4. THE CIRCULATORY SYSTEM
157
60
50
-
N
0
E
.C 40
E
z
X
-
-
2
s
0 )
-
30 t
/
X / x
Stroke volume
20 2
2
4
6
8
1 0
12
Temperature (" C )
12
C
.E
E
8 1
Y
4
Fig. 9. The effect of temperature on heart rate, stroke volume, and cardiac output
In the lingcod, Ophiodon elongutus (Stevens et ul., 1969). From D. J. Randall
(1968), Am. Zoologist 8, 179-189.
to increased levels of catecholamines and increases in venous return to
the heart.
3. THE REGULATION OF VENOUS RETURN
An increase in venous return will increase end diastolic volume and
increase stroke volume as described by Starling's law. An increase in
venous return can be achieved in a number of ways. A decrease in total
peripheral resistance will increase the uis Q tergo effect of the heart on
venous return. In Chondrichthyes, increased cardiac activity will lower
the negative pressure within the rigid pericardium and raise the aspiratory forces moving venous blood back to the heart. Mobilization of blood
from storage organs and venous reservoirs will also aid venous return.
The pumping activity of skeletal muscle in the presence of suitably