Gross Anatomy

The circulatory system is divided into a pulmonary circuit that serves only for gas exchange with the air in the lungs and a [1] circuit that supplies the entire body. The latter is served by the [2] side of the heart. The heart is enclosed in a double-walled membrane called the [3]. The [4] layer of this membrane forms the outermost layer of the heart wall, the [5]. The thick middle, muscular layer of the heart is the [6] and the inner lining is the [7].

The two superior heart chambers are called the [8] and the more muscular inferior chambers are the [9]. Each pair of right and left chambers has a wall, or [10], separating them. The receiving chamber for all systemic blood is the [11]. Blood flows from here through the [12] valve into the [13]. When that chamber contracts, valve 12 closes but stringlike [14] keep it from turning inside out. The blood from this chamber exits through the [15] valve into the pulmonary trunk. Blood returning from the lungs flows by way of four [16] into the left atrium, and passes from here through the [17] valve into the left ventricle. From here, it is pumped through the [18] valve into a large artery called the [19], the start of the systemic circuit.

The heart wall itself is supplied with blood by a right and left [20] arising from the 19. The left one gives off an anterior [21] that descends toward the apex of the heart and a [22] that curves around to the posterior side of the heart. The right one produces a [23] along the right edge of the heart and then a posterior 21. Blood from the cardiac veins collects in the [24] just before entering the right atrium.

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Cardiac Muscle and the Cardiac Conduction System

Cardiac muscle has a more scanty [25] reticulum than skeletal muscle, but has large, abundant [26] that constitute as much as 25% of the cell volume and meet the high energy demands of the heart. Cardiac myocytes are joined end to end by [27], which have both electrical and mechanical intercellular junctions. At rest, the heart gets most of its energy from [28]. It makes little use of the mechanism of [29] and is therefore not as subject to [30] as skeletal muscle. The pacemaker that sets the cardiac rhythm is the [31]. Signals from it eventually reach the [32], which delays the signal long enough for the ventricles to finish filling. Signals then travel very rapidly through the [33] bundle, the right and left [34], and nervelike [35] fibers to the ventricular myocardium.

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Electrical and Contractile Activity of the Heart

The SA node fires at a resting rate of 70–80 beats per minute, called the [36] rhythm. If it is diseased, other pacemakers tend to take over at a slower rate; such sites are called [37] foci. The membrane potentials of SA node cells exhibit a repetitive, gradual depolarization called a [38]. Each time this reaches threshold, membrane gates called [39] open and an action potential is generated. As this spreads through the myocardium of the atria, the atria exhibit contraction, or [40]. When the signal has passed, the atria repolarize and exhibit relaxation, or [41].

Cardiac myocytes exhibit action potentials quite different from those of nerve cells and skeletal muscle fibers. The most noticeable difference is that after the peak, there is a long [42] in which calcium influx causes the cells to remain depolarized for 200 msec or longer. This ensures that the [43] contract long enough to eject blood effectively. The myocardium also has a long (250 msec) [44] after its contraction, which prevents wave summation and myocardial tetany. The [45] is a recording of electrical events in the heart from skin electrodes. At the time the atria depolarize, this record exhibits a [46] wave. Ventricular depolarization, among other factors, produces its [47] complex, and ventricular [48] produces the T wave.

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Blood Flow, Heart Sounds, and the Cardiac Cycle

Any difference in fluid pressure from one point to another is called a [49]. Fluids tend to flow from the point of [50] to the point of [51]. Whenever the volume of an enclosed space increases, its pressure [52] and, if unobstructed, fluid will flow into it. When the volume [53], fluid is forced out.

During the [54] of the cardiac cycle, none of the chambers are contracting, but blood flows into the atria and through the AV valves into the ventricles. The action of [55] then actively pumps the last 30% of the blood or so to complete the filling of the ventricles. The ventricles then contain a quantity of blood known as their [56]. The QRS wave of the ECG indicates the onset of [57], so named because the ventricles contract but don't yet expel blood. A noise called the [58] occurs as the ventricular blood surges back against the closed [59] valves. When the aortic and pulmonary valves open, the phase of [60] begins. The volume of blood expelled during this phase is called the [61]; the fraction of 56 that is expelled is called the [62]; and the amount of blood remaining behind at the end of systole is called the [63].

At the time of the second heart sound, the [64] appears in the ECG, the ventricles begin to expand, and the phase of [65] begins. This means the ventricles are enlarging but not yet taking in blood. The next phase, [66], begins when the AV valves open.

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Cardiac Output

Cardiac output (CO) is the amount of blood ejected by each [67] in one [68]. It can be calculated by multiplying [69] by [70]. A typical resting CO is about 5.25 L/min. If a person with this CO showed a peak CO during exercise of 20 L/min, his or her [71] would be 14.75 L/min. Cardiac output changes under the influence of [72] agents, which raise or lower the heart rate, and [73] agents, which raise or lower the contractility of the myocardium. An abnormally high resting heart rate is called [74], whereas an abnormally low rate is called [75]. Heart rate is regulated by the [76] in the medulla oblongata. This nucleus is sensitive to input from pressure monitors called [77] in the blood vessels, [78] which monitor blood chemistry, and stretch monitors called [79] in the muscles and joints. A subdivision of the 76 called the [80] speeds up the heart with impulses transmitted through sympathetic nerve fibers, and the subdivision called the [81] slows down the heart with impulses transmitted through the vagus nerves. [82] refers to the amount of tension in the myocardium just before it begins to contract. According to the [83] law, this value and the stroke volume increase in proportion to the amount of blood that fills the ventricles during diastole. Pressure in the arteries that opposes the opening of the semilunar valves is called [84].

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