Diastole and how the heart works in physiology

Diastole is the moment when the heart muscle relaxes and blood can collect in the heart again. If systole is ejection, diastole is filling. It is the alternation between the two that keeps the circulation in rhythm.

Why is diastole important for the heart’s work?

The heart is not a simple pump that only pushes blood out. Half of its work happens when it is seemingly “resting”. That resting moment is diastole, the relaxation of the heart muscle. Without it, the ventricles do not fill with blood and the next systole cannot be effective.

The heart is a cone-shaped, hollow, muscular organ. It sits in the anterior mediastinum, mostly in the left side of the chest cavity. A person’s heart is roughly the size of their fist and weighs ~300 g. The heart has a broader part, the base, a narrowed part, the apex, and 3 surfaces: anterior, posterior and inferior.

The human heart has four chambers: 2 atria and 2 ventricles. The main function of diastole is to ensure the continuous movement of blood through the vessels. Blood carries oxygen, nutrients and immune-related substances, and helps keep the body’s internal environment stable.

A simple truth: the heart is not just a number on an athlete’s heart rate watch. It is a rhythmic system in which contraction and relaxation must follow each other precisely.

Systemic and pulmonary circulation

The blood vessels form 2 circuits: the systemic and the pulmonary circulation. The systemic circulation begins in the left ventricle. From there, blood flows into the aorta, arteries, arterioles, capillaries, veins and finally the venae cavae, which empty into the right atrium.

The systemic circulation carries arterial blood to the organs. In the tissues, the oxygen is released and the blood becomes venous. The blood then collects in the veins, travels through the superior and inferior venae cavae and reaches the right atrium. Here, arterial blood flows in the arteries and venous blood in the veins.

The pulmonary circulation begins in the right ventricle. Blood flows into the pulmonary trunk, pulmonary arteries, pulmonary arterioles, capillaries, veins and the pulmonary veins, which empty into the left atrium. In the pulmonary capillaries, the blood becomes oxygen-rich again: O2 is taken up and CO2 is released.

In the pulmonary circulation, the picture is reversed: venous blood flows in the arteries and arterial blood in the veins. An artery is a vessel that carries blood away from the heart. A vein brings blood back toward the heart.

Diastole, systole and the cardiac cycle

The heart’s work consists of rhythmically repeated contractions and relaxations of the atria and ventricles. In the cardiac cycle, contraction is systole. Relaxation is diastole. They do not happen at random, but in a fixed order.

• First, both atria contract simultaneously. Blood moves from the atria into the ventricles, and this phase lasts 0.1 s.
• Next, both ventricles contract at the same time, which lasts 0.3 s.
• After contraction, the ventricles relax. This is diastole, and it lasts 0.4 s.

Together, systole and diastole make up one cardiac cycle. Systole means ejecting blood from the ventricle. Diastole means blood collecting in the heart again. There is no fundamental difference between the right and left sides of the heart in the duration of the cycle or in volume changes. The difference lies mainly in the pressures the ventricles generate.

Atrial contraction adds an extra amount of blood to the ventricles already filled during relaxation. This makes up 8-10% of the total volume. The ventricular filling volume is 70-80 ml, and each ventricle holds 150 ml of blood, known as the end-diastolic volume.

At rest, with each beat the heart pumps about 70 ml of blood from each ventricle into the circulation. If the heart rate is 70 beats/min, about 294 l of blood circulates in one hour. Over 24 hours, that comes to 7096 l. In well-trained people, the volume pumped can be even higher.

The work of the right ventricle is 6-7 times smaller than that of the left ventricle. The reason is simple: the pressure in the pulmonary artery is lower than aortic pressure by the same factor. Even so, stroke volume and cardiac output are equal in both ventricles.

Properties of the heart muscle and diastole

Cardiac muscle has several properties that allow it to work rhythmically and without conscious control. The first is automaticity. This means the heart’s ability to contract rhythmically regardless of external stimulation. Excitation arises in the heart itself and is passed on through the conduction system to different regions of the heart muscle.

The other key properties are excitability, conductivity and contractility. Conductivity means that the impulse travels along the excitation conduction system. Contractility means the ability of the heart muscle to contract. These properties are needed for systole and diastole to alternate precisely.

After being excited, the heart muscle becomes unresponsive to new stimuli for a short time, which is known as the refractory period. Depending on the heart rate, it can last 0.13-0.20 sec. During this time, the heart muscle does not accept new stimuli. This prevents sustained contraction and lets the heart work rhythmically.

The absolute refractory period lasts 0.20 0.13 s. Alongside it, a relative refractory period is distinguished, during which the heart muscle can still respond to a very strong stimulus. This is called supernormality.

Respiratory arrhythmia means a fluctuation in heart rate: the rhythm speeds up on inhalation and slows down on exhalation. Sinus arrhythmia is likewise a change in rhythm, one that originates from the systolic natural rhythm centre.

ECG, heart sounds and mechanical signs

When the electrical phenomena that accompany heart activity are recorded from the body surface, the result is an electrocardiogram, or ECG. Electrodes are usually attached to the arms and legs: I RA, LA; II RA, LL; III LA, LL.

On an electrocardiogram, the deflections are labelled with the letters P, Q, R, S and T. The P wave arises as excitation spreads through the atria. The Q, R and S waves form the QRS complex and correspond to depolarisation of the ventricles. The T wave shows repolarisation of the ventricles. The time measured from the start of the P wave to the start of the Q wave shows the spread of excitation from the atria to the ventricles. The time from the start of the Q wave to the end of the T wave is the electrical systole of the ventricles.

In a healthy person, all 5 waves can be distinguished. If the shape of the waves or the intervals between them change, this can give information about the cardiac cycle, the conduction system and the state of the myocardium. The contractile force of the heart muscle depends, among other things, on its pre-contraction length, on how fully it fills with blood in diastole, and on its energy and oxygen reserves.

Sounds, or heart sounds, are also produced during diastolic work. The first, systolic sound occurs at the start of ventricular systole. The second, diastolic sound occurs when the semilunar valves close during ventricular diastole. A third and a fourth heart sound are also distinguished. The third is linked to vibration of the ventricular walls during the filling phase. The fourth occurs during atrial systole, at the end of the filling phase.

When listening, you can usually hear the first sound, which is lower and longer, and the second sound, which is higher and shorter. Recording heart sounds is called phonocardiography, and the resulting graph is a phonocardiogram. Listening to heart sounds helps a doctor assess the condition of the valves. Murmurs can occur when the valves do not close fully or when the openings between the heart chambers have narrowed.

Mechanical signs include the apex beat, vibrations of the heart sounds transmitted to the chest, the arterial pulse and the venous pulse. The apex beat gives information primarily about the left ventricle. The venous pulse reflects changes in the filling of the right side of the heart and of the veins close to the heart during the cardiac cycle.

Blood pressure, diastole and the movement of blood in the circulation

Blood flows from an area of higher pressure to an area of lower pressure. The volumetric flow rate depends on the pressure difference and on the resistance to blood flow. The volumetric flow rate of blood is the same in the aorta, pulmonary trunk, arteries, capillaries and veins, but the linear velocity differs from place to place.

Linear velocity means the speed at which blood moves forward in the vessels. It depends mainly on the total cross-sectional area of the vessels: the smaller the lumen, the higher the speed. Blood moves fastest in the aorta, at about 0.5 m/s. During contraction, in the ejection phase, the linear velocity of the blood reaches its maximum.

Venous blood flow is supported by the muscle pump, the suction-pressure pumping action of breathing and the valve-plane mechanism of diastole. Venous return is also affected by body position, muscle work and thermal loads.

Blood pressure depends on the volume of blood in the circulation, the viscosity of the blood, the systolic minute volume, and the resistance of the vessels and capillaries. Pressure is highest in the aorta and lower in the venae cavae. The difference between these pressures is what sets the blood moving.

In the aorta and in the large arteries close to the cardiac cycle, pressure is pulsatile. During the ejection phase of the left ventricle, pressure reaches its maximum. This is called systolic pressure. After contraction ends and the semilunar valves close, pressure in the aorta falls to the level of diastolic pressure by the end of diastole.

In an adult, systolic blood pressure is usually 110-125 mmHg and diastolic 65-80 mmHg. Normotension means that blood pressure is within the normal range. Hypotension means lowered blood pressure. Hypertension means raised blood pressure.

Measuring blood pressure and the effect of training load

Blood pressure can be measured 1) directly, that is invasively, by inserting a cannula connected to a manometer into a vessel, or 2) indirectly, that is non-invasively. In everyday practice, the Korotkoff method is used, in which arterial blood pressure is measured on the brachial artery with a cuff and a stethoscope.

The cuff is placed around the upper arm. The pressure is raised until the artery beneath the cuff closes and blood flow stops. The pressure is then released slowly. When the Korotkoff sounds appear, the systolic arterial pressure is recorded. When the sounds fade sharply or disappear, this corresponds to the diastolic arterial pressure.

During physical work, diastolic work becomes more intense. At rest, the frequency of diastolic contractions fluctuates between 60 and 80. In a physically trained person, systolic work increases mainly through greater contraction force and less through a faster rhythm. In an untrained person, contractions become more frequent as the training load increases.

A newborn’s heart beats about 140 beats per minute. A faster heartbeat is also seen in the elderly, for example 90-95 beats/min. Bradycardia means slower contractions. Tachycardia means accelerated heart activity and can occur in illnesses with a high temperature.

Stroke volume is the amount of blood that the left ventricle ejects into the aorta and the right ventricle into the pulmonary artery with a single contraction. Cardiac output shows how much blood the two ventricles send into the circulation in one minute. During physical exertion, the volume of blood flowing through the working muscles increases significantly, reaching up to 85% of all the blood in circulation.

FAQ: diastole and circulation

What is the difference between systole and diastole?

Systole is the contraction of the heart muscle and the ejection of blood from the ventricles. Diastole is the relaxation of the heart muscle and the filling of the heart with blood before the next beat.

Why does diastolic blood pressure matter?

Diastolic blood pressure shows the pressure level when the heart is relaxed. It helps assess the load the blood vessels are under between heartbeats.

How does physical training load change the way the heart works?

Under training load, muscles need more blood and oxygen. The heart responds by changing its heart rate, contractile force and cardiac output. In a trained person, this happens more economically.

Source: WHO – physical activity.

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