Question

Explain the role of pressure gradients in the cardiac cycle. Your answer should state the relationship between pressure gradients and blood flow. For each phase of the cardiac cycle, describe how pressure changes influence the direction of blood flow and the opening and closing of valves in the heart.

Answer 1

• Throughout the cardiac cycle, the atria collect deoxygenated blood returning to the heart from the peripheral circulation and the coronary circulation (right atria) or from the pulmonary circulation (left atria)
• During diastole, the build-up of blood in the atria creates a pressure gradient that forces open the AV valves allowing for about 75% of this blood to pass into the ventricle causing a gradual increase in ventricular diastolic pressure.
• In late diastole, the contraction of the atria propels the remaining 25% of the blood into the ventricles, which produces a further increase in atrial and ventricle pressures
• The contraction of the ventricle then follows, signaling the onset of mechanical systole.
• As the ventricles contract, the pressures within them rapidly exceed atrial pressures.
• This pressure gradient pushes back on the leaflets of the AV valves and forces them closed
• The onset of the ventricular contraction also creates tension on the papillary muscles, which apply extra force to the edges of the leaflets ensuring the proper alignment of the valves, which aids in their closing
• Further ventricular contraction causes ventricular pressure to exceed the diastolic pressures in the pulmonary artery and aorta, which forces the opening of the semilunar valves
• This allows the ventricles to empty their contents into the pulmonary and systemic circulations.
• Because the semilunar valves are open, the continued contraction of the ventricles increases the pressure in the pulmonary artery and aorta.
• The conclusion of ventricular ejection causes the pressure in the ventricles to fall below those of the pulmonary artery and aorta.
• This allows for the blood in the pulmonary artery and aorta to push back on the semilunar valves forcing them closed.
• The cardiac cycle then begins once again with the ventricles filling with blood that has collected in the atria.