Diastole (pron.: //) is the period of time when the heart refills with blood after systole (contraction). Ventricular diastole is the period during which the ventricles are relaxing, while atrial diastole is the period during which the atria are relaxing. The term diastole originates from the Greek word διαστολη, meaning dilation.
Inside the heart 
During ventricular diastole, the pressure in the (left and right) ventricles drops from the peak that it reaches in systole. When the pressure in the left ventricle drops to below the pressure in the left atrium, the mitral valve opens, causing accumulated blood from the atrium to flow into the ventricle.
This is not a suction mechanism. Instead, the heart is refilled by the momentum of the blood still flowing from the previous systolic cycle. As the left and right atria contract during systole, the blood pressure in each atrium increases, forcing additional blood into the ventricles. This is known as atrial kick and it "hoists" the ventricular myocardium over the mass of blood contained within the chamber. 80% of the blood flows passively down to the ventricles, so the atria do not have to contract a great amount.
The ventricular filling velocity or flow into the ventricles have two main components; First an early (E) diastolic one caused by accumulation of blood in the atria during previous systole, and second, a late one created by atrial contraction (A). The E/A ratio can be used as a diagnostic measure, since it is reduced in diastolic dysfunction.
Atria and ventricles undergo diastole at the same time.
Inside the arteries 
The adjective "diastolic" is used to refer to filling of the heart with blood between muscle contractions. It is used to describe the opposite portion of the cardiac cycle related to contraction. More typically it is used as one component of measurement of blood pressure. "Diastolic pressure" refers to the lowest pressure within the arterial blood stream occurring during each heart beat. The other component of blood pressure is systolic pressure, which refers to the highest arterial pressure during each heart beat.
Clinical notation 
When blood pressure is stated for medical purposes, it is usually written as a seeming "ratio" of systolic to diastolic pressure; for example: 120/80 mmHg. This is not intended to be read as a ratio and cannot be legitimately read as a ratio. It is not a display of a numerator over a denominator but rather a medical notation used for quickly showing the two clinically significant pressures involved and cannot be reduced into lower terms.
See also 
- Diastole. Merriam-Webster Online Dictionary. 24 August 2008.
- Advanced Biology for You - Gareth Williams
- Abdul Latif Mohamed, Jun Yong, Jamil Masiyati, Lee Lim, Sze Chec Tee. The Prevalence Of Diastolic Dysfunction In Patients With Hypertension Referred For Echocardiographic Assessment of Left Ventricular Function. Malaysian Journal of Medical Sciences, Vol. 11, No. 1, January 2004, pp. 66-74