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cardiac cycle

cardiac cycle The series of muscular contractions which squeeze blood through the heart. In Amphibia and higher vertebrates, blood is pumped through a double circulatory system. The right side of the heart pumps blood through the pulmonary circuit to reach the lungs, the left side pumps oxygenated blood through the systemic circuit which supplies the body. Deoxygenated blood enters the right atrium via the vena cava and is pumped into the right ventricle. The right ventricle pumps the blood into the pulmonary artery which divides and leads to both lungs. Oxygenated blood returns from the lungs via the pulmonary vein to the left atrium and in turn to the left ventricle. From the left ventricle, the most muscular of the heart chambers, the blood is pumped around the body where it becomes deoxygenated and returns to the right atrium. Amphibia have only one ventricle, which must pump blood around both circuits; deoxygenated blood is kept separate from oxygenated blood by a combination of timing and complex architecture. During the phase of muscle relaxation (diastole), blood flows freely into the atrium until it is full whereupon the sino-atrial node sends out an electrical pulse causing the cardiac muscle of the atrium to contract (systole). Blood is forced into the atrium, which is in diastole, until it is full, whereupon the atrioventricular node sends out an electrical pulse along the Purkinje fibres around the right ventricle and the bundle of His (specialized muscle fibres) around the left ventricle, causing the ventricle to enter systole and force blood out into the arteries of the circulatory system. Nerves of the sympathetic nervous system from the spinal cord connect to the sino-atrial node and can speed up the rate of heartbeat, while the vagus nerve from the medulla also connected to the sino-atrial node slows it down.

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cardiac cycle

cardiac cycle The sequence of events that occurs in the heart during one full heartbeat. These events comprise contraction (see systole) and relaxation (see diastole) of the chambers of the heart, associated with opening and closing of the heart valves. When both the atria and the ventricles are relaxed, pressure in the heart is low and blood flows from the vena cava and pulmonary vein into the atria and through to the ventricles. The aortic and pulmonary semilunar valves, at the junction between the left ventricle and aorta and the right ventricle and pulmonary artery, respectively, are closed; therefore, blood can enter but not leave the heart, which increases the pressure in the chambers. As the pressure in the heart increases, the atria begin to contract, forcing the blood into the ventricles and closing the tricuspid valve and the bicuspid valve. A wave of ventricular contraction follows, expelling the blood into the aorta and pulmonary artery to complete the cardiac cycle. At a resting heart rate, the human cardiac cycle lasts approximately 0.85 second.

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heartbeat

heart·beat / ˈhärtˌbēt/ • n. the pulsation of the heart. ∎  (usu. heartbeats) a single pulsation of the heart: her heartbeats steadied. ∎ fig. a person or thing providing or representing an animating or vital unifying force: conflict is the essential heartbeat of fiction. PHRASES: a heartbeat away from very close to; on the verge of: the man who is just a heartbeat away from the presidency.

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heartbeat

heartbeat a heartbeat (away) from very close to, on the verge of.

In the US, just a heartbeat away from the Presidency describes the vice-president's position; from a reference by Adlai Stevenson (1900–65) to the Republican Vice-Presidential nominee Richard Nixon, in a speech at Cleveland, Ohio, 23 October 1952, in which he referred to ‘The young man who asks you to set him one heart-beat from the Presidency of the United States.’

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cardiac cycle

cardiac cycle n. the sequence of events between one heartbeat and the next, normally occupying less than a second. See diastole, systole.

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heartbeat

heartbeataccrete, beat, beet, bittersweet, bleat, cheat, cleat, clubfeet, compete, compleat, complete, conceit, Crete, deceit, delete, deplete, discreet, discrete, eat, effete, élite, entreat, escheat, estreat, excrete, feat, feet, fleet, gîte, greet, heat, leat, leet, Magritte, maltreat, marguerite, meat, meet, mesquite, mete, mistreat, neat, outcompete, peat, Pete, petite, pleat, receipt, replete, seat, secrete, sheet, skeet, sleet, splay-feet, street, suite, sweet, teat, treat, tweet, wheat •backbeat • heartbeat • deadbeat •breakbeat • offbeat • browbeat •downbeat • drumbeat • upbeat •sugar beet • Blackfeet • flatfeet •forefeet • exegete • polychaete •lorikeet • parakeet •athlete, biathlete, decathlete, heptathlete, pentathlete, triathlete •kick-pleat • paraclete • obsolete •gamete • crabmeat • sweetmeat •mincemeat • forcemeat • backstreet •concrete • window seat

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