Cardiac Anatomy and Physiology

Key Terms and Definitions

• Mediastinum: The central compartment of the thoracic cavity, containing the heart, great vessels, trachea, and other structures.

• Base: The broad, superior aspect of the heart where major vessels attach.

• Apex: The pointed, inferior tip of the heart, directed downward and to the left.

• Anastomoses: Connections between blood vessels that allow for alternate routes of blood flow.

• Functional Syncytium: A group of cells that function as a unit due to electrical coupling via gap junctions.

• Systole: The phase of the cardiac cycle when the heart muscle contracts and pumps blood.

• Diastole: The phase when the heart muscle relaxes and fills with blood.

• Cardiac Reserve: The difference between resting and maximal cardiac output.

• Preload: The degree of stretch of cardiac muscle cells before contraction, related to venous return.

• Afterload: The pressure the heart must overcome to eject blood.

• Frank-Starling Law of the Heart: The relationship between preload and stroke volume; increased stretch leads to increased force of contraction.

Cardiac Circulation

Pulmonary vs. Systemic Circuits

The heart pumps blood through two main circuits: the pulmonary and systemic circuits, each with distinct pathways and functions.

• Pulmonary Circuit:

  • Starts at the right ventricle (pumping chamber).

  • Ends at the left atrium (receiving chamber).

  • Carries deoxygenated blood to the lungs for oxygenation.

  • Operates under low pressure.

• Systemic Circuit:

  • Starts at the left ventricle.

  • Ends at the right atrium.

  • Carries oxygenated blood to the body tissues.

  • Operates under high pressure.

• Both circuits carry equal volumes of blood per cycle.

Heart Structure

Coverings and Layers of the Heart

The heart is protected and supported by several layers and coverings.

• Pericardium: Double-walled sac surrounding the heart.

  • Fibrous Pericardium: Outer tough layer.

  • Serous Pericardium: Inner layer, subdivided into parietal and visceral layers.

• Heart Wall Layers:

  • Epicardium: Outer layer (visceral pericardium).

  • Myocardium: Middle, muscular layer responsible for contraction.

  • Endocardium: Inner endothelial lining.

Cardiac Skeleton

The cardiac skeleton is a framework of dense connective tissue.

• Composition: Dense connective tissue.

• Function: Provides structural support, anchors valves, and insulates electrical impulses.

• Location: Surrounds the heart valves and base of the great vessels.

Flow of Blood Through the Heart

Blood follows a specific pathway through the heart chambers and valves.

1. Right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary trunk → lungs

2. Lungs → pulmonary veins → left atrium → bicuspid (mitral) valve → left ventricle → aortic valve → aorta → body

Heart Valves

Valves ensure unidirectional blood flow and prevent backflow.

• Atrioventricular (AV) Valves:

  • Tricuspid Valve: Between right atrium and right ventricle.

  • Bicuspid (Mitral) Valve: Between left atrium and left ventricle.

• Semilunar Valves:

  • Pulmonary Valve: Between right ventricle and pulmonary trunk.

  • Aortic Valve: Between left ventricle and aorta.

Coronary Circulation

The heart receives its own blood supply via coronary arteries and veins.

• Coronary Arteries: Branch from the aorta and supply oxygenated blood to the heart muscle.

• Cardiac Veins: Collect deoxygenated blood from the myocardium and drain into the coronary sinus, which empties into the right atrium.

• "LA is Great" mnemonic: The Left Atrium receives blood from the Great Cardiac Vein.

Cardiac Muscle Cells

Structure and Comparison to Skeletal Muscle

Cardiac muscle cells have unique features compared to skeletal muscle cells.

• Cardiac Muscle Cells:

  • Usually single nucleus per cell.

  • Striated, short, branched, and interconnected.

  • Connected by intercalated discs containing gap junctions and desmosomes.

  • Calcium comes from both extracellular fluid and sarcoplasmic reticulum.

• Skeletal Muscle Cells:

  • Multiple nuclei per cell.

  • Long, cylindrical, not branched.

  • Calcium comes only from sarcoplasmic reticulum.

Intercalated Discs

• Definition: Specialized junctions between cardiac muscle cells.

• Components: Gap junctions (allow electrical coupling) and desmosomes (provide mechanical strength).

Types of Myocytes

• Contractile Cells: Responsible for contraction and pumping blood.

• Pacemaker Cells: Initiate and conduct electrical impulses.

Cardiac Electrophysiology

Action Potential of Cardiac Pacemaker Cells

Pacemaker cells generate spontaneous action potentials to set the heart's rhythm.

• Pacemaker Potential: Due to slow Na+ channels opening.

• Depolarization: Due to Ca2+ channels opening.

• Repolarization: Due to K+ channels opening.

• No true resting membrane potential; instead, a gradual depolarization.

Sequence of Excitation

The heart's electrical system coordinates contraction.

1. Sinoatrial (SA) Node: Natural pacemaker, located in right atrium; sets the sinus rhythm.

2. Atrioventricular (AV) Node: Located at the junction of atria and ventricles; site of electrical delay.

3. AV Bundle (Bundle of His): Only electrical connection between atria and ventricles.

4. Right and Left Bundle Branches

5. Purkinje Fibers: Spread impulse throughout ventricles.

• Delay at AV Node: Allows atria to contract before ventricles.

• Cardiac Skeleton: Insulates and prevents direct conduction between atria and ventricles.

Autonomic Innervation of the Heart

• Sympathetic Nerves (Cardiac Nerves): Increase heart rate and force of contraction.

• Parasympathetic Nerves (Vagus Nerve): Decrease heart rate.

Action Potential of Contractile Cells

• Depolarization: Due to Na+ channels opening (Na+ influx).

• Plateau: Due to Ca2+ channels opening (Ca2+ influx).

• Repolarization: Due to K+ channels opening (K+ efflux).

Cardiac vs. Skeletal Muscle Contraction

• Cardiac Muscle:

  • Longer contraction and absolute refractory period.

  • Initiation is not required from the nervous system (automaticity).

• Skeletal Muscle:

  • Shorter contraction and refractory period.

  • Requires nervous system stimulation.

Cardiac Cycle and Hemodynamics

Phases of the Cardiac Cycle

The cardiac cycle consists of alternating periods of contraction and relaxation.

1. Total Relaxation (Quiescent Period)

2. Atrial Systole

3. Ventricular Systole

4. Ventricular Diastole

• Valves open or close depending on pressure differences across them.

End Diastolic Volume (EDV) and End Systolic Volume (ESV)

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

• ESV: Volume of blood remaining in a ventricle after systole (contraction).

Stroke Volume (SV)

• Definition: Amount of blood pumped by a ventricle per beat.

• Calculation:

Heart Sounds

• Cause: Closing of heart valves.

• First sound ("lub"): Closure of AV valves at start of ventricular systole.

• Second sound ("dup"): Closure of semilunar valves at start of ventricular diastole.

Cardiac Output (CO)

• Definition: Volume of blood pumped by each ventricle per minute.

• Calculation:

• HR: Heart rate (beats per minute).

• SV: Stroke volume (mL per beat).

Regulation of Cardiac Output

• Preload: Increased by greater venous return; increases EDV and SV (Frank-Starling Law).

• Contractility: Increased by sympathetic stimulation or certain hormones; decreases ESV and increases SV.

• Afterload: Increased by higher arterial pressure; increases ESV and decreases SV.

Summary Table: Effects on Stroke Volume

Fetal Circulation

Structures Bypassing Pulmonary Circulation

• Foramen Ovale: Opening between right and left atria; allows blood to bypass lungs.

• Ductus Arteriosus: Vessel connecting pulmonary trunk to aorta; diverts blood from lungs to systemic circulation.

Both structures close after birth to establish normal adult circulation.

Additional info: Where context was missing, standard textbook details were added for completeness.