心脏(英).ppt

* * Discuss the pressure changes in the heart chambers and great vessels during a complete cardiac cycle. Cardiac Pump OBJECTIVES Describe how the heart enable it to pump blood through the systemic and pulmonary circulations. Explain how electrical excitation of the heart is coupled to its contractions. List the factors that determine cardiac contractile force. The Structures of the Heart is Designed for Optimal Function Several important morphological and functional differences exist between myocardial and skeletal muscle cells A striking difference is that cardiac muscle appears to be a syncytium (a functional syncytium, not a true anatomical syncytium ) with branching and interconnecting fibers, whereas skeletal muscle cells do not interconnect. Cardiac muscle functions as a syncytium because a wave of depolarization, followed by contractions of the atria and ventricles (an all-or-none response), occurs when a uprathreshold stimulus is applied. As the wave of excitation approaches the end of a cardiac cell, the spread of excitation to the next cell depends on the electrical conductance of the boundary between the two cells. Gap junctions (nexuses) with high conductance are present in the intercalated disks between adjacent cells. cardiac impulse from one cell to the next, are made up of connexons, cardiac muscle is richly endowed with mitochondria (sarcosomes) which contain the respiratory enzymes necessary for oxidative phosphorylation. To provide adequate O2 and substrate for its metabolic machinery, the myocardium is also endowed with a rich capillary supply, about one capillary per fiber. The cardiac chambers consist of two atria, two ventricles, and four valves The cardiac valves consist of thin flaps of tough, flexible, endothelium-covered fibrous tissue firmly attached at the base to the fibrous valve rings. Movements of the valve leaflets are essentially passive, and the orientation of the cardiac valves is res