b. Serous Space and Two Serous Pericardia. As in all serous cavities, there
is a serous space between two serous membranes.
(1) The visceral pericardium intimately covers the surface of the heart.
Earlier, we referred to this as the epicardium.
(2) The parietal pericardium is the outer serous membrane.
(3) Between the two serous pericardia is a very thin space containing a thin
film of pericardial fluid. This lubricating fluid makes the action of the heart much less
c. Fibrous Pericardium. The parietal pericardium is covered with a very
dense fibrous envelope. This envelope forms the outer portion of the pericardial sac.
Section V. MOTIVE FORCES INVOLVED IN DRIVING THE BLOOD
THROUGH THE SYSTEM
The blood (vehicle for transporting material) is driven through the blood vessels
(conduits) by a variety of motive forces.
10-33. ARTERIAL BLOOD FLOW
Blood is driven through the arteries by a combination of forces. First, there is
the force produced by the contraction of the ventricular walls. Second, there is the
elastic recoil of the arterial walls.
a. Systole. When the left ventricle contracts (systole), it forces the blood into
the aortic arch. Above the base cylinder, the wall of the aortic arch is mainly elastic
FCT. As the blood fills the aortic arch, the walls are stretched.
b. Diastole. When the ventricle relaxes (diastole), the wall of the arch recoils
and presses against the blood. With the closing of the aortic semilunar valve, the blood
is forced to move out along the arteries in a pressure pulse. Since the elasticity of the
arterial walls produces a continuous pressure, the blood moves continuously throughout
c. Arterial Pressures. The highest pressure is called the systolic pressure,
and the lowest pressure is the diastolic pressure.
d. Vasoconstriction. Vasoconstriction is the actual contraction of the arterial
walls. Vasoconstriction can further increase the pressure on the blood in the arteries.