The Heart: Structure, Function, and Physiology – Study Notes

The Heart: Structure, Function, and Physiology

Overview of the Heart

The heart is a muscular organ that serves as the central pump of the circulatory system, ensuring the delivery of blood throughout the body. Its anatomy and physiology are essential for understanding cardiovascular health and disease.

• Location: The heart is situated within the mediastinum of the thorax, positioned obliquely from the 2nd to the 5th intercostal space.

• Orientation: The base of the heart is directed toward the right shoulder, while the apex points toward the left hip.

• Function: Acts as a transport system pump, moving blood through hollow blood vessels.

Layers of the Heart Wall

The heart wall consists of three distinct layers, each with specialized functions:

• Pericardium: A double-walled, fibro-serous sac that encloses the heart, protecting it and preventing overfilling.

• Myocardium: The thick, muscular middle layer composed of cardiac muscle cells arranged in bundles. Responsible for the contractile force of the heart.

• Endocardium: The innermost layer, made of simple squamous epithelium, lining the heart chambers and continuous with the endothelium of blood vessels.

Pericardium Structure

• Fibrous Pericardium: Outermost layer; protects and anchors the heart.

• Serous Pericardium: Composed of two layers:

  • Parietal Layer: Lines the internal surface of the fibrous pericardium.

  • Visceral Layer (Epicardium): Covers the external surface of the heart.

• Pericardial Cavity: Fluid-filled space between the parietal and visceral layers, reducing friction during heartbeats.

Heart Chambers and Associated Structures

The heart contains four chambers: two atria (receiving chambers) and two ventricles (discharging chambers).

• Atria: Thin-walled chambers that receive blood returning to the heart.

• Ventricles: Thick-walled chambers that pump blood out of the heart.

• Associated Structures: Interatrial septum, interventricular septum, coronary sulcus, anterior and posterior interventricular sulci.

Heart Valves

Valves ensure unidirectional blood flow through the heart:

• 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 artery.

  • Aortic Valve: Between left ventricle and aorta.

• Chordae Tendineae & Papillary Muscles: Prevent valve prolapse during ventricular contraction.

Coronary Circulation

The heart’s own blood supply is provided by the coronary arteries and veins.

• Coronary Arteries: Arise from the base of the aorta and encircle the heart in the atrioventricular groove.

• Coronary Veins: Collect deoxygenated blood from the myocardium and drain into the coronary sinus.

• Anastomoses: Junctions between vessels provide alternate routes for blood delivery.

Cardiac Muscle vs. Skeletal Muscle

Cardiac muscle fibers have unique properties compared to skeletal muscle cells.

Feature	Cardiac Muscle	Skeletal Muscle
Striations	Present	Present
Intercalated Discs	Present (desmosomes, gap junctions)	Absent
Contraction	Involuntary, rhythmic	Voluntary, rapid
Mitochondria	30% of volume	2% of volume
Syncytium	Functional syncytium	Motor unit, not syncytial

Intercalated Discs

• Desmosomes: Provide strong cell-cell adhesion during contraction.

• Gap Junctions: Allow electrical coupling, enabling the heart to function as a syncytium.

Electrical Properties of Cardiac Muscle

Cardiac muscle cells exhibit unique electrical characteristics essential for coordinated contraction.

• Contractile Cells: Respond to action potentials with a twitch contraction.

• Autorhythmic Cells: (1% of cardiac cells) Spontaneously depolarize, initiating heartbeat.

• All-or-None Law: Applies to the entire organ, not individual cells.

• Absolute Refractory Period: Approximately 250 ms, preventing tetanus.

Action Potential in Cardiac Muscle

• Depolarization: Due to Na+ influx through fast voltage-gated channels.

• Plateau Phase: Due to Ca2+ influx through slow channels, maintaining depolarization.

• Repolarization: Due to K+ efflux as channels open.

Intrinsic Conduction System

The heart’s intrinsic conduction system coordinates its rhythmic contractions.

• SA Node: Pacemaker; initiates impulses.

• AV Node: Delays impulse, allowing atria contraction.

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

Extrinsic Innervation

• Sympathetic Nervous System (SNS): Increases heart rate and contractility.

• Parasympathetic Nervous System (PNS): Decreases heart rate.

Electrocardiogram (ECG)

An ECG records the electrical activity of the heart, providing information about its rhythm and conduction.

• P Wave: Atrial depolarization.

• QRS Complex: Ventricular depolarization.

• T Wave: Ventricular repolarization.

Cardiac Cycle

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

• Atrial Systole: Atria contract, filling ventricles.

• Ventricular Systole: Ventricles contract, ejecting blood.

• Quiescent Period: All chambers relaxed.

• Pressure Changes: Drive valve opening/closing and blood flow.

Heart Sounds

• First Heart Sound (S1): Closure of AV valves; marks beginning of systole.

• Second Heart Sound (S2): Closure of semilunar valves; marks end of systole.

• Murmurs: Abnormal sounds due to valve defects (stenosis or insufficiency).

Cardiac Output (CO)

Cardiac output is the volume of blood pumped by the left ventricle per minute.

• Formula:

$CO = HR \times SV$

• HR: Heart rate (beats per minute)

• SV: Stroke volume (volume per beat)

For a resting, healthy person:

• $CO \approx 5$ liters/minute

Regulation of Heart Rate

• Autonomic Nervous System: SNS increases HR (norepinephrine), PNS decreases HR (acetylcholine).

• Hormones: Epinephrine increases HR.

• Electrolytes: Ca2+, K+ affect HR.

• Temperature: Increased body temperature raises HR.

Frank-Starling Law of the Heart

The heart pumps whatever volume of blood it receives, within physiological limits.

• Preload: Degree of stretch of cardiac muscle before contraction; increased preload increases stroke volume.

• Afterload: Pressure ventricles must overcome to eject blood; increased afterload decreases stroke volume.

Extrinsic Influences on Stroke Volume

• Contractility: Increased by sympathetic stimulation and certain drugs (e.g., digoxin).

• Parasympathetic Stimulation: Decreases contractility.

Key Terms and Definitions

• Systole: Contraction phase of the heart.

• Diastole: Relaxation phase of the heart.

• Isovolumetric Contraction/Relaxation: Phases where ventricles contract/relax with no volume change (all valves closed).

• Dicrotic Notch: Brief rise in aortic pressure after closure of aortic valve.

Summary Table: Heart Valves and Blood Flow

Valve	Location	Function
Tricuspid	Right atrioventricular	Prevents backflow into right atrium
Pulmonary	Right ventricle to pulmonary artery	Prevents backflow into right ventricle
Bicuspid (Mitral)	Left atrioventricular	Prevents backflow into left atrium
Aortic	Left ventricle to aorta	Prevents backflow into left ventricle

Example: Pathway of a Red Blood Cell Through the Heart

1. Enters right atrium via superior/inferior vena cava or coronary sinus

2. Through tricuspid valve to right ventricle

3. Through pulmonary valve to pulmonary trunk and arteries

4. To lungs (gas exchange)

5. Returns via pulmonary veins to left atrium

6. Through bicuspid (mitral) valve to left ventricle

7. Through aortic valve to aorta and systemic circulation

Additional info: Some details and terminology have been expanded for clarity and completeness, including definitions, physiological mechanisms, and summary tables.