become closed for whatever reason, that part of the heart wall formerly supplied nutrient
blood by the closed vessel will very likely die.
d. Pericardial Sac. The average heart contracts in what is known as a heart
beat, about 70-80 times a minute. To reduce the frictional forces that would be applied
to its moving surfaces, the heart is enclosed in a special serous sac known as the
pericardium ("around the heart").
9-8. CARDIOVASCULAR CIRCULATORY PATTERNS
See figure 9-4 for an illustration depicting cardiovascular circulatory patterns.
Figure 9-4. Cardiovascular circulatory patterns.