190 
Regal 
Figure 2. MODEL OF BLOOD FLOW THROUGH A LIZARD HEART (after White, 1969). The ventral 
and posterior portions of the ventricle are cut away. 
The heart fills during ventricular diastole with deoxygenated blood from the body crossing the muscular 
ridge (MR) and pooling in the cavum venosum (CV) and cavum plumonare (CP); at the same time 
oxygenated blood from the lungs pools in the cavum arteriosum (CA). Mixing between the dorsal cham- 
ber (CA) and the ventral chambers (CP and CV) via the interventricular canal (IVC) is prevented by the 
atrioventricular valves which are not shown here. 
During ventricular systole the deoxygenated blood exits from the ventricle into the pulmonary artery 
(PA) prior to any ejection into the aortae (owing to the relatively low resistance of the pulmonary circuit) ; 
then the oxygenated blood in the CA is forced through the IVC and into the right and left aortic arches 
(RAA and LAA). (Mixing between the CP and CV is restricted when the muscular ridge presses against 
the ventral wall of the ventricle which is cut away here. Thus the muscular ridge prevents oxygenated 
blood from the CA from entering the low-resistance pulmonary circuit, and this blood is directed immedi- 
ately to the systemic aortic arches. The atrioventricular valves are shifted away from the IVC and seal 
off the atria from the ventricular chambers during this phase.) 
The degree of equality of the distribution of cardiac output between systemic and pulmonary circuits is 
dependent upon the relative resistances of the circuits. High pulmonary vascular resistance during the 
apneic phase of respiration (turtles) or during diving produces a shunt. Such a shunt around the pul- 
monary circuit may also be effective during basking (see text). 
