The Cardiovascular System: The Heart – Structure, Function, and Clinical Correlates

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The Cardiovascular System: The Heart

Location and Basic Structure of the Heart

The heart is a muscular organ located in the thoracic cavity, between the lungs in the mediastinum. It is slightly tilted so that the apex points to the left hip and the base is directed toward the right shoulder. The heart is divided into four chambers: two atria (upper chambers) and two ventricles (lower chambers). The right side of the heart pumps blood to the lungs (pulmonary circuit), while the left side pumps blood to the rest of the body (systemic circuit).

Right Ventricle: Thin-walled, crescent-shaped chamber that pumps blood to the lungs.
Left Ventricle: Thick-walled, circular chamber that pumps blood to the systemic circuit.

Anatomical diagrams showing the location and orientation of the heart in the thoracic cavity Diagram comparing the wall thickness and chamber shape of the right and left ventricles

Functions of the Heart

The heart acts as a double pump, maintaining two separate circuits:

Pulmonary Circuit: Carries deoxygenated blood from the right ventricle to the lungs for gas exchange and returns oxygenated blood to the left atrium.
Systemic Circuit: Delivers oxygenated blood from the left ventricle to the body tissues and returns deoxygenated blood to the right atrium.

Diagram of the pulmonary circuit Diagram of the systemic circuit

The Pericardium and Heart Wall

The Pericardium

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 sublayers (parietal and visceral) separated by serous fluid, reducing friction during heartbeats.

Diagram showing the pericardium and serous fluid around the heart

Layers of the Heart Wall

The heart wall is composed of three layers:

Epicardium (Visceral Pericardium): Outermost layer, provides protection and contains blood vessels.
Myocardium: Middle, muscular layer responsible for contraction and pumping action.
Endocardium: Innermost layer, lines the chambers and valves, minimizing friction.

Diagram showing the layers of the heart wall

Cardiac Tamponade

Cardiac tamponade is a clinical condition where excess fluid accumulates in the pericardial cavity, compressing the heart and reducing its ability to fill and pump blood. Causes include trauma, cancer, kidney failure, or surgery. Treatment involves removing the fluid via pericardiocentesis.

Diagram showing pericardiocentesis procedure for cardiac tamponade

External and Internal Anatomy of the Heart

External Anatomy

The heart's external features include the auricles, major vessels (aorta, pulmonary arteries and veins, vena cavae), and coronary vessels. The coronary arteries supply the heart muscle with oxygenated blood.

Anterior view of the external heart with labeled structures Cadaver photo of the anterior view of the heart Posterior view of the external heart

Internal Anatomy

Internally, the heart is divided by the interventricular septum and contains valves that ensure unidirectional blood flow. The right atrium receives blood from the body, and the left atrium receives blood from the lungs. The ventricles pump blood to the pulmonary and systemic circuits.

Anterior dissection of the heart showing internal structures Cadaver photo of the anterior dissection of the heart

Heart Valves and Valvular Diseases

Heart Valves

The heart contains four main valves:

Atrioventricular (AV) Valves: Tricuspid (right) and mitral (left) valves, prevent backflow into the atria during ventricular contraction.
Semilunar Valves: Pulmonary and aortic valves, prevent backflow into the ventricles after contraction.

Diagram of heart valves

Valvular Heart Diseases

Valvular defects can be congenital or acquired (infection, cancer, immune disorders):

Insufficient Valve (Regurgitation): Valve fails to close fully, causing blood to leak backward.
Stenotic Valve: Valve cusps become stiff and inflexible, restricting blood flow.

Diagram comparing mitral valve stenosis and regurgitation

Heart Sounds

Normal heart sounds (S1 "lub" and S2 "dub") are produced by valve closures. Abnormal sounds (murmurs) indicate defective valves.

Diagram showing locations for auscultation of heart sounds

Coronary Circulation and Disease

Coronary Circulation

The coronary arteries supply oxygenated blood to the myocardium, while coronary veins drain deoxygenated blood. Blockage of these vessels can lead to myocardial infarction (heart attack).

Distribution of coronary arteries Distribution of coronary veins

Coronary Artery Disease (CAD)

CAD is caused by atherosclerosis (plaque buildup) in the coronary arteries, reducing blood flow to the heart muscle. Risk factors include high cholesterol, hypertension, smoking, and diabetes. Symptoms may include chest pain (angina) or myocardial infarction.

Diagram showing narrowing of coronary arteries in CAD Angiogram showing coronary arteries

Treatment of CAD

Treatment options include lifestyle changes, medications, angioplasty with stent placement, and coronary artery bypass grafting (CABG).

Diagram showing stent placement in a coronary artery Diagram of stent with balloon angioplasty Diagram of coronary artery bypass graft

Cardiac Muscle Tissue and Electrophysiology

Cardiac Muscle Tissue

Cardiac muscle exhibits autorhythmicity, meaning it can generate its own action potentials without nervous input. There are two main cell types:

Pacemaker Cells: Generate rhythmic action potentials to set the heart rate.
Contractile Cells: Respond to pacemaker signals and contract to pump blood.

Cardiac Electrophysiology

The heart's electrical activity is coordinated by the cardiac conduction system, including the SA node (primary pacemaker), AV node, AV bundle, bundle branches, and Purkinje fibers. The SA node sets the normal sinus rhythm. Action potentials in contractile cells feature a plateau phase, which prolongs contraction and prevents tetany.

Electrocardiogram (ECG)

An ECG records the electrical activity of the heart, providing information about heart rhythm and conduction. Common abnormalities include bradycardia, tachycardia, heart block, and fibrillation.

The Cardiac Cycle and Cardiac Output

The Cardiac Cycle

The cardiac cycle is the sequence of events in one heartbeat, including atrial and ventricular systole (contraction) and diastole (relaxation). Pressure changes drive blood flow and valve operation.

Stroke Volume and Cardiac Output

Stroke Volume (SV): The amount of blood ejected by a ventricle in one contraction. Calculated as:

where EDV = end-diastolic volume, ESV = end-systolic volume.

Cardiac Output (CO): The volume of blood pumped by each ventricle per minute. Calculated as:

where HR = heart rate (beats per minute).

Regulation of Cardiac Output

Cardiac output is regulated by preload (venous return), contractility (strength of contraction), and afterload (resistance in arteries). The autonomic nervous system and hormones also influence heart rate and stroke volume.

Heart Failure

Heart failure occurs when the heart cannot pump blood effectively. Causes include myocardial infarction, valvular disease, cardiomyopathy, and ventricular hypertrophy. Left-sided failure leads to pulmonary edema, while right-sided failure causes systemic edema. Treatment focuses on improving cardiac output and reducing fluid overload.

Additional info: For a deeper understanding, students should review the mechanisms of action potentials in cardiac muscle, the phases of the cardiac cycle, and the clinical significance of ECG waveforms.