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L18 & 22 The Cardiac Pump Cycle and Output

L18 & 22 The Cardiac Pump Cycle and Output -...

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- 1 - COPYRIGHT Prof. Beyenbach Mammalian Physiology BIOAP 4580 2011 The Cardiac Pump, Cycle and Output 1. The heart as a pump. The heart is a muscular pump. Like skeletal muscle, cardiac muscle is excitable, that is, cardiac muscle is capable of generating and conducting action potentials as prelude to each muscular contraction. Thus, the heart has both electrical and mechanical properties. Consequently, diseases of the heart can be broadly classified as electrical or mechanical malfunctions. The mechanical properties and the mechanical functions of the heart are the subject of this lecture. The heart presents a single structure, but it consists of two pumps in tandem as far as the flow of blood is concerned. The right heart pressurizes the pulmonary arterial circulation and the left heart pressurizes the systemic arterial circulation. Next to the vertical division into right and left hearts, there is also a horizontal division between atria and ventricles. Atria receive blood from the systemic and pulmonary circulations, and ventricles return blood to the systemic and pulmonary circulations. What separates atria and ventricles is the atrio- ventricular septum that serves two functions: a) the septum is an electrical insulator that prevents atrial action potentials from invading the ventricles. Accordingly, the two atria form one functional syncytium and the two ventricles a second syncytium, b) atrial and ventricular muscle fibers begin and terminate in the septum, such that all muscular contractions sweep towards the septum. Accordingly, atrial contractions push blood downward into the ventricles, and ventricular contractions push blood upwards into the pulmonary artery and into the aorta. 2. Anatomy of the myocardial cell. a) Contractile elements. The contractile unit of the heart muscle is the sarcomere. It consists of overlapping myosin and actin filaments between Z-lines (Fig. 1). Myosin is found in the A band, and actin is found in both I and A bands. Cardiac sarcomeres function like those of skeletal muscle. The molecular details of muscular contraction should be reviewed, consulting a physiology text (see readings). The sarcomere develops the greatest tension when its resting length is between 2 and 2.4 μm when there is optimal overlap of thick and thin filaments. The phenomenon is also known as the Starling law of the heart which states that the force of contraction increases upon stretching the muscle, albeit to a limit. Two explanations are offered for this phenomenon: 1. Biological material is elastic. Stretching the muscle before a contraction stretches elastic elements of the muscle to the limit such that contraction is not wasted on stretching the elastic elements of muscle. Accordingly, sliding filaments can now do useful work rather than absorb the slack in the muscle.
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