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Function of the Atrioventricular Valves

The atrioventricular (AV) valves, consisting of the tricuspid valve on the right side of the heart and the mitral valve on the left side, play an essential mechanical role in ensuring unidirectional blood flow from the atria into the ventricles during the cardiac cycle. These delicate, fibrous leaflets open passively when atrial pressure exceeds ventricular pressure during diastole, allowing the ventricles to fill smoothly with blood. During ventricular systole, as the pressure within the ventricles rises sharply above that in the atria, the AV valves snap shut to prevent any retrograde flow back into the atrial chambers. To prevent the high intraventricular pressures from forcing the valve leaflets to bulge backward or invert into the atria a condition known as regurgitation the valves are tethered to papillary muscles extending from the inner ventricular walls via thin, tough fibrous cords called chordae tendineae. The papillary muscles contract simultaneously with the ventricular myocardium, pulling the chordae tendineae taut and holding the valve cusps firmly sealed together, thereby maintaining optimal hemodynamic efficiency throughout every heartbeat.

• A-V valves prevent blood backflow into atria during systole.
• Tricuspid and mitral valves are the   AV valves.
• Semilunar valves prevent backflow into ventricles during diastole.
• Aortic and pulmonary valves are the semilunar valves.
• All heart valves open and close passively based on pressure.
• A-V valves require almost no backflow to close.
• Semilunar valves require rapid backflow for a few milliseconds.

Function of the Papillary Muscles

• Papillary muscles connect to A-V valves via chordae tendineae.
• Papillary muscles contract simultaneously with ventricular walls.
• They do not assist in closing the A-V valves.
• They pull valve vanes inward to prevent backward bulging.
• Ruptured chordae tendineae or paralyzed muscles cause severe leakage.

Aortic and Pulmonary Artery Valves

• Semilunar valves snap closed due to high arterial pressures.
• Blood ejection velocity is higher through smaller semilunar openings.
• Rapid closure and ejection subject semilunar valves to abrasion.
• Semilunar valves lack support from chordae tendineae.
• They are built with strong, pliable fibrous tissue.

Aortic Pressure Curve

• Left ventricular pressure increases rapidly until the aortic valve opens.
• Blood entry stretches arterial walls up to 120 mm Hg.
• Elastic arterial walls maintain high pressure during diastole.
• An incisura occurs when the aortic valve snaps closed.
• Short backflow before closure creates the incisura notch.
• Aortic pressure decreases slowly during diastole to 80 mm Hg.
• Right heart and pulmonary pressures are w of aortic pressure.

Relationship of Heart Sounds to Heart Pumping

• Valve opening is slow and normally makes no audible noise.
• Valve closure creates vibrations that produce heart sounds.
• First heart sound results from closure of A-V valves.
• First heart sound is low-pitched and relatively long-lasting.
• Second heart sound results from closure of semilunar valves.
• The second heart sound is a rapid, short snap.

Work Output of the Heart

• Stroke work output is energy converted per heartbeat.
• Minute work output equals stroke work times heart rate.
• Most work moves blood from veins to arteries.
• This is called external or volume-pressure work.
• A minor portion provides kinetic energy for blood flow.
• Right ventricular work is one-sixth of the left ventricle.
• Left ventricular kinetic energy is about one percent.
• Aortic stenosis can raise kinetic work above 50 percent.

Graphical Analysis of Ventricular Pumping

• The diagram features diastolic and systolic volume-pressure curves.
• Diastolic pressure is measured right before ventricular contraction.
• Systolic pressure is recorded during ventricular contraction at each volume.
• Diastolic pressure stays low until volume reaches 150 ml.
• Above 150 ml, diastolic pressure increases rapidly.
• Systolic pressure peaks at 150 to 170 ml volume.
• Normal left ventricle maximum systolic pressure is 250-300 mm Hg.
• Normal right ventricle maximum systolic pressure is 60-80 mm Hg.

Volume-Pressure Diagram During the Cardiac Cycle

• The red loop shows the left ventricle normal function.
• Phase I is the period of filling.
• End-systolic volume is about 50 ml at 2-3 mm Hg.
• End-diastolic volume reaches about 120 ml during filling.
• Phase II is the period of isovolumic contraction.
• Volume is unchanged while pressure rises to 80 mm Hg.
• Phase III is the period of ejection.
• The aortic valve opens and blood flows into the aorta.
• Phase IV is the period of isovolumic relaxation.
• The aortic valve closes and pressure falls without volume change.
• Diagram loop area represents net external work output (EW).

Concepts of Preload and Afterload

• Preload is ventricular end-diastolic pressure before contraction.
• Afterload is the aortic pressure against ventricular contraction.

Chemical Energy Required for Cardiac Contraction

• Energy mostly derives from fatty acid oxidative metabolism.
• The remainder comes from lactate and glucose.
• Oxygen consumption measures chemical energy used during work.
• Energy expenditure relates to external work plus potential energy.
• Oxygen use relates to the tension-time index.
• Dilated ventricles require more chemical energy in heart failure.

Efficiency of Cardiac Contraction

• Most expended chemical energy converts into heat.
• Efficiency is the ratio of work output to energy expenditure.
• Normal heart maximum efficiency is 20 to 25 percent.
• Heart failure efficiency can drop to 5 to 10 percent.