Heart
Coronary arteries
Cardiac cycle
Conducting system of the heart
The centre of the circulatory system is the heart. It is approximately fist-sized and is made up mostly of cardiac muscle. Its rhythmic contractions keep the blood flowing in the vascular network. It is situated in the thoracic cavity slightly left from the midline of the body. On the outside it is covered by a membranous sac, the pericardium. The inside of the heart is divided into four chambers: two atria and two ventricles. The ventricles are separated by the interventricular septum. Between the atria and the ventricles there are atrioventicular valves, at the exit of the ventricles there are semilunar valves. A resting adult’s heart contracts 60-80 times per minute on average and pumps approximately 60-90 cm3 of blood into the pulmonary artery and the aorta.
Blood is supplied to the heart muscle by its own arteries. These, the coronary arteries stem from the aorta just above the semilunar valves. The coronary arteries have branches supplying the atria, the ventricles and the interventicular septum. There is only minimal overlap in the blood supply of the smaller arteries, so if one artery is clogged, the blood supply of that part of the heart is blocked and the heart muscle gets damaged. This is called a myocardial infarction, or a heart attack. The heart muscle’s capillaries gather into venules as they meet, which then form one large vein that carries the blood to the right atrium.
The cardiac cycle is the repetitive sequence of events of the heart beats. In the first phase of the cardiac cycle the atria get filled with blood. When the pressure in the atria rises above the ventricular pressure the atrioventicular valves open and blood flows into the ventricles. At the end of this phase the atria contract to force the remaining blood to the ventricles. After this the atria relax, the atrioventricular valves close and the ventricles filled with blood contract. As the pressure in the ventricles increase the semilunar valves leading to the aorta and the pulmonary artery open and blood flows to the arteries. This happens parallel with the relaxation of the atria. The duration of the cardiac cycle at rest is 0.8 second on average.
The timely contractions of the heart are ensured by its own electrical conduction system. The main pacemaker of the heart is a small group of cells located in the wall of the right atrium. The signal generated here spreads onto the muscle of the atria, and the atria contract as the result. After this the stimulus reaches the muscles of the ventricles causing contraction there. The signal rate of the excitatory group of cells is controlled by the nervous system and the hormonal system.
