The Heart: Structure and Function in the Cardiovascular System

The Cardiovascular System

Overview and Function

The cardiovascular system is responsible for the transportation of blood, nutrients, gases, and wastes throughout the body. It consists of the heart, blood vessels, and blood, forming a closed circuit that maintains homeostasis and supports cellular function.

• Blood: The primary function is transportation of oxygen, nutrients, hormones, and waste products.

• Circulatory System: Acts as a transportation highway for blood flow.

• Heart: Serves as the pump that propels blood through the vessels.

• Cardiac Activity: The heart beats approximately 100,000 times per day and pumps about 8,000 liters of blood daily.

Heart Anatomy and Location

Position and Structure

The heart is a muscular, triangular organ located in the mediastinum, the central compartment of the thoracic cavity, between the lungs and behind the sternum. Its anatomical position is crucial for its function and protection.

• Location: Two-thirds of the heart's mass lies to the left of the body's midline, with one-third to the right. The apex (pointed tip) rests on the diaphragm and points leftward.

• Size: Comparable to a closed fist, measuring about 5 inches from base to apex.

• Base: The broad superior portion where major vessels attach.

• Apex: The pointed inferior tip, directed downward and to the left.

Thoracic Cavity Relationships

The heart is protected by the rib cage and is situated anterior to the vertebral column and posterior to the sternum. The lungs flank the heart on either side.

• Base of Heart: Located at the level of the second rib.

• Apex of Heart: Located at the level of the fifth intercostal space, just medial to the left midclavicular line.

Pericardial Layers of the Heart

Structure and Function

The heart is enclosed in a double-walled sac called the pericardium, which provides protection, reduces friction, and anchors the heart within the thoracic cavity.

• Parietal Pericardium: The outer, tough, fibrous layer.

• Visceral Pericardium (Epicardium): The inner serous layer that covers the heart surface and is considered the outermost layer of the heart wall.

• Pericardial Cavity: The space between the two layers, filled with pericardial fluid to reduce friction during heartbeats.

Clinical Note: Pericarditis is inflammation of the pericardium, and cardiac tamponade is a life-threatening compression of the heart due to fluid accumulation in the pericardial cavity.

Heart Wall Structure

Three Layers of the Heart Wall

The heart wall is composed of three distinct layers, each with specialized functions:

• Epicardium (Visceral Pericardium): The outermost serous membrane.

• Myocardium: The thick, muscular middle layer composed of cardiac muscle tissue responsible for contraction and pumping action.

• Endocardium: The innermost layer, consisting of smooth simple squamous epithelium, lining the heart chambers and valves.

Cardiac Muscle Tissue

• Cardiac muscle fibers are striated, branched, and interconnected by intercalated discs for synchronized contraction.

• Cells contain a single central nucleus and numerous mitochondria, relying on aerobic respiration.

• Arranged in spiral or circular bundles to maximize pumping efficiency.

Chambers and Valves of the Heart

Heart Chambers

The heart contains four chambers:

• Atria (Right and Left): Upper chambers that receive blood returning to the heart.

• Ventricles (Right and Left): Lower chambers that discharge blood from the heart.

• Interatrial Septum: Separates the right and left atria.

• Interventricular Septum: Separates the right and left ventricles.

Heart Valves

Valves ensure unidirectional blood flow and prevent backflow:

• Atrioventricular (AV) Valves: Between atria and ventricles

  • Tricuspid Valve: Right side, three flaps.

  • Bicuspid (Mitral) Valve: Left side, two flaps.

  • Anchored by chordae tendineae to papillary muscles.

• Semilunar Valves: At the base of major arteries leaving the ventricles

  • Pulmonary Semilunar Valve: Between right ventricle and pulmonary trunk.

  • Aortic Semilunar Valve: Between left ventricle and aorta.

Valve Disorders: Incompetent valves leak, stenosed valves are narrowed, and conditions like mitral valve prolapse can affect function.

Blood Flow Through the Heart

Pulmonary and Systemic Circuits

The heart functions as two separate pumps:

• Pulmonary Circuit: Right side pumps deoxygenated blood to the lungs for gas exchange.

• Systemic Circuit: Left side pumps oxygenated blood to the rest of the body.

Pathway of Blood Flow

1. Deoxygenated blood enters right atrium via superior/inferior vena cava and coronary sinus.

2. Passes through tricuspid valve to right ventricle.

3. Pumped through pulmonary semilunar valve into pulmonary trunk and arteries to lungs.

4. Oxygenated blood returns via pulmonary veins to left atrium.

5. Passes through bicuspid (mitral) valve to left ventricle.

6. Pumped through aortic semilunar valve into aorta and systemic circulation.

Coronary Circulation

Blood Supply to the Heart

The myocardium requires a constant supply of oxygen and nutrients, delivered by the coronary arteries.

• Right Coronary Artery: Supplies right atrium and parts of both ventricles.

• Left Coronary Artery: Supplies left atrium, left ventricle, and interventricular septum.

• Anastomoses: Interconnections between arteries provide alternate pathways for blood flow.

• Cardiac Veins: Drain deoxygenated blood from myocardium into the coronary sinus, which empties into the right atrium.

Clinical Note: Blockage of coronary arteries can cause myocardial infarction (heart attack). Atherosclerosis and angina pectoris are common coronary artery diseases.

Cardiac Cycle and Heart Sounds

Phases of the Cardiac Cycle

• Systole: Contraction phase, chambers eject blood.

• Diastole: Relaxation phase, chambers fill with blood.

The right and left sides of the heart work in synchrony, separated by the septum to prevent mixing of oxygenated and deoxygenated blood.

Heart Sounds

• First Heart Sound (S1, "lubb"): Closure of AV valves during ventricular contraction.

• Second Heart Sound (S2, "dupp"): Closure of semilunar valves during ventricular relaxation.

• Additional faint sounds (S3, S4) may be heard in some individuals.

• Murmurs: Abnormal heart sounds, often due to valve disorders.

Cardiac Output and Regulation

Cardiodynamics

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

• Stroke Volume (SV): Volume of blood ejected by a ventricle with each beat.

• Cardiac Output (CO): $CO = HR \times SV$

Factors Affecting Cardiac Output

• Heart Rate (HR): Influenced by body temperature, neural and hormonal regulation (e.g., sympathetic stimulation increases HR).

• Stroke Volume (SV): Influenced by venous return, contractility, and hormonal factors.

• Frank-Starling Principle: Increased venous return stretches myocardial cells, leading to stronger contractions and increased SV ("more in = more out").

Conducting System of the Heart

Electrical Conduction Pathway

The heart's conducting system allows for automaticity and coordinated contraction without external stimulation.

• SA (Sinoatrial) Node: The pacemaker, initiates impulses (70-80 BPM).

• AV (Atrioventricular) Node: Delays impulse, can act as pacemaker if SA node fails (40-60 BPM).

• AV Bundle (Bundle of His): Conducts impulses to ventricles.

• Right and Left Bundle Branches: Carry impulses through interventricular septum.

• Purkinje Fibers: Distribute impulse through ventricular myocardium.

Terms Related to Conduction

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

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

• Dysrhythmia: Abnormal heart rhythm.

• Fibrillation: Uncoordinated contraction, ineffective pumping.

Electrocardiography (ECG/EKG)

Principles and Interpretation

An electrocardiogram (ECG or EKG) records the electrical activity of the heart and is a key diagnostic tool.

• P Wave: Atrial depolarization (contraction).

• QRS Complex: Ventricular depolarization (contraction).

• T Wave: Ventricular repolarization (relaxation).

ECG is used to detect cardiac muscle damage, conduction abnormalities, and arrhythmias.

Clinical Considerations

Common Heart Disorders

• Coronary Artery Disease (CAD): Blockage or narrowing of coronary arteries, leading to reduced blood flow (ischemia) and possible myocardial infarction.

• Heart Failure: Inability of the heart to pump sufficient blood. Can be right-sided (due to left-sided failure) or left-sided (congestive heart failure).

• Valve Disorders: Incompetence, stenosis, or prolapse affecting blood flow and efficiency.

Cardiopulmonary Resuscitation (CPR)

• Direct rhythmic compression of the heart between the sternum and vertebral column can maintain blood flow during cardiac arrest.

• CPR is often combined with artificial respiration and use of an Automated External Defibrillator (AED).

Dental and Medical Implications

• Heart failure or myocardial infarction may affect patient positioning and treatment planning.

• Patients with heart murmurs or artificial valves may require antibiotic prophylaxis before dental procedures.

• Awareness of potential drug interactions with vasoconstrictors in local anesthetics is important.