Cardiac Anatomy & Physiology: Study Notes

Cardiac Anatomy & Physiology

Mediastinum and Position of the Heart in the Thorax

The mediastinum is the central compartment of the thoracic cavity, containing the heart, great vessels, trachea, esophagus, and other structures. The heart is located within the mediastinum, slightly left of the midline, between the lungs.

• Mediastinum: Divided into superior and inferior regions; the heart lies in the middle mediastinum.

• Position: The apex of the heart points downward, forward, and to the left.

• Clinical relevance: Knowledge of heart position is essential for interpreting imaging and understanding trauma.

Pericardium Structure and Function

The pericardium is a double-walled sac that encloses the heart, providing protection and reducing friction during heartbeats.

• Layers: Fibrous pericardium (outer), serous pericardium (inner: parietal and visceral layers).

• Function: Anchors heart, prevents overfilling, and lubricates movement.

• Pericarditis: Inflammation of the pericardium, often causing chest pain and friction rub.

• Etiology: Can be viral, bacterial, or idiopathic.

Structure and Function of Heart Wall Layers

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

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

• Myocardium: Middle, muscular layer responsible for contraction.

• Endocardium: Inner layer lining the chambers and valves.

Heart Muscle Histology: Intercalated Discs

Cardiac muscle cells are connected by intercalated discs, which facilitate synchronized contraction.

• Intercalated discs: Specialized junctions containing gap junctions and desmosomes.

• Function: Allow rapid transmission of electrical impulses and mechanical strength.

Define Cardiac Hypertrophy: Causes and Risk Factors

Cardiac hypertrophy is the thickening of the heart muscle, often in response to increased workload.

• Causes: Hypertension, valvular disease, athletic training.

• Risk factors: Chronic high blood pressure, genetic predisposition.

• Clinical significance: Can lead to heart failure if excessive.

Internal and External Anatomy of the Heart

The heart has four chambers (two atria, two ventricles) and is surrounded by major vessels.

• Internal: Chambers, valves (tricuspid, mitral, pulmonary, aortic), septa.

• External: Coronary arteries, veins, auricles.

Blood Flow Through the Heart

Blood flows through the heart in a specific sequence, ensuring oxygenation and circulation.

• Deoxygenated blood enters right atrium → right ventricle → pulmonary artery → lungs.

• Oxygenated blood returns via pulmonary veins → left atrium → left ventricle → aorta → body.

Heart Sounds: S1 and S2

Heart sounds are produced by valve closures during the cardiac cycle.

• S1: Closure of mitral and tricuspid valves (start of systole).

• S2: Closure of aortic and pulmonary valves (end of systole).

• Clinical relevance: Abnormal sounds (murmurs) may indicate valve disorders.

MAP and AVS: Description and Etiology

MAP (Mitral/Aortic Valve Prolapse) and AVS (Aortic Valve Stenosis) are common valve disorders.

• MAP: Mitral valve leaflets bulge into the left atrium; may cause regurgitation.

• AVS: Narrowing of the aortic valve, impeding blood flow from left ventricle.

• Etiology: Congenital, degenerative, or rheumatic causes.

Coronary Artery Disease (CAD)

Coronary artery disease is the narrowing or blockage of coronary arteries due to atherosclerosis.

• Definition: Reduced blood flow to myocardium, leading to ischemia.

• Risk factors: High cholesterol, hypertension, smoking, diabetes.

• Clinical significance: Can cause angina, myocardial infarction.

Blood Tests for Myocardial Cell Death

Specific blood tests detect markers of heart muscle damage.

• Troponin: Most sensitive and specific marker for myocardial infarction.

• CK-MB: Creatine kinase isoenzyme, rises after heart injury.

• Clinical use: Diagnosis and monitoring of acute coronary syndromes.

Cardiac Conduction System

The heart's electrical system coordinates contraction through specialized pathways.

• SA node: Pacemaker, initiates impulse.

• AV node: Delays impulse, allowing ventricular filling.

• Bundle of His, bundle branches, Purkinje fibers: Rapidly conduct impulses to ventricles.

ECG Interpretation: Waves and Actions

An electrocardiogram (ECG) records the heart's electrical activity, with distinct waves representing different phases.

• P wave: Atrial depolarization.

• QRS complex: Ventricular depolarization.

• T wave: Ventricular repolarization.

• Clinical use: Diagnosing arrhythmias, ischemia, and conduction defects.

Calcium Channels in Myocardium vs. Skeletal Muscle

Calcium plays a crucial role in muscle contraction, with differences between cardiac and skeletal muscle.

• Cardiac muscle: Relies on extracellular calcium influx through L-type channels for contraction.

• Skeletal muscle: Uses calcium released from sarcoplasmic reticulum.

• Clinical relevance: Calcium channel blockers affect cardiac contractility and rhythm.

Blood Pressure: Diastole and Systole

Blood pressure is the force exerted by circulating blood on vessel walls, measured during two phases.

• Systolic pressure: Pressure during ventricular contraction.

• Diastolic pressure: Pressure during ventricular relaxation.

• Normal values: Approximately 120/80 mmHg.

ANP/BNP Hormones: Origin and Actions

Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are hormones involved in fluid and blood pressure regulation.

• ANP: Released from atria in response to stretch; promotes sodium excretion.

• BNP: Released from ventricles; similar effects, used as a marker for heart failure.

• Targets: Kidneys, blood vessels.

Cardiodynamics: CO, SV, Preload, Afterload, EF

Cardiodynamics refers to the study of heart function and performance, including key parameters.

• Cardiac Output (CO): Volume of blood pumped per minute. Equation: $CO = SV \times HR$

• Stroke Volume (SV): Volume of blood pumped per beat.

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

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

• Ejection Fraction (EF): Percentage of blood ejected from ventricle per beat. Equation: $EF = \frac{SV}{EDV} \times 100\%$

• Clinical relevance: Low EF indicates heart failure.

Table: Key Cardiodynamic Parameters

Parameter	Definition	Normal Value
Cardiac Output (CO)	Blood pumped per minute	4-8 L/min
Stroke Volume (SV)	Blood pumped per beat	60-100 mL/beat
Ejection Fraction (EF)	Fraction of blood ejected per beat	55-70%
Preload	Ventricular filling/stretch	Depends on venous return
Afterload	Resistance to ejection	Depends on arterial pressure

Additional info: Academic context and definitions have been expanded for clarity and completeness.