Cardiac Anatomy & Physiology: Structure, Function, and Clinical Concepts

Cardiac Anatomy and Physiology

Structural Differences Between Right and Left Sides of the Heart

The heart is divided into right and left sides, each with distinct roles in circulation.

• Right Side: Pumps blood to the lungs via the pulmonary circuit.

• Left Side: Pumps blood to the rest of the body via the systemic circuit.

• Pressure Differences: The left ventricle generates higher pressure to overcome greater resistance in systemic circulation, while the right ventricle is shorter and crescent-shaped due to lower resistance in pulmonary circulation.

Example: The left ventricle's thicker wall is an adaptation for pumping blood throughout the body.

Pericardium and Heart Enclosure

The heart is enclosed by the serous pericardium, which provides protection and reduces friction.

• Structure: The pericardium consists of two continuous layers: the parietal pericardium (fused to the inner surface of the fibrous pericardium) and the visceral pericardium (epicardium).

• Function: The epicardium is the most superficial layer of the heart wall.

Coronary Arteries and Myocardial Infarction

Blood flow through the coronary arteries is essential for heart muscle function.

• Anastomoses: Connections between arteries can provide alternate routes for blood flow.

• Blockage: Blockage of major coronary arteries (e.g., by a clot) can cause myocardial infarction (heart attack).

• Clinical Note: Blockage of more distal arteries may allow alternate routes, but blockage of main coronary arteries is more likely to cause infarction.

Example: A clot in a main coronary artery can obstruct blood flow to the myocardium, causing tissue death.

Heart Valves and Prevention of Backflow

Valves in the heart ensure unidirectional blood flow and prevent backflow.

• Valves: The tricuspid and mitral (bicuspid) valves separate atria from ventricles; semilunar valves (aortic and pulmonary) separate ventricles from arteries.

• Function: Valves close when pressure changes, preventing blood from flowing backward.

Example: When the ventricles contract, the AV valves close to prevent backflow into the atria.

Specialized Cardiac Muscle and Intercalated Discs

Cardiac muscle fibers are interconnected by intercalated discs, which facilitate coordinated contraction.

• Intercalated Discs: Contain gap junctions for electrical synapses, allowing rapid transmission of action potentials.

• Function: Ensures the heart contracts as a functional syncytium.

Electrophysiology: Action Potentials and the Plateau Phase

Cardiac muscle cells exhibit a unique action potential with a plateau phase.

• Plateau Phase: Ca2+ channels open, slowing repolarization and prolonging contraction.

• Importance: Prevents tetany and allows time for the heart to fill with blood.

Equation: $I_{Ca} = g_{Ca} (V - E_{Ca})$

Example: The plateau phase lengthens the cardiac action potential to about 200–300 ms.

AV Node Delay and Cardiac Conduction

The AV node introduces a delay in the transmission of electrical impulses from the atria to the ventricles.

• Purpose: Allows time for the atria to contract and fill the ventricles before ventricular contraction.

• Clinical Note: Too short a delay can lead to inefficient filling; too long a delay can cause arrhythmias.

ECG Interpretation: P Wave and QRS Complex

The electrocardiogram (ECG) records the electrical activity of the heart.

• P Wave: Represents atrial depolarization.

• QRS Complex: Represents ventricular depolarization.

• Clinical Note: Abnormal P waves may indicate atrial fibrillation; QRS complexes indicate ventricular activity.

Pressure Gradients and Blood Flow

Blood flows through the heart due to pressure differences created by contraction.

• Contraction: Increases pressure in the chambers, driving blood into the arteries.

• Valves: Open and close in response to pressure changes.

Equation: $\Delta P = Q \times R$ (where $\Delta P$ is pressure difference, $Q$ is flow, $R$ is resistance)

Stroke Volume and Cardiac Output

Stroke volume is the amount of blood ejected by a ventricle per beat; cardiac output is the total volume pumped per minute.

• Normal Values: Stroke volume ≈ 55 mL; heart rate ≈ 80 bpm; cardiac output ≈ 4.4 L/min.

• Influencing Factors: Preload, afterload, contractility, heart rate, blood pressure, temperature, and oxygen levels.

Equation: $\text{Cardiac Output} = \text{Stroke Volume} \times \text{Heart Rate}$

Factors Influencing Stroke Volume

Several factors affect stroke volume, including preload, contractility, and afterload.

Factor	Effect on Stroke Volume
Preload	Increases stroke volume by stretching cardiac muscle fibers (Frank-Starling law)
Contractility	Increased contractility raises stroke volume
Afterload	Increased afterload decreases stroke volume

Example: High blood pressure increases afterload, reducing stroke volume.

End-Systolic Volume (ESV) and Cardiac Output

ESV is the volume of blood remaining in the ventricle after contraction.

• High ESV: Indicates low stroke volume and reduced cardiac output.

• Low ESV: Indicates efficient ejection of blood.

Heart Failure: Pulmonary and Peripheral Edema

Heart failure can lead to fluid accumulation in the lungs (pulmonary edema) or peripheral tissues (peripheral edema).

• Left Ventricular Failure: Causes pulmonary congestion by increasing pressure in pulmonary capillaries.

• Right and Left Ventricular Failure: Both can cause peripheral edema by increasing systemic capillary pressure.

Example: Fluid backs up into the lungs in left-sided heart failure, and into peripheral tissues in right-sided failure.

Summary Table: Heart Failure and Edema

Type of Failure	Primary Effect	Resulting Edema
Left Ventricular Failure	Increased pulmonary capillary pressure	Pulmonary edema
Right Ventricular Failure	Increased systemic capillary pressure	Peripheral edema
Both	Generalized increase in capillary pressure	Pulmonary and peripheral edema

Additional info: Academic context and definitions have been expanded for clarity and completeness.