Comparative physiology of the right and left heart
Although both halves of the heart work in coordination, there are pronounced functional and morphological differences between them. The right ventricle performs significantly less work compared to the left, which is directly reflected in the structure of its wall—it is much thinner.
Key hemodynamic differences:
- Minimum diastolic pressure: In the left half, it never drops below 5 mmHg. Meanwhile, in the right half, the pressure approaches zero and even becomes negative (below atmospheric level) due to the suction effect of the chest during inspiration.
- Contraction asynchrony: Electrical excitation reaches the right atrium first, so its systole begins 10 ms earlier than that of the left.
- Ventricular work: The onset of systole in both ventricles occurs simultaneously. However, due to the low resistance and pressure in the pulmonary circulation, the tension and relaxation phases in the right ventricle proceed faster. As a result, right ventricular systole itself lasts longer, while diastole (the rest period) is shorter. Despite the right side resting less and working slower, its overall energy expenditure is lower.
Cardiac output balance and physiological shunting
A mandatory rule of normal hemodynamics is the equality of minute cardiac output between both halves of the heart. At the same time, stroke volumes (SV) differ slightly: the left ventricle ejects about 65 ml of blood per contraction, whereas the right ejects approximately 64 ml.
This slight difference is due to the presence of a physiological shunt, which accounts for about 1–2% of the total blood volume. This portion of blood completely bypasses the right chambers of the heart.
Shunting mechanisms:
- Direct drainage of some coronary veins directly into the left atrial cavity.
- Blood diversion from the systemic circulation (bronchial arteries) into the pulmonary vascular bed, followed by drainage via pulmonary veins directly into the left atrium.
Pressure-Volume loop and phases of the cardiac cycle
Hemodynamic processes in the left ventricle are clearly illustrated by the Pressure-Volume Loop. It describes the relationship between intraventricular pressure and blood volume throughout a single cardiac cycle. The shape and position of this loop change depending on hemodynamic conditions: it shifts with increased venous pressure (increased preload) or increased arterial pressure (increased afterload).
The left ventricular cardiac cycle includes the following sequential phases:
- Asynchronous contraction.
- Isometric contraction.
- Rapid ejection.
- Reduced ejection.
- Protodiastole.
- Isometric relaxation.
- Rapid filling.
- Reduced filling.
- Atrial systole.
To assess left ventricular pump function, standard normative volume parameters are used: end-diastolic volume (EDV) is 125 ml, end-systolic volume (ESV) is 60 ml, and the difference between them forms the stroke volume (SV) of 65 ml. Pressure assessment includes the analysis of systolic (SP), diastolic (DP), and pulse pressure (PP) components.
Briefly: answers to topical questions
Why does right atrial systole begin earlier than the left?
Right atrial systole begins 10 ms earlier than the left because excitation spreads to the right atrium earlier. Excitation originates in the sinoatrial node and is conducted via specialized atrial fibers at a rate of 1 m/s through the right and left atria.
What factors determine left ventricular preload and afterload?
Left ventricular preload and afterload are determined by the state of the venous and arterial vascular beds.
- Preload depends on the magnitude of venous return to the heart, venous pressure, and the tone of capacitance vessels (veins).
- Afterload is determined by arterial pressure, the tone of resistance vessels (arteries), and total peripheral vascular resistance, which the ventricle must overcome to eject blood into the aorta.
What mechanisms provide the suction action of the chest during inspiration?
The suction action of the chest during inspiration is provided by the respiratory pump and the pressure gradient. During inspiration, intrathoracic pressure decreases and becomes more negative, leading to the expansion of intrathoracic veins and facilitating blood inflow. Simultaneously, intra-abdominal pressure increases, creating a pressure gradient that draws blood from the vena cava into the thoracic cavity.
Class and exam questions
Why can diastolic pressure in the right side of the heart be negative?
This occurs under the influence of the act of inspiration. Chest expansion creates a suction effect capable of lowering the pressure in the right heart chambers below atmospheric level (below zero).
Do the systoles of the right and left ventricles coincide in time?
The onset of systole in both ventricles occurs absolutely simultaneously. However, the total duration of the phases differs: in the right ventricle, the tension and relaxation phases are shorter, and systole itself lasts longer than in the left.
What is a physiological shunt in hemodynamics?
This is the blood volume (about 1–2%) that enters the left heart, bypassing the right. It is formed by blood from the coronary veins and bronchial arteries that drains directly into the left atrium.
What else to study on this topic
- Mechanisms of preload influence on the Pressure-Volume loop shape
- Changes in hemodynamic parameters with increased afterload
- Detailed characteristics of isometric contraction and relaxation
- Myocardial energetics and oxygen consumption in the right and left ventricles
- The relationship between systolic, diastolic, and pulse pressure
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