Importance of blood circulation; structure of the heart
The heart and vessels form the circulatory system; circulation delivers oxygen and nutrients to tissues and carries away waste products. The heart is a muscular organ about the size of its owner’s fist, lying in the chest behind the sternum, mostly to the left. It has four chambers: two upper atria (right and left) and two lower ventricles. Between each atrium and ventricle is a cuspid valve, and between the ventricles and the large arteries (aorta, pulmonary artery) there are semilunar valves; valves let blood pass in one direction only. The left ventricle wall is the thickest because it pumps blood to the whole body. The cardiac cycle lasts about 0.8 seconds: the atria contract (0.1 s), the ventricles contract (0.3 s), then all rest (0.4 s). A calm adult’s heart beats 60–80 times per minute, ejecting about 70 ml per beat. The heart contracts because of excitation arising in special cells in its own muscle – cardiac automaticity.
Slowly clench and open your fist to imagine contraction (systole) and relaxation (diastole) of the heart. Count your pulse for one minute at rest and write it down.