diastole
Summary
Diastole is the phase of the cardiac cycle in which the heart muscle relaxes and the chambers fill with blood. In ventricular diastole the semilunar valves are closed and the AV valves open. Diastolic pressure is the lower number in a blood pressure reading.
Detail
Meaning: Diastole is the relaxation phase of the cardiac cycle. At its start the ventricles relax, and pressure in the aorta and pulmonary artery exceeds ventricular pressure, so the semilunar valves snap shut (the second heart sound, "dub"). Ventricular pressure then falls below atrial pressure, the AV valves (tricuspid and mitral) open, and blood flows in. Most filling is passive. Atrial contraction adds the final portion. Between valve closure and AV valve opening there is a brief isovolumetric relaxation when all valves are closed. Arterial pressure falls during diastole but stays above zero because elastic arteries recoil.
Equation: MAP ≈ Pdia + ⅓(Psys − Pdia), where MAP is mean arterial pressure, Pdia is diastolic pressure and Psys is systolic pressure. All are in Pa, though mmHg is used clinically.
On the MCAT: Passages show a pressure-time or pressure-volume graph of the left ventricle, atrium and aorta. You identify which valves are open at a given point. You may also predict that a faster heart rate shortens diastole more than systole, which reduces filling time and end-diastolic volume. Higher peripheral resistance raises diastolic pressure.
Common confusion: Diastole is not a time when the whole heart is closed off. The AV valves are open for most of it. The 120/80 reading is systolic over diastolic, and the 80 is the arterial pressure during ventricular relaxation.
Sources
- Campbell Biology
- Human Physiology: An Integrated Approach
- Principles of Anatomy and Physiology
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.