The Heart: Structure, Function, and Clinical Relevance

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The Heart: Structure and Function

Overview of the Heart

The heart is a muscular organ responsible for pumping blood throughout the body via two main circuits: the pulmonary and systemic circuits. It is located in the mediastinum of the thoracic cavity, between the lungs and above the diaphragm.

Pulmonary Circuit: Carries blood between the heart and lungs for gas exchange.
Systemic Circuit: Delivers oxygenated blood from the heart to the rest of the body and returns deoxygenated blood back to the heart.

Diagram of the heart showing pulmonary and systemic circuits

Location and Orientation of the Heart

The heart lies obliquely in the thoracic cavity, with its base directed toward the right shoulder and its apex pointing to the left hip. It is protected by the rib cage and rests on the superior surface of the diaphragm.

Location of the heart in the thorax

Layers of the Heart Wall and Pericardium

The heart wall consists of three layers, and the heart is enclosed in a double-walled sac called the pericardium.

Epicardium: The outermost layer, also known as the visceral layer of the serous pericardium.
Myocardium: The thick, middle layer composed of cardiac muscle responsible for contraction.
Endocardium: The innermost layer lining the heart chambers and valves.
Pericardium: Includes the fibrous pericardium (outer) and serous pericardium (parietal and visceral layers).

Layers of the pericardium and heart wall

Gross Anatomy of the Heart

The heart has four chambers: two atria (upper chambers) and two ventricles (lower chambers). Major vessels enter and leave the heart at its base.

Right Atrium: Receives deoxygenated blood from the body via the superior and inferior vena cava.
Right Ventricle: Pumps blood to the lungs via the pulmonary trunk.
Left Atrium: Receives oxygenated blood from the lungs via the pulmonary veins.
Left Ventricle: Pumps oxygenated blood to the body via the aorta.

Anterior view of the heart showing major vessels and coronary arteries Inferior view of the heart showing major vessels and coronary arteries Frontal section of the heart showing internal structures and valves

Cardiac Muscle Structure

The myocardium is composed of cardiac muscle cells arranged in circular and spiral bundles, allowing efficient contraction and ejection of blood.

Cardiac Muscle Cells: Striated, branched, and interconnected by intercalated discs containing gap junctions and fasciae adherens.
Function: Enables coordinated contraction for effective pumping action.

Circular and spiral arrangement of cardiac muscle bundles Microscopic anatomy of cardiac muscle

Heart Valves and Blood Flow

Heart Valves

The heart contains four main valves that ensure unidirectional blood flow:

Atrioventricular (AV) Valves: Tricuspid (right) and mitral/bicuspid (left) valves between atria and ventricles.
Semilunar Valves: Pulmonary (right) and aortic (left) valves at the exits of the ventricles.

Heart valves and cardiac skeleton

Function of the Atrioventricular Valves

AV valves prevent backflow of blood into the atria during ventricular contraction.

When atrial pressure exceeds ventricular pressure, AV valves open, allowing blood to flow into the ventricles.
When ventricles contract, AV valves close, and chordae tendineae with papillary muscles prevent valve prolapse.

AV valves open during ventricular filling AV valves closed during ventricular contraction

Function of the Semilunar Valves

Semilunar valves prevent backflow of blood from the arteries into the ventricles after contraction.

Open when ventricular pressure exceeds arterial pressure during systole.
Close when ventricles relax and arterial pressure exceeds ventricular pressure.

Semilunar valves open and closed during the cardiac cycle

Blood Flow Through the Heart

Blood flows through the heart in a specific sequence, passing through all four chambers and valves:

Deoxygenated blood enters the right atrium from the superior and inferior vena cava.
Passes through the tricuspid valve into the right ventricle.
Pumped through the pulmonary valve into the pulmonary trunk and arteries to the lungs.
Oxygenated blood returns via pulmonary veins to the left atrium.
Passes through the mitral valve into the left ventricle.
Pumped through the aortic valve into the aorta and systemic circulation.

Diagram of blood flow through the heart and circuits

Electrical Conducting System of the Heart

Intrinsic Conducting System

The heart's intrinsic conducting system coordinates the heartbeat, ensuring efficient contraction of the atria and ventricles.

Sinoatrial (SA) Node: Pacemaker of the heart, initiates electrical impulses.
Atrioventricular (AV) Node: Delays impulse, allowing atrial contraction before ventricular contraction.
AV Bundle (Bundle of His): Only electrical connection between atria and ventricles.
Bundle Branches: Conduct impulses through the interventricular septum.
Purkinje Fibers: Stimulate ventricular muscle cells for contraction.

*Additional info: The delay at the AV node ensures that the atria have ejected their blood into the ventricles before the ventricles contract.*

Autonomic Innervation of the Heart

The heart rate and force of contraction are modulated by the autonomic nervous system:

Sympathetic Stimulation: Increases heart rate and contractility via cardiac nerves.
Parasympathetic Stimulation: Decreases heart rate via the vagus nerve.

Autonomic innervation of the heart

Clinical Scenarios and Applications

Atrial Fibrillation

Atrial fibrillation is an arrhythmia affecting the atria, leading to irregular and often rapid heart rhythm. It can cause inefficient blood flow and increase the risk of stroke and heart failure.

Affected Chambers: Both right and left atria.
Potential Complications: Thrombus formation, embolic stroke, heart failure.

Postural Orthostatic Tachycardia Syndrome (POTS)

POTS is characterized by an excessive increase in heart rate upon standing, often accompanied by dizziness, palpitations, and fatigue. Symptoms improve when lying down.

Diagnosis: Based on heart rate and blood pressure changes with position.
Treatment: Increased fluid and salt intake, physical counter-maneuvers, medications as needed.

Heart Sounds and Auscultation

Heart sounds are produced by the closing of valves. The first heart sound (S1) is due to AV valve closure, and the second heart sound (S2) is due to semilunar valve closure. Auscultation is used to assess heart function and detect abnormalities.

S1: Closure of mitral and tricuspid valves.
S2: Closure of aortic and pulmonary valves.

Summary Table: Heart Valves and Their Locations

Valve	Location	Function
Tricuspid (Right AV)	Between right atrium and right ventricle	Prevents backflow into right atrium
Mitral (Left AV)	Between left atrium and left ventricle	Prevents backflow into left atrium
Pulmonary Semilunar	Between right ventricle and pulmonary trunk	Prevents backflow into right ventricle
Aortic Semilunar	Between left ventricle and aorta	Prevents backflow into left ventricle

Key Terms

Myocardium: Muscular layer of the heart wall.
Pericardium: Double-walled sac enclosing the heart.
SA Node: Pacemaker of the heart.
AV Node: Electrical relay station between atria and ventricles.
Chordae Tendineae: Tendinous cords anchoring AV valve cusps to papillary muscles.