How do pressure changes in the left atrium, left ventricle, and aorta occur during the cardiac cycle?

Sourav PanNovember 8, 2024

Answered

During the cardiac cycle, pressure changes in the left atrium, left ventricle, and aorta occur in a coordinated manner that facilitates efficient blood flow through the heart and into systemic circulation. Here’s a detailed overview of these pressure changes throughout the cardiac cycle:

1. Phases of the Cardiac Cycle

The cardiac cycle consists of two main phases: systole (contraction) and diastole (relaxation). Each phase involves specific pressure changes in the left atrium, left ventricle, and aorta.

A. Diastole

• Early Diastole:
  • Left Atrium: As the heart relaxes, blood returns to the left atrium from the pulmonary veins, causing atrial pressure to gradually rise.
  • Left Ventricle: The left ventricle is also relaxed, and its pressure is low (approximately 5-10 mmHg). Blood passively fills the ventricle from the atrium through the open mitral valve.
  • Aorta: Aortic pressure remains relatively high (around 80 mmHg) due to elastic recoil from previous ventricular contractions.
• Atrial Systole:
  • When the left atrium contracts (atrial systole), there is a further increase in atrial pressure, which helps push additional blood into the left ventricle. This contraction contributes to what is known as the “atrial kick,” which accounts for about 20-30% of ventricular filling at rest.

B. Systole

• Isovolumetric Contraction:
  • As ventricular systole begins, the left ventricle contracts while both the mitral and aortic valves are closed. This causes a rapid increase in ventricular pressure (up to about 80 mmHg initially) without any change in volume since all valves are closed.
• Ventricular Ejection:
  • Once ventricular pressure exceeds aortic pressure (approximately 80 mmHg), the aortic valve opens. Blood is ejected into the aorta, causing a sharp rise in both left ventricular and aortic pressures. Left ventricular pressure can peak around 120 mmHg during this phase.
  • As blood exits into the aorta, aortic pressure rises significantly, reaching approximately 120 mmHg.

2. Pressure Changes Summary

• Left Atrium:
  • Pressure rises during diastole as it fills with blood; peaks slightly during atrial systole.
• Left Ventricle:
  • Pressure begins low during diastole, rises sharply during isovolumetric contraction, peaks during ventricular ejection, and then falls rapidly during isovolumetric relaxation as it prepares for another filling phase.
• Aorta:
  • Aortic pressure remains relatively high due to elastic recoil but increases further when blood is ejected from the left ventricle. After ventricular ejection, it briefly drops due to backflow before stabilizing again during diastole.

3. Closure of Valves and Dicrotic Notch

• After ventricular systole ends and ventricular pressure falls below aortic pressure, the aortic valve closes, preventing backflow of blood into the ventricle. This closure creates a characteristic dip in aortic pressure known as the dicrotic notch, which reflects transient backflow against the closed valve.