Cardiac Anatomy and Physiology: Study Guide

Cardiac Anatomy and Physiology

Gross Anatomy of the Heart

The heart is a muscular organ responsible for pumping blood throughout the body. Understanding its structure is essential for comprehending its function.

• Location: The heart is located in the thoracic cavity, within the mediastinum, between the lungs.

• Orientation: The apex points inferiorly and to the left, while the base is superior and directed toward the right shoulder.

• Coverings: The heart is enclosed by the pericardium, a double-walled sac that protects and anchors the heart.

• Layers of the Heart Wall:

  • Epicardium: The outermost layer, also known as the visceral pericardium.

  • Myocardium: The thick, muscular middle layer responsible for contraction.

  • Endocardium: The innermost layer lining the heart chambers and valves.

Example: The pericardium prevents overfilling of the heart and reduces friction during heartbeats.

Microscopic Anatomy of Cardiac Muscle

Cardiac muscle cells, or cardiomyocytes, are specialized for continuous rhythmic contraction.

• Intercalated Discs: Specialized junctions between cardiac cells that contain desmosomes (for mechanical strength) and gap junctions (for electrical connectivity).

• Desmosomes: Anchor adjacent cells together, preventing separation during contraction.

• Gap Junctions: Allow ions to pass between cells, enabling synchronized contraction.

Example: The presence of gap junctions allows the heart to function as a syncytium, with coordinated contraction of the myocardium.

Intrinsic Cardiac Conduction System

The heart's rhythmic contractions are coordinated by a specialized conduction system.

• Sinoatrial (SA) Node: The primary pacemaker, located in the right atrium.

• Atrioventricular (AV) Node: Delays the impulse before it passes to the ventricles.

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

Example: Damage to the SA node can result in arrhythmias, requiring artificial pacemakers.

Action Potential of Contractile Cardiac Muscle Cells

Cardiac muscle cells generate action potentials that trigger contraction.

• Phases: Rapid depolarization (Na+ influx), plateau phase (Ca2+ influx), and repolarization (K+ efflux).

• Significance: The plateau phase prolongs the action potential, preventing tetanus and ensuring rhythmic contractions.

Example: The unique action potential of cardiac muscle allows for a refractory period that protects against abnormal rhythms.

The Cardiac Cycle

The cardiac cycle describes the sequence of events in one heartbeat, including contraction and relaxation of the atria and ventricles.

• Systole: Contraction phase, during which blood is ejected from the chambers.

• Diastole: Relaxation phase, during which the chambers fill with blood.

• Blood Flow: Blood flows from veins into atria, through AV valves into ventricles, and out through semilunar valves to arteries.

Example: During ventricular systole, the left ventricle pumps blood into the aorta.

Blood Flow Through the Heart

Blood follows a specific pathway through the heart, ensuring separation of oxygenated and deoxygenated blood.

• Right atrium → right ventricle → pulmonary arteries → lungs (oxygenation)

• Lungs → pulmonary veins → left atrium → left ventricle → aorta → systemic circulation

Example: The pulmonary circuit carries blood to and from the lungs, while the systemic circuit supplies the rest of the body.

Heart Pathology

Diseases of the heart can affect its coverings, valves, or muscle tissue.

• Pericarditis: Inflammation of the pericardium.

• Valve Disorders: Such as stenosis (narrowing) or regurgitation (leakage).

• Myocardial Infarction (Heart Attack): Death of cardiac muscle tissue due to blocked blood supply.

Example: A myocardial infarction can result from blockage of a coronary artery.

Autonomic Regulation of the Heart

The heart rate and force of contraction are regulated by the autonomic nervous system and cardiac centers in the brain.

• Cardioacceleratory Center: Increases heart rate via sympathetic stimulation.

• Cardioinhibitory Center: Decreases heart rate via parasympathetic (vagus nerve) stimulation.

• Location: These centers are located in the medulla oblongata.

Example: During exercise, sympathetic stimulation increases heart rate and contractility.

Calculation of Stroke Volume

Stroke volume (SV) is the amount of blood ejected by a ventricle in one contraction.

• Formula:

$SV = EDV - ESV$

• EDV (End-Diastolic Volume): Volume of blood in the ventricle at the end of filling (diastole).

• ESV (End-Systolic Volume): Volume of blood remaining in the ventricle after contraction (systole).

Example: If EDV = 120 mL and ESV = 50 mL, then $SV = 120 - 50 = 70$ mL.

Summary Table: Key Features of Cardiac Anatomy and Physiology

Feature	Description	Example/Significance
Pericardium	Protective sac around the heart	Reduces friction during heartbeats
Myocardium	Muscular layer responsible for contraction	Pumps blood throughout the body
Intercalated Discs	Junctions between cardiac cells	Enable synchronized contraction
SA Node	Primary pacemaker of the heart	Initiates heartbeat
Stroke Volume	Blood ejected per beat	Calculated as $SV = EDV - ESV$