The heart (Latin: cor; Greek: cardia) is a hollow muscular organ that maintains blood circulation by rhythmic contractions. The organism can survive cardiac arrest for only a few minutes.
What is the heart?
The human heart is a hollow organ about the size of a fist in the chest cavity. Acting as a pressure and suction pump, it moves blood throughout the body. In a normal constitution, the heart weighs about 250-300 g. Its contour usually projects slightly to the left behind the sternum between the 2nd and 5th ribs.
Anatomy and structure
Schematic anatomical representation of the heart showing the ventricles. The heart is located within the thoracic cavity in the lower mediastinum. The pleural cavities are adjacent on the right and left, the esophagus is posterior, and the thymus and sternum are anterior. The base of the heart rests on the diaphragm. The pericardium is a connective tissue sac that surrounds the heart and gives it sliding freedom of movement through 10-15 ml of serous fluid. The heart itself is divided into right and left halves, each with an atrium and a ventricle. Between the atria and the ventricles are leaflet valves (atrioventricular valves), and at the outlets of the ventricles are pocket valves (right: pulmonary valve; left: aortic valve). The wall of the heart has three layers: the coronary vessels run in the outer epicardium, which consists of epithelial, fatty and connective tissue. Below this is the myocardium of cardiac muscle fibers; this also contains the complex excitation formation and conduction system. Inside, the heart is lined with endocardium of connective tissue and endothelium.
Functions and Tasks
The heart’s task is to pump oxygen-depleted blood through the pulmonary circulation and then to pump the blood, freshly enriched with oxygen there, through the systemic circulation. The blood takes the following route: from the great vena cavae it flows into the right atrium, from there into the right ventricle and on into the pulmonary arteries. After flowing through the lungs, the now oxygen-rich blood reaches the left atrium via the pulmonary veins. From here, it continues to flow into the left ventricle and is ejected into the aorta. To enable these flow conditions, the atria and ventricles must contract in a staggered manner. A distinction is made in the cardiac cycle between systole and diastole:
In systole, the ventricles contract while the closed leaflet valves prevent backflow into the relaxed atria. During diastole, in turn, the atria pump blood into the relaxed ventricles, whose outlets are closed by the leaflet valves. The contraction of the heart is based on spontaneous electrical excitation in the sinus condyles, the natural pacemaker of the heart. The excitation propagates through the atrial myocardium to the AV node, which itself can step in as a secondary pacemaker at a lower frequency if the sinus node fails. After a time delay, the excitation reaches the ventricular muscles. The frequency and strength of spontaneous cardiac contractions can be influenced by the autonomic nervous system. At rest, the heart beats 50-80 times per minute, pumping about 5 l – once the entire blood volume – through the circulation. During great exertion, it can even move 20-25 l per minute.
Diseases
The heart does considerable work through its regular contraction and thus has a high demand for oxygen. The so-called coronary heart disease, an oxygen deficiency of the heart mostly caused by arteriosclerosis, is by far the most common cause of death in industrialized nations. In the case of infarctions in the coronary arteries, heart muscle cells perish irreparably after just a few minutes. But it is not only age and an unhealthy lifestyle that endanger the oxygen supply to the heart: even in competitive athletes with enlarged hearts, the myocardium is threatened by oxygen deficiency from a heart weight of 500 g and up. Cardiac arrhythmias can be congenital or acquired through previous ischemic diseases. They are highly variable and differ in cause, site of origin, dangerousness, and resulting heart rate (increased: tachycardia; decreased: bradycardia). In case of insufficiency or stenosis (narrowing) of the heart valves, these can be replaced today by artificial heart valves. Congenital heart defects such as short circuits between the chambers are also not uncommon – they affect approximately 0.8% of all newborns. Cardiology deals with the entire spectrum of heart diseases.
Typical and common diseases
- Heart attack
- Pericarditis
- Heart failure
- Atrial fibrillation
- Heart muscle inflammation
