PY 105 Chapter Notes - Chapter 11: Heart Valve, Electrical Conduction System Of The Heart, Tricuspid Valve
Valves
• Necessary to ensure one-way flow through the circulatory system
• Ventricular pressure is very high and atrial pressure is lower
• Atrioventricular valve (AV valve) - between each ventricle and its atrium is necessary to prevent
backflow
• The AV valve between the left atrium and left ventricle is the bicuspid (mitral) valve
• The AV valve between the right atrium and right ventricle is the tricuspid valve
• Pulmonary and aortic semilunar valves are between the large arteries and ventricles
• Venous valves prevent backflow
The Cardiac Cycle
• Two periods:
o Diastole and systole
• Diastole - the ventricles are relaxed, and blood is able to flow into them from the atria
o At the end of diastole, the ventricles contract, initiating systole
o The ensuing buildup of pressure causes the AV valves to slam shut
o The pressure in the ventricles increases rapidly, until the semilunar valves fly open and
blood rushes into the aorta and pulmonary artery
• Systole - period of time which the ventricles are contracting, beginning at the "lub" sound and
ending at the "dup"
o At the end, the ventricles are nearly empty, and stop contracting
o The pressure inside falls rapidly, and blood begins to flow backward, from the pulmonary
artery into the right ventricle, and the aorta into the left ventricle
Heart Sounds, Heart Rate and Cardiac Output
• The lub-dup sound is produced by valves slamming shut
• The lub is the closure of the AV valves at the beginning of systole
• The dup is the sound of the semilunar valves closing at the end of systole
• The heart rate (HR) or pulse is the number of time the "lub-dup" cardiac cycle is repeated per
minute
• The amount of blood pumped with each systole is known as the stroke volume (SV)
• Total amount of blood pumped per minute is termed the cardiac output (CO)
The Frank-Starling Mechanism and Venous Return
• A stronger heart has a larger stroke volume
• Frank-Starling mechanism - if the heart muscle is stretched, it will contract more forcefully
• The return of blood to the heart by the vena cava is termed venous return
• If venous return is increased, the heart fills more
• As a result, its muscle fibers are stretched, and they respond by contracting more forcefully
• A large volume of blood enters the heart and the heart contracts better
• Two ways to increase venous return:
o Increase the total volume of blood in the circulatory system
• Retain water
o Contraction of large veins can propel blood toward the heart
Cardiac Muscle
• Cardiac muscle is a functional syncytium
o Syncytium - tissue in which the cytoplasm of different cells can communicate via gap
junctions
• Gap junctions in cardiac muscle is intercalated disks
• No chemical synapses between cardiac muscle cells, only electrical
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