest blood vessels.
In fish, blood from the heart passes first through the ventral aorta into
the gill circulation and then via the dorsal aorta into the general body
circulation. In the majority of fish, the capillary bed of the gills represents a considerable proportion of the total vascular resistance to flow,
and it is in series with all other capillary networks. In the lungfish, blood
passes from the heart through the branchial vessels into the systemic
and pulmonary circulations. The pulmonary vascular bed is in series
with the gill circulation but in parallel with the systemic circulation
(Johansen and Hanson, 1968). In the lungfish Neoceratodus forsteri
there are capillaries in all gill arches, but in other species of lungfish,
Protopterus aethiopicus and Lepidosiren paradoxa, there are no capillaries
in the first and second gill arches, and branchial vessels connect the
ventral and dorsal aortae directly.
In all fish, except the hagfish, the highest blood pressures have been
recorded from the ventricle during systole; The resistance to flow between ventricle and ventral aorta is low; therefore, when the intervening valves are open, pressures in the ventricle and ventral aorta
do not differ from each other by more than 1 mm Hg. Arterial
pressures reported by earlier workers (see Mott, 1957) are sometimes unreliable because the fish were either restrained, out of water,
ventral side up, anesthetized, or not intact. Johansen (1962) recorded