Cardiac Structure and Coronary Circulation

Valvular Mechanics and Pressure Gradients

Blood flow through the heart is not driven by muscle contraction alone, but by pressure gradients that cause valves to open and close passively.

Atrioventricular (AV) Valves

The tricuspid valve (right side) and the mitral/bicuspid valve (left side) regulate flow between the atria and ventricles.

• Opening: These valves open when atrial pressure exceeds ventricular pressure.
• Closing: They close during systole (ventricular contraction) when ventricular pressure exceeds atrial pressure, preventing backflow. To prevent the valve leaflets from bulging backward (prolapsing) into the atria during high-pressure contraction, the chordae tendineae (fibrous cords) connect the leaflets to the papillary muscles, which project from the ventricular walls.

Semilunar Valves

The pulmonary valve and the aortic valve regulate blood exiting the ventricles into the great vessels.

• Opening: These valves open when ventricular pressure exceeds the pressure in the pulmonary artery or aorta.
• Closing: They close during diastole (ventricular relaxation) to prevent blood from flowing backward from the arteries into the ventricles.

The Cardiac Cycle and Wiggers Diagram

The cardiac cycle is a continuous loop of electrical and mechanical events. Crucially, electrical events (represented by the P wave, QRS complex, and T wave on an ECG) always precede the mechanical events of contraction and relaxation. The cycle is divided into systole (contraction/ejection) and diastole (relaxation/filling) through six distinct phases:

1. Isovolumetric contraction (systole): All valves are closed. LV pressure rises sharply, but volume remains constant at the End-Diastolic Volume (EDV). The closure of the mitral valve produces the first heart sound (S1).
2. Ventricular ejection (systole): LV pressure exceeds aortic pressure, opening the aortic valve and decreasing volume as blood exits.
3. Isovolumetric relaxation (diastole): All valves are closed. LV pressure falls rapidly while volume remains constant at the End-Systolic Volume (ESV). Aortic closure produces the second heart sound (S2) and the dicrotic notch in the aortic pressure trace.
4. Rapid ventricular filling (diastole): The mitral valve opens as LA pressure exceeds LV pressure, and volume rises quickly.
5. Diastasis (diastole): A period of slow, passive ventricular filling as the pressure gradient narrows.
6. Atrial systole (late diastole): Atrial contraction provides the final "atrial kick," contributing approximately 15–20% of the total ventricular filling to reach EDV.

Exam Tip: Remember that "isovolumetric" means the volume is constant. During these phases, both the inlet (AV) and outlet (semilunar) valves are closed, so no blood can enter or leave the chamber.

Pressure-Volume (PV) Loops

A PV loop is a powerful tool for visualizing cardiac function by plotting LV pressure (Y-axis) against LV volume (X-axis).

• The width of the loop represents the Stroke Volume (SV), which is the amount of blood ejected per beat.
• The mathematical relationship is: $\begin{aligned} SV &= EDV - ESV \end{aligned}$
• The loop's four limbs represent: isovolumetric contraction (right vertical), ejection (top), isovolumetric relaxation (left vertical), and ventricular filling (bottom).

Coronary Circulation and Perfusion

Because the myocardium is highly metabolic, it requires a constant supply of oxygen and nutrients via the coronary arteries.

Perfusion Timing and Demand

A critical distinction exists in how the two ventricles are perfused:

• Left Ventricle (LV): Perfusion occurs predominantly during diastole. During the forceful systolic contraction of the LV, the intramuscular and subendocardial vessels are compressed, significantly reducing blood flow.
• Right Ventricle (RV): Perfusion occurs during both systole and diastole because the RV operates at much lower pressures, resulting in less vessel compression.

The myocardium has a very high resting oxygen extraction rate (approximately 70%). Because there is little "reserve" to extract more oxygen from the blood, the primary way the heart meets increased oxygen demand (such as during exercise) is by increasing coronary blood flow.

Regulation of Flow

Coronary blood flow is primarily regulated by local metabolic mediators. When oxygen levels drop, the breakdown of ATP leads to the production of adenosine, a potent vasodilator that increases blood flow to the needy tissue. While the autonomic nervous system can influence the heart (e.g., sympathetic activity increasing heart rate and contractility), the local metabolic signals are the dominant regulators and can outweigh direct vasoconstrictor effects.

Coronary Anatomy and Territories

The coronary arteries arise from the aortic root. Key territories include:

• Left Anterior Descending (LAD): Supplies the anterior two-thirds of the interventricular septum and the anterolateral/apical LV walls.
• Left Circumflex (LCx): Supplies the left atrium and the lateral/posterior LV walls.
• Right Coronary Artery (RCA): Supplies the right atrium, right ventricle, and the posterior third of the interventricular septum.

If a major artery is obstructed, the body may undergo angiogenesis—the formation of new blood vessels—to create collateral circulation. While collateral vessels might maintain blood flow at rest, they may be insufficient during strenuous activity, leading to ischemia and chest pain (angina).