Overview

Diastole is the portion of the cardiac cycle during which the heart muscle relaxes after contraction and the chambers refill with blood. It contrasts with systole, when the heart contracts to eject blood. Proper diastolic function is essential for adequate cardiac output because filling during relaxation determines the volume available for the next beat. In everyday language, diastolic pressure is often referred to as the lower number in a blood pressure reading, reflecting arterial pressure while the heart is relaxed.

Phases and mechanics

Diastole is not a single moment but a sequence of events that coordinate the left and right sides of the heart. The main subphases commonly described are:

  • Isovolumetric relaxation — valves close and the ventricles relax without changing volume.
  • Rapid ventricular filling — blood flows from the atria into the ventricles when the atrioventricular valves open.
  • Diastasis — a slower filling phase as pressures equilibrate.
  • Atrial contraction (atrial systole) — a final push from the atria tops up ventricular volume just before the next systole.
These steps depend on coordinated valve function, chamber compliance (the ability of the ventricle to stretch), and appropriate timing between atrial and ventricular activity.

Relation to blood pressure and heart rate

Diastolic arterial pressure is the pressure in the large arteries when the heart is relaxed and is measured in millimetres of mercury (mmHg) in clinical practice. The length of diastole varies with heart rate: when heart rate increases, diastole shortens proportionally more than systole. Because filling time falls at higher rates, very rapid heartbeats can reduce ventricular filling and cardiac output if diastolic time becomes too brief.

Clinical importance and common disorders

Abnormal diastolic function can cause symptoms of breathlessness and fatigue even when systolic contraction (the pumping ability) appears normal. Conditions associated with impaired diastole include increased stiffness of the ventricle due to aging or hypertrophy, high blood pressure, ischemic heart disease, and restrictive cardiomyopathies. Clinicians may refer to "diastolic dysfunction" or to heart failure with preserved ejection fraction (HFpEF) when relaxation or filling is inadequate despite a preserved ejection fraction.

Measurement and assessment

Diastolic performance is assessed in several ways. Blood pressure measurement provides the arterial diastolic value. Echocardiography offers detailed assessment of ventricular filling patterns, chamber sizes, valve function and estimates of filling pressures. Electrocardiography and invasive catheter studies can give additional timing and pressure information when needed. Interpreting these tests requires integrating symptoms, imaging, and risk factors such as hypertension and coronary disease.

History, terminology and notable facts

The word "diastole" comes from the Greek term for stretching or opening. The concept of diastole as a distinct part of the heartbeat developed over centuries as instruments and physiological understanding improved. Notable practical points:

  • In blood pressure readings the diastolic value is the lower number and indicates arterial pressure during relaxation.
  • Diastolic murmurs and extra sounds can signal specific valve or filling abnormalities.
  • Age and chronic high blood pressure commonly alter diastolic properties of the heart and vessels.
Understanding diastole is central to diagnosing and managing many forms of cardiovascular disease because both filling and relaxation directly influence how the heart meets the body's demands.