Comparative Physiology of the Right and Left Heart
Although both halves of the heart work in coordination, distinct functional and morphological differences exist between them. The right ventricle performs significantly less mechanical work than the left, which is directly reflected in its thinner wall structure.
Key hemodynamic differences:
- Minimum Diastolic Pressure: In the left heart, pressure never drops below 5 mm Hg. In contrast, pressure in the right heart approaches zero and can even become negative (sub-atmospheric) due to the suction effect of the thoracic cavity during inspiration.
- Contractile Asynchrony: Electrical excitation reaches the right atrium first; therefore, right atrial systole begins 10 ms earlier than left atrial systole.
- Ventricular Work: The onset of systole occurs simultaneously in both ventricles. However, due to the low resistance and low pressure of the pulmonary circulation, the phases of tension and relaxation are shorter in the right ventricle. Consequently, right ventricular systole lasts longer, while its diastole (relaxation period) is shorter. Although the right heart rests less and contracts more slowly, its overall energy expenditure is lower.
Cardiac Output Balance and Physiological Shunting
A fundamental rule of normal hemodynamics is that the minute cardiac output of both sides of the heart must be equal. However, stroke volumes (SV) differ slightly: the left ventricle ejects approximately 65 mL of blood per contraction, while the right ejects roughly 64 mL.
This small discrepancy is due to the physiological shunt, which accounts for about 1–2% of total cardiac output. This fraction of blood completely bypasses the right heart chambers.
Mechanisms of shunting:
- Direct drainage of a portion of the coronary veins (vv. cordis minimae) directly into the left atrial cavity.
- Drainage of blood from the systemic circulation (bronchial arteries) into the pulmonary venous system, subsequently returning directly to the left atrium via the pulmonary veins.
Pressure-Volume Loop and Phases of the Cardiac Cycle
Left ventricular hemodynamic processes are clearly illustrated by the Pressure-Volume Loop. This graph plots intraventricular pressure against blood volume throughout a single cardiac cycle. The shape and position of the loop shift according to hemodynamic conditions: it shifts to the right with increased venous return (increased preload) or upward/outward with increased arterial pressure (increased afterload).
The left ventricular cardiac cycle includes the following sequential phases:
- Asynchronous contraction
- Isovolumetric contraction
- Rapid ejection
- Reduced ejection
- Protodiastole
- Isovolumetric relaxation
- Rapid filling
- Reduced filling (diastasis)
- Atrial systole
Standard volumetric parameters are used to assess left ventricular pump function: end-diastolic volume (EDV) is 125 mL, end-systolic volume (ESV) is 60 mL, and the difference yields a stroke volume (SV) of 65 mL. Pressure evaluations analyze systolic, diastolic, and pulse pressure components.