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

Heart Anatomy and Physiology

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, and rests on the diaphragm. This position protects the heart and allows for efficient circulation.

• Superior border: At the level of the second rib

• Apex: Points downward, left, and anteriorly

• Base: Directed toward the right shoulder

Clinical relevance: The mediastinal position is important for interpreting chest X-rays and understanding trauma impacts.

Pericardium: Structure and Function

The pericardium is a double-walled sac that surrounds and protects the heart. It consists of two main layers:

• Fibrous pericardium: Tough, outer layer that anchors the heart and prevents overfilling.

• Serous pericardium: Thin, inner layer with two parts: parietal (lines the fibrous pericardium) and visceral (epicardium, covers the heart surface).

The pericardial cavity between these layers contains fluid to reduce friction during heartbeats.

Pericarditis: Definition and Etiology

Pericarditis is inflammation of the pericardium, often caused by infection, autoimmune disorders, or trauma. It can lead to chest pain and pericardial effusion (fluid accumulation).

• Etiology: Viral infections, bacterial infections, myocardial infarction, or systemic diseases (e.g., lupus).

Structure and Function of Heart Wall Layers

The heart wall is composed of three layers, each with distinct functions:

• Epicardium: Outer layer; provides protection and contains blood vessels.

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

• Endocardium: Inner layer; lines chambers and valves, minimizing friction.

Heart Muscle Histology: Intercalated Discs

Cardiac muscle cells are connected by intercalated discs, which contain gap junctions and desmosomes. These structures allow rapid electrical communication and strong mechanical connections, enabling coordinated contractions.

Cardiac Hypertrophy: Definition, Causes, and Risk Factors

Cardiac hypertrophy is the thickening of the heart muscle, usually the myocardium. It can be physiological (athletic training) or pathological (hypertension, valve disease).

• Causes: Increased workload, high blood pressure, valve disorders.

• Risks: Heart failure, arrhythmias.

Internal and External Anatomy of the Heart

The heart has four chambers (right/left atria, right/left ventricles) and major vessels (aorta, pulmonary arteries/veins, vena cavae). Externally, grooves called sulci mark chamber boundaries.

Blood Flow Through the Heart

Blood flows through the heart in a specific sequence:

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 arteries (to 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 (systemic circulation).

Heart Sounds, Valves, and Disorders

Heart Sounds (S1 and S2)

Heart sounds are produced by valve closures:

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

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

Valve Disorders: MVP and AVS

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

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

• Etiology: Congenital defects, aging, rheumatic fever.

Coronary Artery Disease (CAD)

CAD is the narrowing or blockage of coronary arteries due to atherosclerosis, reducing blood flow to the myocardium and increasing risk of myocardial infarction (heart attack).

Blood Tests for Myocardial Infarction (MI)

Key blood tests for MI include:

• Troponin: Most specific marker; elevated within hours of MI.

• CK-MB: Cardiac-specific creatine kinase; rises within 4-6 hours.

• Myoglobin: Early marker but less specific.

Electrical Activity and Cardiac Function

Conduction System Pathway

The heart's conduction system ensures coordinated contractions:

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

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

3. 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.

Electrocardiogram (ECG) Interpretation

An ECG records the heart's electrical activity:

• P wave: Atrial depolarization (contraction).

• QRS complex: Ventricular depolarization (contraction).

• T wave: Ventricular repolarization (relaxation).

Importance of Slow Calcium Channels in Myocardium vs Skeletal Muscle

Slow calcium channels in cardiac muscle prolong the action potential, allowing sustained contraction and preventing tetanus. In skeletal muscle, action potentials are shorter and do not rely on slow calcium influx.

ST Elevation in Myocardial Infarction (MI)

ST elevation on ECG indicates acute myocardial injury, often due to complete coronary artery blockage. It is a key sign of ST-elevation MI (STEMI).

Cardiovascular Dynamics and Clinical Measures

Blood Pressure: Diastole and Systole

Blood pressure is the force exerted by blood on vessel walls. It is measured as:

• Systolic pressure: During ventricular contraction (systole).

• Diastolic pressure: During ventricular relaxation (diastole).

ANP/BNP Hormones: Origin and Actions

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

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

Cardio-dynamics: CO, SV, Preload, Afterload

Cardio-dynamics refers to the forces and factors affecting cardiac output:

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

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

• Preload: Degree of ventricular stretch before contraction.

• 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 per beat. Normal EF is 55-70%.

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

Congestive Heart Failure (CHF): Occurs when the heart cannot pump enough blood to meet the body's needs; often associated with reduced EF.

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)	Percent of EDV ejected	55-70%

Additional info: Understanding these values is critical for diagnosing and managing heart failure and other cardiac conditions.