Human heart

Introduction

The human heart is a muscular organ responsible for pumping blood throughout the body, supplying oxygen and nutrients to tissues and removing waste products. As the central component of the circulatory system, it maintains systemic and pulmonary circulation, ensuring the body’s homeostasis. Understanding the structure and function of the heart is essential for diagnosing and managing cardiovascular diseases.

Anatomy of the Heart

External Structure

The heart is a cone-shaped organ located in the mediastinum of the thoracic cavity, slightly tilted with its apex pointing downward and to the left. It is enclosed within the pericardium, which provides protection and reduces friction during contractions.

• Heart shape, size, and location: approximately the size of a fist, located between the lungs, behind the sternum.
• Pericardium: consists of a fibrous outer layer and a serous inner layer providing lubrication.
• Major vessels entering and exiting the heart: include the aorta, pulmonary arteries, pulmonary veins, and superior and inferior vena cava.

Internal Structure

The interior of the heart is divided into chambers and separated by septa and valves, ensuring unidirectional blood flow.

• Atria: right and left atrium receive blood from systemic and pulmonary circulation.
• Ventricles: right and left ventricle pump blood into the pulmonary artery and aorta respectively.
• Interatrial and interventricular septa: muscular walls that prevent mixing of oxygenated and deoxygenated blood.
• Valves: atrioventricular valves (tricuspid and mitral) regulate blood flow from atria to ventricles, semilunar valves (pulmonary and aortic) control flow out of ventricles.

Coronary Circulation

The heart receives its own blood supply through coronary arteries and veins, which deliver oxygen and nutrients to the myocardium and remove metabolic waste.

• Coronary arteries: right and left coronary arteries supply the atria and ventricles.
• Coronary veins: collect deoxygenated blood from the myocardium into the coronary sinus.
• Collateral circulation: secondary pathways provide alternative routes for blood flow in case of arterial blockage.

Histology of the Heart

The heart is composed of specialized tissues that enable its continuous rhythmic contractions and efficient blood pumping.

• Cardiac muscle structure and intercalated discs: cardiac myocytes are striated and connected by intercalated discs, allowing synchronized contraction.
• Endocardium, myocardium, and epicardium: the endocardium lines the chambers, the myocardium forms the muscular wall, and the epicardium covers the outer surface.
• Conduction system histology (SA node, AV node, Purkinje fibers): specialized pacemaker cells initiate and conduct electrical impulses throughout the heart.

Physiology of the Heart

Cardiac Cycle

The cardiac cycle consists of coordinated contraction and relaxation of the atria and ventricles, enabling blood flow through the heart and circulation.

• Systole and diastole phases: systole is the contraction phase pumping blood out, diastole is the relaxation phase allowing chambers to fill.
• Pressure-volume relationships: ventricular pressure changes correspond with volume changes to facilitate efficient ejection of blood.
• Heart sounds and their origins: the “lub-dub” sounds correspond to closure of atrioventricular and semilunar valves.

Electrical Conduction

The heart’s rhythm is controlled by its intrinsic conduction system, ensuring timely contraction and coordination between chambers.

• Pacemaker activity and SA node function: the sinoatrial node initiates electrical impulses, setting the heart rate.
• Conduction pathways: AV node, bundle of His, and Purkinje fibers transmit impulses to ventricles.
• Electrocardiography (ECG) interpretation: ECG records electrical activity and helps detect arrhythmias or conduction abnormalities.

Cardiac Output and Regulation

Cardiac output represents the volume of blood pumped by the heart per minute and is tightly regulated by multiple mechanisms.

• Stroke volume and heart rate determinants: cardiac output is the product of stroke volume and heart rate.
• Autonomic nervous system control: sympathetic stimulation increases heart rate and contractility, parasympathetic decreases heart rate.
• Hormonal regulation (e.g., adrenaline, ANP): hormones adjust heart performance and blood pressure based on physiological needs.

Development of the Heart

Embryonic Heart Formation

The heart develops early in embryogenesis, beginning as a simple tubular structure that undergoes looping and septation to form a four-chambered organ.

• Heart tube formation: arises from mesodermal cells in the cardiogenic region.
• Looping and chamber formation: the linear heart tube folds to establish spatial arrangement of atria and ventricles.
• Septation: formation of interatrial and interventricular septa separates systemic and pulmonary circulation.

Fetal Circulation Patterns

Fetal circulation includes shunts that bypass non-functioning lungs and deliver oxygenated blood from the placenta to systemic circulation.

• Foramen ovale: allows blood flow from right atrium to left atrium.
• Ductus arteriosus: connects the pulmonary artery to the aorta, bypassing the lungs.
• Ductus venosus: directs oxygenated blood from the umbilical vein to the inferior vena cava.

Postnatal Changes

After birth, the circulatory system adapts to lung function, and fetal shunts close to establish normal adult circulation.

• Closure of foramen ovale and ductus arteriosus
• Increase in pulmonary blood flow
• Adaptation of cardiac output to systemic demands

Pathophysiology

Congenital Heart Diseases

Congenital anomalies of the heart arise during development and can affect structure and function, leading to significant morbidity.

• Septal defects: atrial septal defect and ventricular septal defect
• Transposition of great arteries: aorta and pulmonary artery connections are reversed
• Tetralogy of Fallot: combination of ventricular septal defect, pulmonary stenosis, overriding aorta, and right ventricular hypertrophy

Acquired Heart Diseases

Diseases that develop after birth can compromise heart function and systemic circulation.

• Coronary artery disease and myocardial infarction: atherosclerosis reduces blood supply to myocardium, causing ischemia and necrosis
• Heart failure: inability of the heart to meet systemic demands
• Valvular disorders: stenosis or regurgitation of heart valves affecting blood flow
• Cardiomyopathies: structural and functional disorders of the myocardium
• Arrhythmias: abnormalities in heart rhythm due to conduction disturbances

Diagnostic Methods

Accurate assessment of heart structure and function is essential for diagnosis and management of cardiovascular diseases. Several diagnostic tools are employed to evaluate cardiac health.

• Electrocardiography (ECG): records the electrical activity of the heart to detect arrhythmias, conduction abnormalities, and ischemic changes.
• Echocardiography: ultrasound imaging used to visualize heart chambers, valves, wall motion, and ejection fraction.
• Cardiac catheterization and angiography: invasive procedures to assess coronary artery patency, measure pressures, and perform interventions.
• CT and MRI of the heart: advanced imaging modalities providing detailed anatomical and functional information, including myocardial perfusion and fibrosis.

Treatment and Management

Management of heart diseases involves a combination of medical, interventional, and lifestyle approaches tailored to the underlying condition.

• Medical therapy: includes antiarrhythmics, antihypertensives, anticoagulants, and lipid-lowering agents.
• Surgical interventions: valve replacement, repair of congenital defects, and coronary artery bypass grafting.
• Interventional cardiology: procedures such as angioplasty and stenting to restore coronary blood flow.
• Heart transplantation: considered in end-stage heart failure unresponsive to other treatments.
• Lifestyle and preventive measures: diet, exercise, smoking cessation, and management of comorbidities to reduce cardiovascular risk.

Clinical Significance

The human heart is central to overall health, and its function affects all organ systems. Early recognition and management of heart conditions are crucial for preventing morbidity and mortality.

• Importance of early detection of cardiac diseases: prompt diagnosis allows timely intervention, reducing complications and improving prognosis.
• Impact on systemic health and other organs: cardiac dysfunction can lead to renal impairment, hepatic congestion, and cerebral hypoperfusion.
• Public health implications: cardiovascular diseases are leading causes of death globally, highlighting the importance of prevention and awareness programs.

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