Cardiovascular System Overview

Introduction to the Cardiovascular System

The cardiovascular system is essential for transporting blood, nutrients, gases, and wastes throughout the body. It consists of the heart and an extensive network of blood vessels.

• Heart: The muscular organ that pumps blood through the body.

• Blood Vessels: Tubular structures (arteries, veins, capillaries) that carry blood to and from all body tissues.

• Cardiology: The study of the heart and diseases associated with it.

• The heart beats approximately 7,000 liters of blood daily and contracts about 2.5 billion times in an average lifetime.

Heart Anatomy

Size and Location of the Heart

The heart is located in the thoracic cavity within the mediastinum, between the lungs and deep to the sternum. It is about the size of a fist (14 cm long, 9 cm wide) and weighs approximately 300 g.

• Base: The broad superior portion beneath the 2nd rib, point of attachment for great vessels.

• Apex: The inferior end, points to the left and rests above the diaphragm at the 5th intercostal space.

Coverings of the Heart

The heart is enclosed by a double-walled sac called the pericardium:

• Fibrous Pericardium: Tough, outer layer of dense irregular connective tissue.

• Serous Pericardium: Thin, inner layer with two parts:

  • Parietal Layer: Lines the internal surface of the fibrous pericardium.

  • Visceral Layer (Epicardium): Covers the external heart surface.

• Pericardial Cavity: Space between parietal and visceral layers, filled with pericardial fluid to reduce friction.

• Pericarditis: Inflammation of the pericardium, causing painful friction with each heartbeat.

Heart Wall Structure

• Epicardium: Outer layer, also the visceral pericardium.

• Myocardium: Middle, thick muscular layer composed of cardiac muscle fibers.

  • Cardiac muscle fibers are shorter, branched, and interconnected by intercalated discs (contain desmosomes and gap junctions).

  • Cardiac muscle has a spiral arrangement, allowing a twisting contraction.

  • Cardiac muscle cells have more mitochondria and a longer refractory period than skeletal muscle.

  • Cardiac muscle cells contract as a unit (syncytium) due to gap junctions.

  • Repair of damaged cardiac muscle is by fibrosis (scarring), not regeneration.

• Endocardium: Inner layer, simple squamous epithelium lining the heart chambers and valves.

Heart Chambers and Valves

Chambers of the Heart

The heart has four chambers: two atria (superior) and two ventricles (inferior).

• Right Atrium: Receives deoxygenated blood from the superior and inferior vena cava and the coronary sinus.

• Left Atrium: Receives oxygenated blood from the pulmonary veins.

• Right Ventricle: Pumps blood to the lungs via the pulmonary trunk.

• Left Ventricle: Pumps blood to the systemic circulation via the aorta; has the thickest wall.

Valves of the Heart

Valves ensure one-way blood flow through the heart:

• Atrioventricular (AV) Valves:

  • Tricuspid Valve: Between right atrium and right ventricle.

  • Bicuspid (Mitral) Valve: Between left atrium and left ventricle.

  • AV valves are anchored by chordae tendineae to papillary muscles to prevent prolapse.

• Semilunar Valves:

  • Pulmonary Valve: Between right ventricle and pulmonary trunk.

  • Aortic Valve: Between left ventricle and aorta.

  • Semilunar valves prevent backflow from arteries into ventricles.

Pathway of Blood Through the Heart

1. Deoxygenated blood enters the right atrium from the body (via superior/inferior vena cava).

2. Passes through the tricuspid valve into the right ventricle.

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

4. Oxygenated blood returns from the lungs via pulmonary veins to the left atrium.

5. Passes through the mitral valve into the left ventricle.

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

Fibrous Skeleton of the Heart

The fibrous skeleton is a framework of dense connective tissue that:

• Provides structural support, especially around valves and openings of great vessels.

• Anchors cardiac muscle fibers and provides electrical insulation between atria and ventricles.

Coronary Circulation

The heart muscle (myocardium) receives its own blood supply via the coronary arteries.

• Left Coronary Artery (LCA):

  • Anterior Interventricular Branch (LAD/"Widow Maker"): Supplies both ventricles and anterior two-thirds of the interventricular septum.

  • Circumflex Branch: Supplies the left atrium and left ventricle.

• Right Coronary Artery (RCA):

  • Right Marginal Branch: Supplies the right atrium and ventricle.

  • Posterior Interventricular Branch: Supplies both ventricles' posterior walls.

• Coronary Veins: Collect deoxygenated blood from the myocardium and return it to the right atrium via the coronary sinus.

Note: Blood flow in coronary arteries increases during ventricular relaxation (diastole).

Cardiac Muscle Physiology

• Cardiac muscle cells are autorhythmic, meaning they can generate their own action potentials.

• Contraction is coordinated by the conduction system (see below).

• Cardiac muscle has a long refractory period, preventing tetanus.

Conduction System of the Heart

The heart's electrical system ensures coordinated contraction:

1. Sinoatrial (SA) Node: Pacemaker; initiates heartbeat (60–100 bpm).

2. Atrioventricular (AV) Node: Delays impulse, allowing atria to contract before ventricles.

3. AV Bundle (Bundle of His): Conducts impulse from AV node to ventricles.

4. Right and Left Bundle Branches: Carry impulse through interventricular septum.

5. Purkinje Fibers: Distribute impulse through ventricular myocardium.

Autonomic Regulation: Sympathetic stimulation increases heart rate and force; parasympathetic (vagus nerve) decreases heart rate.

Clinical Correlations

• Pericarditis: Inflammation of the pericardium, causing chest pain and friction rub.

• Coronary Artery Disease (CAD): Narrowing or blockage of coronary arteries, leading to myocardial infarction (heart attack).

• Heart Failure: The heart cannot meet the body's demand for oxygenated blood.

• Arrhythmias: Disorders of the conduction system causing irregular heartbeats.

• Congenital Heart Defects: Structural abnormalities present at birth.

Table: Comparison of Cardiac, Skeletal, and Smooth Muscle

Key Equations

• Cardiac Output (CO): The amount of blood pumped by each ventricle per minute. Where = heart rate (beats per minute), = stroke volume (mL per beat)

Summary

• The heart is a muscular organ with four chambers and a double-layered pericardial covering.

• Valves ensure unidirectional blood flow.

• Coronary circulation supplies the heart muscle itself.

• The conduction system coordinates the heartbeat.

• Disorders of the heart can affect its structure, function, or electrical activity.