Heart Sounds and Cardiac Cycle

Heart Sounds and Valves

• Heart sounds (lub-dup) are associated with the closing of heart valves
• First sound occurs when AV valves close, signaling the beginning of systole
• Second sound occurs when SL valves close at the beginning of ventricular diastole

Cardiac Cycle

• Cardiac cycle refers to all events associated with blood flow through the heart
• Systole: contraction of heart muscle
• Diastole: relaxation of heart muscle
• Ventricular filling: mid-to-late diastole

Phases of Cardiac Cycle

• Heart blood pressure is low as blood enters atria and flows into ventricles
• AV valves are open, then atrial systole occurs
• Ventricular systole: atria relax, rising ventricular pressure results in closing of AV valves
• Isovolumetric contraction phase
• Ventricular ejection phase: opens semilunar valves

Cardiac Output (CO)

• CO is the amount of blood pumped by each ventricle in one minute
• CO is the product of heart rate (HR) and stroke volume (SV)
• Formula: CO (ml/min) = HR (beats/min) x SV (ml/beat)
• Example: CO = 5250 ml/min (5.25 L/min) = 75 beats/min x 70 ml/beat

Cardiac Reserve and Stroke Volume

• Cardiac reserve is the difference between resting and maximal CO
• SV = end diastolic volume (EDV) minus end systolic volume (ESV)
• Factors affecting SV: preload, contractility, and afterload

Factors Affecting Stroke Volume

• Preload: amount ventricles are stretched by contained blood
• Contractility: cardiac cell contractile force due to factors other than EDV
• Afterload: back pressure exerted by blood in the large arteries leaving the heart
• Increase in preload: slow heartbeat and exercise
• Decrease in preload: blood loss and extremely rapid heartbeat
• Increase in contractility: sympathetic stimuli, certain hormones, Ca2+, and some drugs

Autonomic Nervous System

• Sympathetic nervous system (SNS) stimulation: increases heart rate, activated by stress, anxiety, excitement, or exercise
• Parasympathetic nervous system (PNS) stimulation: mediated by acetylcholine, slows heart rate, and causes vagal tone
• Hormones: epinephrine and thyroxine increase heart rate

Ion Concentrations and Heart Function

• Intra- and extracellular ion concentrations must be maintained for normal heart function