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Cardio 3 – Mechanical Properties of the Heart I Anil Chopra 1. Describe the relationship between ventricular wall tension, chamber radius and chamber pressure. (Law of Laplace) Law of Laplace “When pressure in a cylinder is constant, the tension ion the walls increases with increasing chamber pressure” Ventricular volume increases causing circumference to increase and length of cells to increase. This increases intraventricular pressure and so increases tension in the ventricular cells. Larger forces are needed to put pressure on the ventricles as their volume increases. This means that the L.V. is able to generate higher pressures with similar wall stress on the R.V. In dilated cardiomyopathy, ventricles dilate and so the pressure generated with each pulse is low. 2. List the sequence of events from excitation that bring about contraction and relaxation of a ventricular cell. Ventricular cells are 100μm wide and 15 μm across with small finger-like invaginations called T-tubules (200nm) which carry depolarisation into the cell. T-tubules lie along each Z-line. Contraction of cardiac muscle is dependent on the amount of extracellular Ca2+ and intracellular Ca2+ stores i.e. calcium couples electrical and mechanical events. Ca2+ ions enter through L-type Ca2+ channels and induce the opening of the sarcoplasmic reticulum Ca2+ release channels. These cause Ca2+ ions to flood out and consequently cause muscle contraction. ATP is used to pump Ca2+ ions back into the sarcoplasmic reticulum (SR-CaATPase) and an Na/Ca exchanger mediates Ca efflux. The amount of calcium entering the cell must be equal to that leaving with each beat if a steady state is to be maintained. 3. State Starlings Law of the Heart. “The more a ventricle is filled during diastole, the more blood is pumped out in that stroke as the fibres increase in length” 4. Understand the mechanisms underlying Starling’s law of the heart. Due to 2 factors: (1) Changes in the number of myofilament cross-bridges that interact. The length of the sarcomeres determines the force of contraction in that the more interactions between the thin actin and thick myosin filaments, the greater the force of contraction. Increase in ventricular volume increases sarcomeres length and so produces more interactions. Cardiac muscle is much more resistant to stretch than skeletal muscle. (2) Changes in the Ca sensitivity of myofilaments As the sarcomeres length increases, the Troponin C has a greater affinity for Ca2+ , therefore less Ca2+ is required for the same amount of force. 5. Use a graph to compare the length-tension relationships for cardiac and skeletal muscle. Cardiac muscle has a much greater stiffness due to properties of the extracellular matrix and cytoskeleton. 6. Understand the concepts of preload and afterload. Preload: weight that stretches a muscle before it contracts i.e. the filling of ventricles in diastole causing cells to stretch. This produces isometric contraction. ( Measured by end diastolic volume, end diastolic pressure, & right atrial pressure ) Afterload: weight not apparent when muscle is resting but encountered when muscle begins to contract. i.e. the load against which the left ventricle ejects blood after opening the aortic valve. Produces isotonic contraction. ( Measured by diastolic atrial pressure )