The contractility of the heart is the force with which the heart contracts and causes blood to move. It depends on several factors and can be influenced by medications.
What is the force of contraction?
The contractile force of the heart is the force with which the heart contracts and causes blood to move. A physiological contractile force of the heart is a prerequisite for the heart to pump enough blood into the systemic circulation to supply the entire body with sufficient blood. At rest, the human heart pumps the entire volume of blood through the circulatory system about once per minute. With each pumping action, each of the heart chambers transports about 50 to 100 milliliters of blood. Per minute, the heart contracts about 50 to 80 times. The higher the contraction force of the heart, the more blood can be ejected. The force of contraction is controlled, among other things, by the influence of sympathetic nerve fibers. The force of contraction can also be influenced by medication.
Function and task
The heartbeat is triggered by action potentials. These propagate through the specialized muscle tissue of the heart. During a pumping cycle, the first to fill are the atria of the heart. At the same time, the ventricles eject blood into the systemic circulation. Then the heart muscles of the ventricles relax again and blood can flow from the atria into the ventricles. This phase is called ventricular diastole. The filling of the ventricles is supported by a contraction of the atria (atrial systole). When the ventricles are sufficiently filled, the ventricular muscles contract. The pocket valves of the ventricles open and blood can flow into the arteries. This phase is called ventricular systole. How much the ventricles contract, and how much blood they then eject, is influenced by several factors. During physical exertion, the heart’s action is stimulated by the influence of nerve fibers of the sympathetic nervous system. The neurotransmitter norepinephrine is released at the cells of the heart muscles. Adrenaline also reaches the heart via the blood. The effect of the transmitters and hormones on the heart muscle is mediated via so-called β1-adrenoceptors. Through various mechanisms, calcium channels open in the cells so that increased calcium can flow into the cells. This leads to increased muscle contraction of the heart. Norepinephrine and epinephrine thus influence the contractile force of the heart. They have a positive inotropic effect. The contractile force of the heart usually adjusts automatically to the physical demands. Additional blood volume stretches the heart muscle. This also improves the function of the muscle cells. This mechanism is called the Frank-Starling mechanism. It states that there is a relationship between the filling and the ejection capacity of the heart. The greater the volume of blood that enters the heart during diastole, the greater the volume of blood that is ejected during systole. Increasing filling of the atria, therefore, leads to a stronger contraction of the heart with an increase in stroke volume. Thus, it could be said that the contractile force of the heart depends on the preload. The Frank-Starling mechanism is used to adapt the heart’s activity to variations in pressure and volume. The goal is for the right and left ventricles to always pump the same volume. If this were to malfunction, complications would arise within a very short time. The consequence would be pulmonary edema, for example.
Diseases and complaints
A condition in which the contractile force of the heart is reduced is heart failure. Heart failure is also called cardiac insufficiency or heart muscle weakness. It can result from almost any heart disease. Typical causes include coronary artery disease (CAD), inflammation of the heart muscle (myocarditis), valvular heart disease, valvular heart disease or pericarditis. Chronic lung disease can also cause heart failure. Risk factors also include elevated cholesterol, diabetes mellitus (diabetes), smoking, alcohol addiction, and severe obesity. In heart failure, cardiac output is reduced due to a decrease in stroke volume. The contractile force of the heart is no longer sufficient to supply the body with sufficient blood. The body responds by releasing adrenaline and noradrenaline.On the one hand, this causes the blood vessels to constrict, and on the other, it increases the contractile force of the heart. However, since the heart muscle is insufficient, the hormones and transmitters at the heart’s receptors no longer work. The vessels, on the other hand, contract. This causes the blood pressure to rise. The heart now has to pump against high pressure in the vessels despite reduced contractile force. As a result, the condition of the heart progressively worsens (vicious circle). Digitalis drugs are often used to treat heart failure. These are cardiac glycosides that are usually extracted from the foxglove. Digitalis has a positive inotropic effect. The contractile force of the heart is increased, which also increases the stroke volume. A life-threatening condition associated with decreased contractility of the heart is cardiac tamponade. In cardiac tamponade, the heart is compressed. The cause is usually fluid accumulation in the pericardium. These can be caused by pericardial inflammation, hemorrhage, aortic aneurysm, and heart attack. Due to the compression caused by the fluid in the pericardium, the heart can no longer relax in diastole. As a result, adequate filling is no longer possible. According to the Frank-Starling mechanism, the contractile force of the heart decreases when atrial filling is reduced. As a result, there is a lower stroke volume. The result is a backlog of blood in front of the heart. In addition, the body is not adequately supplied with arterial blood. Typical symptoms of cardiac tamponade are low blood pressure, rapid heartbeat, rapid breathing and blue discoloration of the skin. Cardiac tamponade is a medical emergency. There is a threat of cardiogenic shock.
