What is the function of the heart valves?
The four heart valves work in a pattern to keep blood flowing in one direction. They are made of thin tissue and open and close easily. The tricuspid, pulmonic, and aortic valves all have three cusps (leaflets). The mitral valve has only two leaflets. The mitral and tricuspid valves are larger than the others, and their margins, or free edges, are supported by strands of heart tissue called chordae tendinae. These small cords keep the valves closed during contraction (squeezing) of the heart. What causes the heart to beat? Every normal heartbeat starts in a group of specialized cells in the right atrium, called the sinus node. These “pacemaker” cells (P cells) automatically discharge a regular electrical impulse to start the contraction of the heart. The usual rate is between 50 and 100 beats per minute. The rate of beating can be made faster or slower by the body’s nervous system, hormones, or drugs. The heart will continue to beat even if all the nerves leading to it are cut. The sin
