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Transcript
Interactive Biology, Making Biology Fun Find Videos on www.youtube.com Cardiac Muscles Functioning of the heart Arsalan Yousuf BS 4th Semester Comparison between human and mouse heart Mouse Heart Human Heart The heart is composed of three major types of cardiac muscle: • Atrial muscle, • Ventricular muscle, • Specialized excitatory and conductive muscle fibers. Cardiac muscle is striated in the same manner as in typical skeletal muscle. Cardiac muscle has typical myofibrils that contain actin and myosin filaments almost identical to those found in skeletal muscle Cardiac muscle is a syncytium of many heart muscle cells Purkinje fibers are located in the inner ventricular walls of the heart, just beneath the endocardium. These are specialized myocardial fibers that conduct an electrical stimulus or impulse that enables the heart to contract in a coordinated fashion. The atria are separated from the ventricles by fibrous tissue that surrounds the atrioventricular (A-V) valvular openings between the atria and ventricles. Division of the cardiac muscle into two functional syncytiums allows the atria to contract a short time ahead of ventricular contraction. Action Potentials in Cardiac Muscle (Here we go again….) Conduction of the excitatory action potential signal along both atrial and ventricular muscle fibers is about 0.3 to 0.5 m/sec A large quantity of extra calcium ions also diffuses into the sarcoplasm from the T tubules themselves at the time of the action potential. The Cardiac Cycle Cardiac events that occur from the beginning of one heartbeat to the beginning of the next are called the cardiac cycle. Initiated by spontaneous generation of an action potential in the sinus node. Sino Atrial Node: Impulse-generating (pacemaker) tissue located in the right atrium of the heart. These cells are modified cardiac myocytes. The Electrocardiogram Electrocardiography (ECG or EKG) is a transthoracic interpretation of the electrical activity of the heart over a period of time. Events during an ECG • Action potentials spread from the SA node to the AV node, causing atrial depolarization. • Atrial depolarization, seen as the P wave, induces atrial systole. • Action potential spread through the bundle of His bundle branches and Purkinje fibers causing ventricular depolarization. • Ventricular depolarization (QRS complex) induces ventricular systole. • As action potential pass out of the ventricles, ventricular diastole is induced. • Ventricular repolarization is shown by the T-wave Cardiac Cycle Heart Valves A-V valves: The tricuspid and mitral valves Semilunar valves: the aortic and pulmonary artery valves Papillary Muscles STROKE WORK OUTPUT of the heart is the amount of energy that the heart converts to work during each heartbeat while pumping blood into the arteries. • Major proportion is used to move the blood from the low-pressure veins to the high-pressure arteries. • Second, a minor proportion of the energy is used to accelerate the blood through the aortic and pulmonary valves. Ventricular Pumping-Graphical Analysis “Volume-Pressure Diagram” Phase I: Period of filling. Phase II: Period of isovolumic contraction. Phase III: Period of ejection. Phase IV: Period of isovolumic relaxation. Chemical Energy Required for Cardiac Contraction Energy is derived mainly from oxidative metabolism of fatty acids and, to a lesser extent, of other nutrients, especially lactate and glucose. The Frank-Starling Mechanism Ability of the heart to adapt to increasing volumes of inflowing blood is called the Frank- Starling mechanism of the heart. Effect of Potassium and Calcium Ions on Heart Function • Excess potassium in the extracellular fluids causes the heart to become dilated and slows heart rate. • Elevation of potassium concentration to only 8 to 12 mEq/L— two to three times the normal value—can cause weakness and abnormal rhythm of the heart. • Excess of calcium ions causes effects almost exactly opposite to those of potassium ions, causing the heart to go toward spastic contraction. Effect of Temperature • Increased body temperature causes a greatly increased heart rate, sometimes to as fast as double normal. • Decreased temperature causes a greatly decreased heart rate, falling to as low as a few beats per minute