Diastole is the relaxation phase of the heart muscle during which blood flows from the atria into the ventricles during the early filling phase when the leaflet valves are open. In the subsequent late filling phase, further blood is actively delivered to the ventricles by contraction of the atria. In the systole that follows, blood is pumped from the ventricles into the systemic circulation and pulmonary circulation by contraction of the heart muscle.
What is diastole?
Diastole is the relaxation phase of the heart muscle during which blood flows from the atria into the ventricles during the early filling phase when the leaflet valves are open. Diastole, the relaxation and filling phase of the two chambers of the heart (ventricles), is followed by systole, the tightening, contraction and ejection phase of the ventricles. Diastole and systole together form a complete sequence of a heartbeat sequence that repeats (almost) regularly. The timing of the contraction and relaxation phases of the heart muscles within a complete heartbeat sequence reveals the heart rhythm. In healthy people, it follows a specific pattern that can be measured by means of an electrocardiogram (ECG). The repetition rate per minute in humans at rest is about 60 to 70 beats, depending on athletic fitness and age. The two atria of the heart undergo a comparable rhythm that is out of phase with the rhythm of the ventricles. During diastole of the ventricles, the atria undergo their systolic phase and vice versa. Diastole of the ventricles can be divided into three main phases. It begins with the relaxation phase immediately following the contraction phase. During the relaxation or unwinding phase, all 4 heart valves are briefly closed. During the early filling phase that follows, the two leaflet valves that provide the connection between the left atrium and left ventricle, or right atrium and right ventricle, open. Blood flows from the atria into the ventricles. During the ensuing systole of the atria, another volume of blood is actively pumped from the atria into the ventricles.
Function and Purpose
The heart’s beating sequence of systole and diastole maintains the necessary blood circulation. Oxygen-rich blood from the pulmonary veins is pumped into the aorta, the major artery of the body, and oxygen-poor blood from the body’s veins is pumped into the pulmonary arteries. The main phases of the ventricles occur almost in parallel and are initiated electrically by the sinus node in the right atrium. The electrical contraction impulses reach the ventricular muscles with a time delay via the AV node, the His bundle, and the Purkinje fibers, which respond accordingly by initiating systole. Diastole and systole must be considered virtually as a unit, since they cannot proceed independently. The relaxation phase during diastole forms the prerequisite for the subsequent contraction phase, because after the contraction phase the cardiac muscle cells need a short time of about 100 milliseconds for their repolarization, the prerequisite for receiving a renewed contraction impulse. Diastole is responsible for filling the ventricles with blood. To ensure that the blood in each case is venous blood and not blood that the ventricles have previously pumped into the large artery of the body, the aorta, and into the pulmonary artery, the two pocket valves, the pulmonary valve and the aortic valve, must close and remain closed throughout diastole. Because the two pocket valves operate on the principle of a check valve, they passively close when the residual blood pressure in the arteries, the diastolic blood pressure, exceeds the pressure in the ventricles. During the pressure buildup in the systolic phase, the blood pressure in the chambers exceeds the diastolic pressure in the arteries, allowing them to reopen and pump blood into the arteries. The heart rate is adaptable according to the demands of the body, especially the muscles, within a spectrum of about 60 to a maximum of 200 beats per minute. However, because a disruption in the succession of diastole and systole can immediately become life-threatening, it has evolved evolutionarily so that the succession of the heart rhythm is largely autonomous, with its own electrical stimulus generation including two backup systems and its own stimulus transmission through modified heart muscle cells.
Diseases and ailments
Arterial blood pressure is composed of the separate systolic and diastolic values. Normal values are approximately 80 mmHg (diastolic arterial blood pressure) to 120 – 140 mmHg (systolic arterial blood pressure). Deviations may occur due to a variable demand profile during increased physical stress to which the cardiovascular system reacts. The “residual pressure” prevailing in the arteries during diastole depends mainly on the factors of physical demand, hormonal status, elasticity of the arterial vessel walls, thickness and elasticity of the ventricular muscles, and the functional efficiency of the pulmonary and aortic valves. The phase sequence of the largely autonomously controlled heart rhythm can also influence diastolic blood pressure in the arteries. The multitude of influencing factors already suggest that malfunctions in one or more organs that affect blood pressure and or heart rate can lead to symptoms and discomfort. One of the most common problems is cardiac arrhythmia, which leads to a kind of dysfunction of the beating phases. The best known cardiac arrhythmia is the so-called atrial fibrillation, which is usually triggered by chronic high blood pressure. Infographic on the anatomy and causes of cardiovascular disease. Click on image to enlarge. Atrial fibrillation usually manifests itself in a permanently high pulse rate of about 150 beats per minute, whereby the atria can move the blood “in circles” in a completely disordered manner, which is associated with considerable loss of power and the risk of blood clot formation that can trigger a stroke. Atrial fibrillation, unlike ventricular fibrillation, is not immediately life-threatening and can usually be treated with medication (beta blockers) and electrocardioversion (electric shock).
