Study Guide: The Heart (Chapter 20) – Structure, Function, and Clinical Aspects

The Heart: Structure, Function, and Clinical Aspects

Mediastinum and Position of the Heart in the Thorax

The mediastinum is the central compartment of the thoracic cavity, located between the lungs. The heart is situated within the mediastinum, slightly left of the midline, posterior to the sternum, and superior to the diaphragm.

• Position: The heart's apex points downward and to the left; the base is superior and directed toward the right shoulder.

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

Pericardium: Structure and Function

The pericardium is a double-walled sac that encloses the heart, providing protection and anchoring it within the thorax.

• Fibrous pericardium: Tough outer layer; prevents overexpansion.

• Serous pericardium: Thin, double-layered membrane (parietal and visceral layers) with pericardial fluid in between to reduce friction.

• Function: Protects, anchors, and reduces friction during heartbeats.

Pericarditis: Definition and Etiology

• Pericarditis: Inflammation of the pericardium.

• Etiology: Causes include viral or bacterial infection, autoimmune disorders, or post-myocardial infarction (Dressler's syndrome).

Structure and Function of Heart Wall Layers

• Epicardium: Outer layer; also called visceral pericardium.

• Myocardium: Middle, muscular layer; responsible for contraction.

• Endocardium: Inner endothelial lining; smooth surface for blood flow.

Heart Muscle Histology: Intercalated Discs

• Intercalated discs: Specialized connections between cardiac muscle cells containing gap junctions and desmosomes.

• Function: Allow rapid electrical communication and mechanical stability during contraction.

Cardiac Hypertrophy: Definition and Causes

• Cardiac hypertrophy: Thickening of the heart muscle, usually the left ventricle.

• Causes/Risk factors: Chronic hypertension, valvular disease, athletic training, or genetic conditions.

Internal and External Anatomy of the Heart

• External: Four chambers (right/left atria, right/left ventricles), major vessels (aorta, pulmonary trunk, vena cavae, pulmonary veins), coronary arteries and veins.

• Internal: Valves (tricuspid, bicuspid/mitral, pulmonary, aortic), septa (interatrial, interventricular), chordae tendineae, papillary muscles.

Blood Flow Through the Heart

1. Deoxygenated blood enters right atrium via superior/inferior vena cava.

2. Passes through tricuspid valve to right ventricle.

3. Pumped through pulmonary valve to pulmonary trunk and lungs.

4. Oxygenated blood returns via pulmonary veins to left atrium.

5. Passes through mitral (bicuspid) valve to left ventricle.

6. Pumped through aortic valve to aorta and systemic circulation.

Heart Sounds (S1 and S2)

• S1 ("lub"): Closure of AV valves (tricuspid and mitral) at start of ventricular systole.

• S2 ("dub"): Closure of semilunar valves (aortic and pulmonary) at start of ventricular diastole.

Valve Disorders: MVP and AVS

• Mitral Valve Prolapse (MVP): Mitral valve leaflets bulge into left atrium during systole; may cause regurgitation.

• Aortic Valve Stenosis (AVS): Narrowing of aortic valve, impeding blood flow from left ventricle to aorta.

• Etiology: MVP often due to connective tissue disorders; AVS commonly from age-related calcification or congenital defects.

Coronary Artery Disease (CAD)

• Definition: Narrowing or blockage of coronary arteries due to atherosclerosis.

• Consequences: Reduced blood flow to myocardium, leading to angina or myocardial infarction (heart attack).

Blood Tests for Myocardial Infarction (MI)

• Troponin I/T: Most specific and sensitive markers; elevated within hours of MI.

• CK-MB: Cardiac-specific isoenzyme of creatine kinase; rises within 3-6 hours.

• Myoglobin: Early marker but less specific.

Conduction System Pathway of the Heart

1. Sinoatrial (SA) node: Pacemaker; initiates impulse.

2. Atrioventricular (AV) node: Delays impulse, allowing atrial contraction.

3. AV bundle (Bundle of His): Conducts impulse to ventricles.

4. Right and left bundle branches: Carry impulse through interventricular septum.

5. Purkinje fibers: Distribute impulse to ventricular myocardium.

ECG Interpretation: P, QRS, T Waves

• P wave: Atrial depolarization (contraction).

• QRS complex: Ventricular depolarization (contraction); atrial repolarization occurs here but is masked.

• T wave: Ventricular repolarization (relaxation).

Importance of Slow Calcium Channels in Myocardium vs Skeletal Muscle

• Myocardium: Slow calcium channels prolong action potential, allowing sustained contraction and preventing tetanus.

• Skeletal muscle: Lacks this plateau phase; contractions are shorter and can summate to tetanus.

ST Elevation in Myocardial Infarction (MI)

• ST elevation: Indicates acute transmural myocardial injury; seen in STEMI (ST-Elevation Myocardial Infarction).

• Clinical importance: Requires urgent intervention to restore blood flow.

Blood Pressure: Diastole and Systole

• Systolic pressure: Pressure during ventricular contraction.

• Diastolic pressure: Pressure during ventricular relaxation.

• Normal values: Approximately 120/80 mmHg (systolic/diastolic).

ANP and BNP Hormones: Origin and Actions

• Atrial Natriuretic Peptide (ANP): Released from atrial myocytes in response to stretch; promotes sodium and water excretion, lowering blood pressure.

• Brain Natriuretic Peptide (BNP): Released from ventricular myocytes; similar actions to ANP, used as a marker for heart failure.

Cardiodynamics: Cardiac Output, Stroke Volume, Preload, Afterload

• Cardiac Output (CO): Volume of blood pumped by each ventricle per minute.

• Formula: $CO = HR \times SV$

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

• Preload: Degree of stretch of ventricular muscle at end of diastole.

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

Ejection Fraction and Congestive Heart Failure (CHF)

• Ejection Fraction (EF): Percentage of end-diastolic volume ejected with each contraction.

• Formula: $EF = \frac{SV}{EDV} \times 100\%$

• Normal EF: 55-70%.

• CHF: EF < 40% suggests systolic heart failure.

• Calculation example: If SV = 70 mL, EDV = 120 mL, $EF = \frac{70}{120} \times 100\% = 58\%$

Summary Table: Key Heart 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
Ejection Fraction (EF)	SV/EDV × 100%	55-70%
Systolic BP	Pressure during contraction	~120 mmHg
Diastolic BP	Pressure during relaxation	~80 mmHg

Additional info: Some explanations and normal values were expanded for clarity and completeness.