|
| |
Cardiac Cycle - Isovolumetric Relaxation (Phase
5)
All Valves Closed
-
When the intraventricular pressures fall
sufficiently at the end of phase 4, the aortic and pulmonic valves abruptly
close (aortic precedes pulmonic) causing the second
heart sound (S2) and the beginning of isovolumetric
relaxation. Valve closure is associated with a small backflow of
blood into the ventricles and a characteristic notch (incisura or dicrotic
notch) in the aortic and
pulmonary artery pressure tracings.
-
After valve closure, the aortic and pulmonary
artery
pressures rise slightly (dicrotic wave) following by a slow decline in
pressure.
-
The rate of pressure decline in the ventricles
is determined by the rate of relaxation of the muscle fibers, which is
termed lusitropy. This relaxation is regulated largely by the
sarcoplasmic reticulum that are responsible for rapidly re-sequestering
calcium following contraction (see
excitation-contraction coupling).
-
Although ventricular pressures decrease during
this phase, volumes remain constant because all valves are
closed. The volume of blood that remains in a ventricle is called the end-systolic
volume and is ~50 ml in the left ventricle. The difference between the end-diastolic
volume and the end-systolic volume is ~70 ml and represents the stroke volume.
-
Left atrial pressure (LAP) continues to rise
because of venous return
from the lungs. The peak LAP at the end of this phase is termed the v-wave.
Jump to other phases:
Phase 1 - Atrial Contraction
Phase 2 - Isovolumetric Contraction
Phase 3 - Rapid Ejection
Phase 4 - Reduced Ejection
Phase 6 - Rapid Filling
Phase 7 - Reduced Filling
RK Revised
04/13/07
|
Cardiovascular Physiology Concepts
