Anatomy & Physiology

Anatomy & Physiology Study Guide • Valves normally permit blood flow in only one direction, preventing the regurgitation (backflow) of blood. • The connective tissues of the heart (mainly collagen and elastic fibers) and the cardiac skeleton support the heart’s contractile cells and valves. • The coronary circulation meets the high oxygen and nutrient demands of cardiac muscle cells. The coronary arteries originate at the base of the ascending aorta. Interconnections between arteries, called arterial anastomoses, ensure a constant blood supply. The great, posterior, small, anterior, and middle cardiac veins are epicardial vessels that carry blood from the coronary capillaries to the coronary sinus. • In coronary artery disease (CAD), portions of the coronary circulation undergo partial or complete blockage. The conducting system distributes electrical impulses through the heart, and an electrocardiogram records the associated electrical events: • Two general kinds of cardiac muscle cells are involved in the normal heartbeat: contractile cells and cells of the conducting system. • The conducting system is composed of the sinoatrial node, the atrioventricular node, and conducting cells. The conducting system initiates and distributes electrical impulses within the heart. Nodal cells establish the rate of cardiac contraction, and conducting cells distribute the contractile stimulus from the SA node to the atrial myocardium and the AV node (along internodal pathways), and from the AV node to the ventricular myocardium. • Unlike skeletal muscle, cardiac muscle contracts without neural or hormonal stimulation. Pacemaker cells in the sinoatrial (SA) node, the cardiac pacemaker, establish the rate of contraction. From the SA node, the stimulus travels to the atrioventricular (AV) node, and then to the AV bundle, which divides into bundle branches. From there, Purkinje fibers convey the impulses to the ventricular myocardium. • A recording of electrical activities in the heart is an electrocardiogram (ECG or EKG). Important landmarks of an ECG include the P wave (atrial depolarization), the QRS complex (ventricular depolarization), and the T wave (ventricular repolarization). • Contractile cells form the bulk of the atrial and ventricular walls. Cardiac muscle cells have a long refractory period, so rapid stimulation produces twitches rather than tetanic contractions. Events during a complete heartbeat constitute a cardiac cycle: • The cardiac cycle contains periods of atrial and ventricular systole (contraction) and atrial and ventricular diastole (relaxation). • When the heart beats, the two ventricles eject equal volumes of blood. • The closing of valves and rushing of blood through the heart cause characteristic heart sounds, which can be heard during auscultation.

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