The Heart: Structure, Function, and Disease

The Heart: Structure, Function, and Disease

Location, Size, and Position of the Heart

The heart is a muscular organ located in the mediastinum, the central compartment of the thoracic cavity. Approximately two-thirds of the heart's mass lies to the left of the body's midline, with the apex resting on the diaphragm. Its shape and size are comparable to a closed fist, making it compact yet powerful in function.

Functional Anatomy of the Heart

The heart consists of four chambers: two upper atria (receiving chambers) and two lower ventricles (discharging chambers). The walls of these chambers are composed of myocardium, a specialized cardiac muscle tissue. The inner lining of the heart is called the endocardium, which provides a smooth surface for blood flow and helps prevent clot formation. Inflammation of the endocardium is termed endocarditis, which can lead to thrombus (clot) formation.

Pericardium and Pericarditis

The heart is enclosed in a double-layered sac called the pericardium. The inner layer, or visceral pericardium (epicardium), is in direct contact with the heart, while the outer layer, or parietal pericardium, forms a protective outer covering. Between these layers is a lubricated space containing pericardial fluid, which reduces friction during heartbeats. Inflammation of the pericardium is known as pericarditis, and fluid accumulation can lead to cardiac tamponade, a life-threatening compression of the heart.

Internal Structure of the Heart

The heart's internal anatomy includes the atria, ventricles, valves, and associated structures. The myocardium forms the bulk of the heart wall, while the endocardium lines the chambers. The pericardial sac surrounds the heart, providing protection and reducing friction.

Heart Action: Systole and Diastole

The heart functions through a coordinated cycle of contraction and relaxation. Systole refers to the contraction phase, while diastole is the relaxation phase. During each heartbeat, the atria contract first (atrial systole), pushing blood into the ventricles. This is followed by ventricular contraction (ventricular systole), which propels blood out of the heart. Efficient pumping requires precise timing and the proper functioning of heart valves to direct blood flow.

Heart Valves and Valve Disorders

Valves ensure unidirectional blood flow through the heart and prevent backflow. The atrioventricular (AV) valves include the tricuspid (right side) and bicuspid/mitral (left side) valves. The semilunar (SL) valves are the pulmonary and aortic valves, located at the exits of the ventricles. Valve disorders include:

• Incompetent valves: Allow backflow of blood (regurgitation).

• Stenosed valves: Narrowed, restricting blood flow.

• Rheumatic heart disease: Cardiac damage from a delayed inflammatory response to streptococcal infection.

• Mitral valve prolapse (MVP): The mitral valve bulges into the left atrium during ventricular contraction, causing incompetence.

Heart Sounds and Murmurs

Each cardiac cycle produces two main heart sounds: "lub" (closure of AV valves during ventricular contraction) and "dup" (closure of SL valves during ventricular relaxation). Heart murmurs are abnormal sounds often caused by valve disorders.

Blood Flow Through the Heart and Cardiovascular System

The heart acts as two separate pumps: the right side pumps blood to the lungs (pulmonary circulation), and the left side pumps blood to the body (systemic circulation). The sequence of blood flow is as follows:

• Venous blood enters the right atrium via the superior and inferior venae cavae.

• Passes through the tricuspid valve to the right ventricle.

• Pumped through the pulmonary SL valve to the pulmonary artery and lungs.

• Oxygenated blood returns to the left atrium, passes through the mitral valve to the left ventricle.

• Pumped through the aortic SL valve into the aorta and distributed to the body.

Blood Supply to the Heart Muscle (Coronary Circulation)

The myocardium receives oxygen and nutrients from the right and left coronary arteries. Blockage of these arteries can cause myocardial infarction (MI), or heart attack. Venous blood from the myocardium returns to the right atrium via the coronary sinus.

Coronary Heart Disease and Treatments

Coronary heart disease results from blockage of coronary arteries by thrombosis, embolism, or atherosclerosis (lipid buildup). This can lead to angina pectoris (chest pain) or MI. Treatments include:

• Coronary bypass surgery: Using vessels from other parts of the body to bypass blockages.

• Coronary angioplasty: Inserting a device to open blocked arteries.

Cardiac Cycle

The cardiac cycle is the sequence of events in one heartbeat, typically lasting about 0.8 seconds. It includes systole (contraction) and diastole (relaxation) phases. The cycle ensures efficient blood flow and is regulated by electrical impulses.

Conduction System of the Heart and Electrocardiography (ECG)

The heart's conduction system coordinates contraction through specialized structures:

• SA (sinoatrial) node: Pacemaker, initiates impulse in right atrium.

• AV (atrioventricular) node: Receives impulse from SA node.

• AV bundle (bundle of His): Transmits impulse to ventricles.

• Purkinje fibers: Distribute impulse throughout ventricular myocardium.

Electrical activity is recorded as an electrocardiogram (ECG/EKG), which displays three main waves:

• P wave: Atrial depolarization

• QRS complex: Ventricular depolarization

• T wave: Ventricular repolarization

Cardiac Dysrhythmias

Dysrhythmias are abnormalities in heart rhythm. Types include:

• Heart block: Impaired conduction, may require artificial pacemaker.

• Bradycardia: Slow heart rate (<60 bpm).

• Tachycardia: Rapid heart rate (>100 bpm).

• Sinus dysrhythmia: Variation in heart rate during breathing.

• Premature contraction (extrasystole): Early contraction.

• Fibrillation: Uncoordinated contraction, ineffective pumping. Defibrillation (electric shock) can restore rhythm.

• Atrial ablation: Destruction of tissue to treat atrial fibrillation.

Cardiac Output

Cardiac output (CO) is the volume of blood pumped by one ventricle per minute. It is determined by heart rate (HR) and stroke volume (SV):

$CO = HR \times SV$

• Stroke volume (SV): Volume ejected per beat, influenced by venous return and myocardial contractility.

• Heart rate (HR): Beats per minute, regulated by the autonomic nervous system and physiological demands.

Factors such as valve disorders, coronary artery disease, and myocardial infarction can decrease stroke volume and cardiac output.

Heart Failure

Heart failure is the inability of the heart to pump sufficient blood to sustain life. It can result from various heart diseases, including cardiomyopathy (disease of the myocardium). Types include:

• Right heart failure: Often secondary to left heart failure; leads to systemic congestion.

• Left heart failure (congestive heart failure, CHF): Inability of the left ventricle to pump effectively, causing pulmonary and systemic congestion.

Treatment options include heart transplants and artificial hearts, though these are not yet perfected.

Review Questions

Question	Answer
What is endocarditis?	Inflammation of the endocardium, the heart's inner lining.
What is rheumatic heart disease?	Cardiac damage from a delayed inflammatory response to streptococcal infection.
What causes heart murmurs?	Often caused by valve disorders.
Where does impulse conduction in the heart begin?	At the sinoatrial (SA) node.
What do ECG deflections represent?	Electrical activity regulating atrial and ventricular contraction and relaxation.
What is sinus dysrhythmia?	Variation in heart rate during the breathing cycle.
What factors affect stroke volume?	Hormones, ion imbalances, valve disorders, among others.