Heart Anatomy and Location

Mediastinum and Heart Size

The heart is a muscular organ located in the mediastinum, the central compartment of the thoracic cavity. It typically weighs between 250-350 grams in adults.

• Mediastinum: The area between the lungs where the heart resides.

• Heart Size: Average adult heart mass is 250-350 g.

Pericardium and Heart Wall Layers

The heart is enclosed by a double-walled sac called the pericardium, which protects and anchors the heart within the mediastinum.

• Fibrous Pericardium: The tough, external layer providing protection and anchorage.

• Serous Pericardium: The internal layer, subdivided into:

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

  • Visceral Pericardium (Epicardium): Covers the heart surface.

• Pericardial Cavity: Space between parietal and visceral layers, filled with serous fluid to reduce friction.

• Myocardium: The thick, contractile middle layer composed of cardiac muscle tissue.

• Endocardium: The shiny, inner layer made of simple squamous epithelium.

Heart Chambers and Circulation

Chambers and Septa

The heart consists of four chambers separated by septa and valves, facilitating unidirectional blood flow.

• Atria (Superior Chambers): Right and left atria receive blood returning to the heart.

• Ventricles (Inferior Chambers): Right and left ventricles pump blood out of the heart.

• Interatrial and Interventricular Septa: Internal walls separating the right and left sides.

• Auricles: Small, ear-like extensions of the atria that increase volume capacity.

• Remnants of Foramen Ovale: A fetal structure in the interatrial septum, which closes after birth.

Major Vessels and Circuits

Blood enters and leaves the heart through major vessels, following two main circuits.

• Right Atrium: Receives blood from the superior and inferior venae cavae and coronary sinus.

• Left Atrium: Receives blood from the four pulmonary veins.

• Right Ventricle: Pumps blood into the pulmonary trunk (pulmonary circuit).

• Left Ventricle: Pumps blood into the aorta (systemic circuit).

Pulmonary Circuit: Carries deoxygenated blood from the right ventricle to the lungs and returns oxygenated blood to the left atrium.

Systemic Circuit: Delivers oxygenated blood from the left ventricle to the body and returns deoxygenated blood to the right atrium.

Heart Valves

Types and Functions

Heart valves ensure unidirectional blood flow and prevent backflow.

• Atrioventricular (AV) Valves: Located between atria and ventricles.

  • Tricuspid Valve: Between right atrium and right ventricle.

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

  • Chordae Tendineae: Tendinous cords connecting valves to papillary muscles, preventing valve prolapse.

• Semi-lunar (SL) Valves: Located between ventricles and arteries.

  • Pulmonary Valve: Between right ventricle and pulmonary trunk.

  • Aortic Valve: Between left ventricle and aorta.

Coronary Circulation

Arteries and Veins

The heart muscle (myocardium) receives its blood supply from the coronary arteries and is drained by cardiac veins.

• Coronary Arteries: Arise from the base of the aorta.

  • Right Coronary Artery: Branches into posterior interventricular and right marginal arteries.

  • Left Coronary Artery: Branches into anterior interventricular and circumflex arteries.

• Cardiac Veins: Drain blood from the myocardium into the coronary sinus, which empties into the right atrium.

Cardiac Muscle and Physiology

Structure and Function

Cardiac muscle cells are specialized for continuous, rhythmic contraction.

• Striated, Short, Branched Cells: Usually mono- or binucleated.

• Intercalated Discs: Specialized junctions containing:

  • Gap Junctions: Allow electrochemical communication.

  • Desmosomes: Provide physical strength and stability.

• High Mitochondrial Volume: Cardiac muscle cells have abundant mitochondria (about 25-35% of cell volume), supporting high energy demands.

• Fatigue Resistance: Cardiac muscle is highly resistant to fatigue but requires a constant oxygen supply.

• Autorhythmicity: Some cardiac cells can spontaneously depolarize, initiating heart contractions.

• Long Refractory Period: Prevents tetanic contractions, allowing chambers to refill with blood.

Cardiac Conduction System

Pathway of Electrical Impulses

The heart's conduction system coordinates the sequence of cardiac muscle contraction.

• Sinoatrial (SA) Node: Located in the right atrium; acts as the pacemaker, setting heart rate (~75 beats/min).

• Internodal Pathways: Transmit impulses from the SA node to the atrioventricular (AV) node.

• Atrioventricular (AV) Node: Located at the base of the right atrium; delays impulse (~0.1 sec) to allow atrial contraction before ventricular contraction.

• AV Bundle (Bundle of His): Conducts impulses down the interventricular septum.

• Bundle Branches: Split into right and left branches, traveling toward the apex of the heart.

• Purkinje Fibers: Spread throughout ventricular walls, causing coordinated ventricular contraction.

Electrocardiogram (ECG/EKG)

Electrical Events and Waves

An ECG records the electrical activity of the heart, providing information about cardiac function.

• P Wave: Atrial depolarization (leads to atrial systole).

• QRS Complex: Ventricular depolarization (leads to ventricular systole) and atrial repolarization.

• T Wave: Ventricular repolarization (leads to ventricular diastole).

• P-R Interval: Time from beginning of atrial depolarization to beginning of ventricular depolarization.

• S-T Segment: Time when ventricles are completely depolarized.

• Q-T Interval: Time from beginning of ventricular depolarization to end of ventricular repolarization.

Note: These electrical signals result in mechanical events—contraction and relaxation of heart chambers.

Cardiac Cycle

Phases and Heart Sounds

The cardiac cycle includes all events associated with one heartbeat, including contraction (systole) and relaxation (diastole) of the atria and ventricles.

• Systole: Contraction phase; ventricular systole ejects blood into arteries, causing systolic blood pressure.

• Diastole: Relaxation phase; ventricular diastole allows chambers to fill with blood, causing diastolic blood pressure.

• Heart Sounds: "Lub-dup" sounds are produced by valve closures:

  • Lub: AV valves closing during ventricular systole.

  • Dup: SL valves closing during ventricular diastole.

  • Murmurs: Abnormal sounds usually due to valve issues.

Cardiac Output and Regulation

Definitions and Equations

Cardiac output (CO) is the amount of blood pumped by each ventricle per minute.

• Heart Rate (HR): Number of beats per minute (~75 b/m).

• Stroke Volume (SV): Volume of blood pumped by each ventricle per beat (~70 ml).

• Cardiac Output (CO):

• Example Calculation:

• Cardiac Reserve (CR):

• End Diastolic Volume (EDV): Volume in ventricle after filling (~120 ml).

• End Systolic Volume (ESV): Volume in ventricle after contraction (~50 ml).

• Stroke Volume Equation:

Factors Affecting Stroke Volume

• Preload: Degree of stretch of ventricular muscle before contraction; increased preload increases SV.

• Contractility: Strength of ventricular contraction; increased contractility increases SV.

• Afterload: Backpressure from arterial blood; increased afterload decreases SV.

Heart Rate Regulation

• Autonomic Nervous System:

  • Parasympathetic (Vagal Tone): Decreases HR.

  • Sympathetic Stimulation: Increases HR and SV.

• Hormones: Epinephrine and thyroid hormone increase HR and SV.

• Ions: Calcium and potassium levels affect HR and contractility.

• Other Factors: Age, gender, fitness, temperature, and electrolyte balance.

Heart Pathologies and Development

Common Disorders

• Heart Failure: Inadequate cardiac output for body needs.

• Atherosclerosis: Fatty buildup in coronary arteries.

• Hypertension: Elevated afterload strains the heart.

• Myocardial Infarction: Heart attack; fibrous tissue replaces cardiac muscle.

• Cardiomyopathy: Stretched, flabby ventricles.

• Pulmonary/Systemic Congestion: Blood backs up in respective circuits.

Congenital and Age-Related Changes

• Congenital Defects: Septal defects (holes in septum), aortic coarctation, patent ductus arteriosus.

• Age-Related Changes: Valves thicken and stiffen, cardiac reserve declines, myocardium becomes fibrosed, and atherosclerosis increases.

Summary Table: Heart Chambers and Major Vessels

Additional info:

• Some context and terminology were inferred from standard Anatomy & Physiology curriculum.

• Images referenced in the original file were described and integrated into the notes for completeness.