Functions and electrocardiographic pattern
Diastole is not a simple passive waiting for the next contraction. It is a critically important stage that performs several key functions at once:
- Provides the myocardium with the necessary time to rest.
- Ensures the restoration of energy reserves in heart muscle cells.
- Guarantees full preparation for the next systole through adequate chamber filling.
On the electrocardiogram, the relaxation process is closely associated with the isoelectric line. The TP segment, which lasts about 370 ms, reflects the resting membrane potential of cardiomyocytes and coincides with the phase of total diastole (when both the atria and ventricles are relaxed). If we add the P wave of the next cycle to this time, we get the TP interval. This is what lasts 470 ms and strictly corresponds to the total duration of ventricular diastole.
Relaxation period (Phases 6 and 7)
The dynamics of diastole begin with a sharp drop in myocardial tension. The relaxation period is divided into two sequential phases:
- Phase 6: Protodiastolic period. This is the very beginning of relaxation. In the right ventricle, this process takes 50 ms, and in the left, 40 ms. At this moment, the pressure in the aorta and pulmonary trunk continues to decrease smoothly as blood flows peripherally to organs and tissues. Protodiastole ends with the closure of the semilunar valves.
- Phase 7: Isovolumic (isometric) relaxation.
Mechanism and hemodynamics: As the ventricles relax, the pressure inside them rapidly drops and becomes lower than in the large outflow vessels (aorta and pulmonary trunk). Due to the resulting pressure difference, blood rushes back toward the heart. This backflow instantly slams the cusps of the semilunar valves shut. The impact of the blood against the closed cusps causes them to vibrate, which we hear as the II sound (diastolic).
Throughout the entire relaxation period, the atrioventricular valves remain closed.
Filling period (Phases 8 and 9)
As soon as the pressure in the ventricles drops below the atrial pressure, the filling period begins. It is also divided into two phases:
- Phase 8: Rapid filling.
- Phase 9: Slow filling (diastasis).
Mechanism: Filling occurs passively. Blood that has accumulated in the atria flows down the pressure gradient into the empty and relaxed ventricles.
During this period, the state of the valvular apparatus changes: the atrioventricular valves open, allowing blood flow through, while the semilunar valves remain securely closed, preventing the return of blood from the arterial bed.
Briefly: answers to topical questions
What is the duration of the isovolumic relaxation phase?
The duration of the isovolumic (isometric) relaxation phase is 60 ms for the right ventricle and 80 ms for the left ventricle. During this period, the following processes are observed:
- Preservation of volume — the ventricular volume remains constant because all valves are closed and blood is incompressible.
- Drop in pressure — the metric sharply decreases to 5 mmHg in the right ventricle and 10 mmHg in the left.
- Myocardial changes — muscle relaxation occurs with a decrease in tension, but without fiber elongation.
To avoid confusing the valves at the beginning of diastole, remember: the relaxing ventricle first "closes the exit door" (the semilunar valves snap shut, the II sound is heard), and only then "opens the entrance door" (the atrioventricular valves open for filling).
Class and exam questions
What causes the generation of the II heart sound?
The second sound arises due to the impact of the blood backflow against the closed cusps of the semilunar valves. The resulting vibration of the cusps creates the acoustic phenomenon.
What is the difference between the TP segment and the TP interval?
The TP segment (370 ms) corresponds to total diastole and the resting potential, while the TP interval (470 ms) includes the TP segment and the subsequent P wave, reflecting the total duration of ventricular diastole.
What happens to aortic pressure during protodiastole?
The pressure in the aorta (and pulmonary trunk) continues to decrease. This happens due to the continuous outflow of blood through the vascular bed to organs and tissues.
How does the state of the valves change during the filling period?
During the filling period, the atrioventricular valves open, allowing blood to flow from the atria. Meanwhile, the semilunar valves remain closed.
What else to study on this topic
- Mechanisms of isometric myocardial relaxation
- Features of the slow filling phase (diastasis)
- Formation of the resting membrane potential (TP segment)
- Hemodynamics of the pulmonary trunk and aorta in protodiastole
Master the entire topic in the "Physiology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Ventricular diastole" and another 4,128 pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 114 video lessons on the subject
Other topics in the "Cardiac Physiology" section
Free video lessons: PhysiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.