Cardiac Anatomy

Heart Chambers and Circulation

The heart is a muscular organ responsible for pumping blood throughout the body via two main circuits: the pulmonary and systemic circuits. It consists of four chambers: two atria and two ventricles.

• Atria: The right atrium receives deoxygenated blood from the systemic circuit via the superior and inferior vena cava. The left atrium receives oxygenated blood from the pulmonary circuit via the pulmonary veins.

• Ventricles: The right ventricle pumps deoxygenated blood into the pulmonary circuit through the pulmonary artery. The left ventricle pumps oxygenated blood into the systemic circuit through the aorta.

• Receiving Chambers: Atria are considered the receiving chambers, collecting blood returning to the heart.

• Pumping Chambers: Ventricles are the pumping chambers, propelling blood out of the heart.

• Wall Thickness: The left ventricle has the thickest wall, as it must generate higher pressure to pump blood throughout the body.

Example: Blood flows from the right atrium → right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium → left ventricle → aorta → body tissues.

Heart Valves

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

• Atrioventricular (AV) Valves: Tricuspid (right) and bicuspid/mitral (left) valves separate atria from ventricles.

• Semilunar Valves: Pulmonary (right) and aortic (left) valves separate ventricles from major arteries.

• Valve Sounds: The closing of valves produces heart sounds ("lub-dub") detectable by stethoscope.

• Valve Pathologies: Stenosis (narrowing) and regurgitation (leakage) are common valve disorders.

Additional info: Valve closure timing is associated with phases of the cardiac cycle (systole and diastole).

Body Cavities and Membranes

The heart is located in the mediastinum, within the thoracic cavity, and is surrounded by the pericardium.

• Thoracic Cavity: Contains the heart and lungs.

• Mediastinum: Central compartment of the thoracic cavity housing the heart.

• Pericardium: A double-walled sac enclosing the heart, consisting of fibrous and serous layers.

• Pericardial Cavity: Space between serous layers containing pericardial fluid to reduce friction.

• Pericardial Effusion: Accumulation of fluid in the pericardial cavity can impair heart function.

Heart Wall Structure

Layers of the Heart Wall

The heart wall is composed of three layers, each with distinct tissue types and functions.

• Epicardium (outermost): Also called the visceral pericardium; provides protection and contains blood vessels.

• Myocardium (middle): Composed of cardiac muscle tissue; responsible for contraction and pumping action.

• Endocardium (innermost): Thin layer lining the heart chambers; consists of endothelial cells.

Additional info: The myocardium is the thickest layer, especially in the left ventricle.

Cardiac Muscle and Cellular Specializations

Cardiac Muscle Cell Structure

Cardiac muscle cells (cardiomyocytes) are specialized for both mechanical and electrical functions.

• Intercalated Discs: Specialized junctions containing desmosomes (mechanical coupling) and gap junctions (electrical coupling).

• Shape: Short, branched cells with a single central nucleus.

• Membrane Structures: Sarcolemma, T-tubules, and sarcoplasmic reticulum facilitate excitation-contraction coupling.

Additional info: Intercalated discs allow rapid transmission of action potentials and synchronized contraction.

Excitation-Contraction Coupling

Cardiac muscle contraction is initiated by action potentials and involves calcium ion movement.

• Calcium Source: Most Ca2+ for contraction comes from the extracellular space, unlike skeletal muscle.

• Process: Action potential → Ca2+ influx → sarcoplasmic reticulum release → contraction.

Cardiac Electrophysiology

Action Potentials in Cardiac Cells

Cardiac action potentials (APs) differ between contractile cells and pacemaker cells.

• Contractile Cells: Generate APs with distinct phases (depolarization, plateau, repolarization).

• Pacemaker Cells: Located in the sinoatrial (SA) node; generate spontaneous APs (pacemaker potential).

• Phases of Cardiac AP:

  • Phase 0: Rapid depolarization (Na+ influx)

  • Phase 1: Initial repolarization (K+ efflux)

  • Phase 2: Plateau (Ca2+ influx)

  • Phase 3: Repolarization (K+ efflux)

  • Phase 4: Resting potential

• Refractory Period: Long refractory period prevents tetanus in cardiac muscle.

Equation:

(ionic currents responsible for phases of the cardiac AP)

Conduction System of the Heart

The heart's conduction system coordinates the heartbeat.

• SA Node: Primary pacemaker; initiates APs.

• AV Node: Delays impulse, allowing atrial contraction before ventricular contraction.

• Bundle of His, Bundle Branches, Purkinje Fibers: Distribute impulse throughout ventricles.

Additional info: The conduction pathway ensures efficient, coordinated contraction of the heart chambers.

Electrocardiography (ECG/EKG)

An ECG records the electrical activity of the heart and is used to diagnose cardiac function and pathology.

• Waves and Intervals:

  • P wave: Atrial depolarization

  • QRS complex: Ventricular depolarization

  • T wave: Ventricular repolarization

• Clinical Use: Detects arrhythmias, myocardial infarction, and conduction abnormalities.

Cardiac Cycle and Hemodynamics

Phases of the Cardiac Cycle

The cardiac cycle consists of systole (contraction) and diastole (relaxation).

• Isovolumetric Contraction: Ventricles contract with all valves closed; pressure rises but volume remains constant.

• Stroke Volume (SV): Amount of blood ejected by a ventricle per beat.

• Equation:

where EDV = end-diastolic volume, ESV = end-systolic volume.

• Major Factors Affecting SV:

  • Preload: Degree of stretch of cardiac muscle before contraction.

  • Contractility: Strength of contraction.

  • Afterload: Resistance the heart must overcome to eject blood.

Heart Rate (HR) and Regulation

Heart rate is regulated by autonomic innervation and hormonal influences.

• Autonomic Nervous System:

  • Parasympathetic (via vagus nerve, cranial nerve X): Decreases HR; neurotransmitter is acetylcholine.

  • Sympathetic (via cardiac nerves from thoracic spinal cord): Increases HR and contractility; neurotransmitter is norepinephrine.

• Chronotropic Effect: Change in heart rate.

• Inotropic Effect: Change in contractility.

• Receptors: Muscarinic (parasympathetic), beta-adrenergic (sympathetic).

Additional info: The cardiac centers in the medulla oblongata regulate autonomic input to the heart.

Pathologies and Clinical Correlations

Common Cardiac Pathologies

Several conditions can affect heart structure and function.

• Myocardial Infarction (MI): Also called heart attack; caused by blockage of coronary arteries.

• Valve Disorders: Stenosis and regurgitation affect blood flow and pressure.

• Congenital Defects: Structural abnormalities present at birth.

• Pericardial Effusion: Fluid accumulation in the pericardial cavity.

HTML Table: Heart Wall Layers and Features

Summary

This study guide covers the essential anatomy and physiology of the heart, including its chambers, valves, wall structure, conduction system, cardiac cycle, and regulatory mechanisms. Understanding these concepts is fundamental for further study in cardiovascular physiology and clinical medicine.