Introduction
The heart is the central organ of the circulatory system, functioning as a muscular pump that maintains the continuous flow of blood throughout the body. Through rhythmic contractions, it ensures that oxygen, nutrients, and hormones are delivered to tissues while carbon dioxide and metabolic wastes are removed. The heart’s constant activity sustains life by providing the energy and materials needed for every cell to function properly.
Although it is roughly the size of a closed fist and weighs less than 400 grams in adults, the heart performs an extraordinary task. It beats about 100,000 times each day, pumping approximately 7,000 to 8,000 liters of blood. The efficiency of this system relies on both the heart’s anatomical structure and its physiological function, both intricately adapted to ensure uninterrupted blood circulation.
This article explores in detail the structure and function of the heart, including its anatomy, layers, chambers, valves, blood flow, electrical conduction, and the physiological mechanisms that regulate its activity.
Location and Position of the Heart
The heart is located in the thoracic cavity, specifically in the mediastinum, the central compartment between the lungs. It rests on the diaphragm and lies slightly to the left of the midline, with about two-thirds of its mass positioned on the left side of the body.
The heart’s base is directed upward, posteriorly, and to the right, while the apex points downward, forward, and to the left, lying near the fifth intercostal space. This orientation allows efficient pumping into both the pulmonary and systemic circulations.
The heart is enclosed within a double-layered protective sac called the pericardium, which anchors it in place, provides lubrication, and prevents friction during contractions.
The Pericardium and Layers of the Heart Wall
The heart wall consists of three main layers — the epicardium, myocardium, and endocardium — surrounded by the pericardium.
Pericardium
The pericardium has two layers:
- Fibrous pericardium: A tough outer layer made of dense connective tissue that anchors the heart to surrounding structures such as the diaphragm and sternum.
- Serous pericardium: A thin, double-layered membrane consisting of:
- The parietal layer, which lines the fibrous pericardium.
- The visceral layer (epicardium), which covers the heart surface.
Between the two serous layers lies the pericardial cavity, containing a small amount of lubricating pericardial fluid that reduces friction during heartbeats.
Epicardium
The epicardium forms the outer layer of the heart wall. It consists of connective tissue and fat that cushion the heart and house coronary blood vessels.
Myocardium
The myocardium is the thick, muscular middle layer responsible for the heart’s contractile power. It is composed of cardiac muscle tissue, specialized for endurance and rhythmic contraction. The myocardium’s thickness varies; it is thickest in the left ventricle, which must pump blood throughout the body.
Endocardium
The endocardium lines the inner surfaces of the heart chambers and valves. It is made of endothelial cells that provide a smooth surface to minimize friction and prevent blood clot formation.
Together, these layers ensure structural integrity, contraction efficiency, and protection of the heart.
The Four Chambers of the Heart
The human heart is a four-chambered organ consisting of two upper chambers, the atria, and two lower chambers, the ventricles. The right side handles deoxygenated blood, while the left side manages oxygenated blood.
Right Atrium
The right atrium receives deoxygenated blood from three major sources:
- The superior vena cava (blood from the upper body),
- The inferior vena cava (blood from the lower body),
- The coronary sinus (blood from the heart muscle itself).
The right atrium passes this blood through the tricuspid valve into the right ventricle.
Right Ventricle
The right ventricle receives blood from the right atrium and pumps it into the pulmonary trunk, which divides into the right and left pulmonary arteries, carrying blood to the lungs for oxygenation.
The inner surface of the right ventricle contains ridges called trabeculae carneae, and papillary muscles connected to the tricuspid valve via chordae tendineae that prevent valve inversion during contraction.
Left Atrium
The left atrium receives oxygenated blood from the lungs through four pulmonary veins (two from each lung). This chamber acts as a reservoir and pumps blood through the bicuspid (mitral) valve into the left ventricle.
Left Ventricle
The left ventricle is the thickest and most muscular chamber, responsible for pumping oxygenated blood through the aortic valve into the aorta, the body’s main artery. From there, blood is distributed to all tissues.
The left ventricle’s thick myocardium reflects the high pressure required to propel blood through the systemic circulation.
The Heart Valves
Heart valves ensure unidirectional blood flow through the heart by opening and closing in response to pressure changes during contraction and relaxation.
Atrioventricular (AV) Valves
These valves separate the atria from the ventricles:
- Tricuspid valve: Between the right atrium and right ventricle, consisting of three flaps (cusps).
- Bicuspid (mitral) valve: Between the left atrium and left ventricle, consisting of two flaps.
During ventricular relaxation, the AV valves open, allowing blood to flow into the ventricles. When ventricles contract, the valves close, preventing backflow into the atria.
Semilunar Valves
These valves guard the exits of the ventricles:
- Pulmonary valve: Located between the right ventricle and the pulmonary trunk.
- Aortic valve: Located between the left ventricle and the aorta.
Semilunar valves open when ventricular pressure exceeds arterial pressure, allowing blood ejection. They close when pressure drops, preventing backflow into the ventricles.
Proper valve function ensures efficient circulation and prevents regurgitation of blood.
Circulatory Pathways of the Heart
The heart functions as two pumps operating in sequence: the right side for pulmonary circulation and the left side for systemic circulation.
Pulmonary Circulation
Deoxygenated blood from the body enters the right atrium, flows to the right ventricle, and is pumped to the lungs via the pulmonary arteries. In the lungs, carbon dioxide is exchanged for oxygen. Oxygenated blood then returns to the left atrium through the pulmonary veins.
Systemic Circulation
Oxygen-rich blood from the left atrium enters the left ventricle, which pumps it into the aorta. The aorta branches into arteries that carry blood throughout the body. After delivering oxygen and nutrients, deoxygenated blood returns to the right atrium through the veins, completing the cycle.
This dual-circuit design ensures continuous and efficient blood flow between the lungs and the rest of the body.
The Coronary Circulation
The coronary arteries supply oxygen and nutrients to the myocardium, as the heart muscle requires its own blood supply due to its thickness.
Coronary Arteries
The left and right coronary arteries branch from the base of the aorta.
- Left coronary artery divides into the anterior interventricular (left anterior descending) artery and the circumflex artery, supplying the anterior and lateral walls of the left ventricle.
- Right coronary artery gives rise to the marginal artery and posterior interventricular artery, supplying the right atrium, right ventricle, and parts of the conduction system.
Cardiac Veins and Coronary Sinus
Deoxygenated blood from the myocardium drains into cardiac veins, which empty into the coronary sinus on the posterior side of the heart. The coronary sinus then returns blood to the right atrium.
Efficient coronary circulation is essential, as any blockage can lead to myocardial infarction (heart attack).
Microscopic Structure of Cardiac Muscle
Cardiac muscle tissue is unique and highly specialized for endurance, rhythmicity, and coordinated contraction.
Characteristics
- Striated fibers similar to skeletal muscle, but shorter and branched.
- Each cell has a single central nucleus.
- Cells are connected by intercalated discs, which contain gap junctions and desmosomes.
- Gap junctions allow electrical impulses to pass quickly from cell to cell.
- Desmosomes provide mechanical strength during contraction.
Functional Syncytium
Because of intercellular connections, the myocardium acts as a functional syncytium — when one cell is stimulated, the entire network contracts together. This ensures uniform contraction of the atria and ventricles.
The Cardiac Conduction System
The heart has its own intrinsic electrical system that generates and transmits impulses, ensuring coordinated contraction. This system allows the heart to beat independently of direct nervous stimulation.
Components of the Conduction System
Sinoatrial (SA) Node
Located in the right atrium near the superior vena cava, the SA node acts as the natural pacemaker of the heart. It initiates electrical impulses that spread through the atria, causing them to contract.
Atrioventricular (AV) Node
Located at the junction between atria and ventricles, the AV node delays the impulse slightly to allow the atria to complete contraction before the ventricles begin.
Atrioventricular (AV) Bundle (Bundle of His)
From the AV node, impulses travel through the AV bundle, which passes into the interventricular septum.
Bundle Branches and Purkinje Fibers
The AV bundle divides into right and left bundle branches, which further distribute impulses to Purkinje fibers in the ventricular walls. This network ensures synchronized contraction of both ventricles.
Cardiac Cycle Coordination
This conduction system produces a regular sequence of atrial contraction (systole) followed by ventricular contraction (systole) and relaxation (diastole), forming the heartbeat.
The Cardiac Cycle
The cardiac cycle refers to one complete sequence of contraction and relaxation of the heart chambers. It lasts about 0.8 seconds in a person with a heart rate of 75 beats per minute.
Phases of the Cardiac Cycle
Atrial Systole
The atria contract, pushing blood into the ventricles. The AV valves are open, and semilunar valves are closed.
Ventricular Systole
The ventricles contract, closing AV valves and opening semilunar valves to eject blood into the pulmonary trunk and aorta.
Ventricular Diastole
The ventricles relax, semilunar valves close to prevent backflow, and AV valves reopen as the chambers fill with blood again.
Heart Sounds
Two primary heart sounds are heard through a stethoscope:
- “Lub” (S1): Closure of AV valves at the beginning of ventricular systole.
- “Dub” (S2): Closure of semilunar valves at the beginning of ventricular diastole.
Abnormal sounds, known as murmurs, may indicate valve defects or turbulent blood flow.
Electrical Activity and Electrocardiogram (ECG)
The electrical impulses generated by the heart can be recorded as an electrocardiogram (ECG or EKG), a diagnostic tool used to assess cardiac function.
Components of an ECG Waveform
- P wave: Atrial depolarization.
- QRS complex: Ventricular depolarization (and hidden atrial repolarization).
- T wave: Ventricular repolarization.
The ECG helps detect abnormalities in rhythm, conduction, and cardiac muscle health.
Blood Supply and Oxygen Demand
The heart is a highly aerobic organ, requiring a constant supply of oxygen. Its metabolism relies primarily on fatty acids and glucose, using oxygen delivered by coronary circulation. Any interruption in oxygen supply — even for a few minutes — can cause irreversible tissue damage.
Ischemia (reduced blood flow) leads to angina pectoris (chest pain), while complete blockage results in myocardial infarction (heart attack).
Regulation of Heart Function
Heart rate and strength of contraction are regulated by intrinsic and extrinsic mechanisms.
Intrinsic Regulation
The Frank-Starling law states that the greater the stretch of cardiac muscle fibers (due to increased venous return), the stronger the contraction. This ensures balanced output between both sides of the heart.
Extrinsic Regulation
The autonomic nervous system (ANS) and hormonal control adjust cardiac performance to body needs.
- Sympathetic stimulation increases heart rate and force of contraction.
- Parasympathetic stimulation (via the vagus nerve) decreases heart rate.
- Hormones such as epinephrine, norepinephrine, and thyroxine increase cardiac output during stress or exercise.
Cardiac Output and Blood Flow
Cardiac output (CO) is the volume of blood pumped by each ventricle per minute.
It is calculated as:
CO = Heart Rate (HR) × Stroke Volume (SV)
Average resting cardiac output is about 5 liters per minute but can increase fivefold during intense exercise.
Stroke Volume
Stroke volume depends on:
- Preload: The degree of ventricular stretch.
- Contractility: The force of contraction.
- Afterload: The resistance the ventricles must overcome to eject blood.
These factors ensure that the heart adjusts its output to meet the body’s metabolic demands.
Development and Adaptations of the Heart
Embryonic Development
The heart begins as a simple tubular structure that starts beating around the fourth week of embryonic life. It undergoes folding, septation, and valve formation to become a four-chambered organ by the end of the first trimester.
Physiological Adaptations
- Athlete’s Heart: Regular exercise leads to mild enlargement and increased efficiency.
- Aging Heart: With age, heart rate and elasticity decline, and valves may thicken, increasing cardiovascular risk.
Common Disorders of the Heart
Several diseases can affect the heart’s structure and function:
Coronary Artery Disease (CAD)
Caused by atherosclerosis, leading to reduced blood flow to the myocardium.
Myocardial Infarction
Death of heart tissue due to prolonged ischemia.
Heart Failure
The heart’s pumping ability becomes inadequate, leading to fluid accumulation.
Arrhythmias
Abnormal electrical activity causing irregular heartbeat.
Valvular Heart Disease
Malfunction of valves resulting in stenosis or regurgitation.
Understanding the structure and physiology of the heart is essential for diagnosing and treating such conditions.
Importance of the Heart in Homeostasis
The heart maintains homeostasis by ensuring efficient distribution of oxygen, nutrients, and hormones while removing waste products. It supports all other organ systems by maintaining adequate blood pressure and perfusion, adapting instantly to changes in physical activity, temperature, or stress.
Leave a Reply