Cardiovascular System

General Shape and Location of the Heart

The heart is a muscular organ roughly the size of a fist, located in the mediastinum of the thoracic cavity, between the lungs and slightly to the left of the midline.

• Shape: Cone-shaped, with a broad base and a pointed apex.

• Location: Posterior to the sternum, anterior to the vertebral column, and superior to the diaphragm.

• Orientation: The apex points downward and to the left.

Pulmonary and Systemic Circuits

The heart pumps blood through two main circuits: the pulmonary and systemic circuits.

• Pulmonary Circuit: Carries deoxygenated blood from the right side of the heart to the lungs for oxygenation, then returns oxygenated blood to the left side of the heart.

• Systemic Circuit: Delivers oxygenated blood from the left side of the heart to the body tissues and returns deoxygenated blood to the right side of the heart.

• Key Difference: Pulmonary circuit exchanges gases in the lungs; systemic circuit supplies nutrients and removes wastes from body tissues.

Layers of the Pericardium and Heart Wall

The heart is enclosed by the pericardium and composed of three main layers.

• Pericardium: Double-walled sac surrounding the heart, consisting of:

  • Fibrous pericardium: Outer tough layer.

  • Serous pericardium: Inner layer, subdivided into parietal and visceral layers.

• Heart Wall Layers:

  • Epicardium: Outer layer (visceral pericardium).

  • Myocardium: Middle, muscular layer responsible for contraction.

  • Endocardium: Inner endothelial lining.

Function of Valves in the Heart

Heart valves ensure unidirectional blood flow and prevent backflow.

• Atrioventricular (AV) Valves: Tricuspid (right) and mitral (left) valves separate atria from ventricles.

• Semilunar Valves: Pulmonary and aortic valves separate ventricles from major arteries.

• Function: Open and close in response to pressure changes, maintaining efficient circulation.

Blood Flow Through the Heart

Blood follows a specific pathway through the heart, with distinct structures carrying oxygenated or deoxygenated blood.

• Deoxygenated blood enters the right atrium via the superior and inferior vena cava.

• Passes through the tricuspid valve into the right ventricle.

• Pumped through the pulmonary valve into the pulmonary arteries to the lungs.

• Oxygenated blood returns via pulmonary veins to the left atrium.

• Passes through the mitral valve into the left ventricle.

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

• Oxygenated Structures: Left atrium, left ventricle, aorta, pulmonary veins.

• Deoxygenated Structures: Right atrium, right ventricle, vena cava, pulmonary arteries.

Structures of the Heart

The heart contains several key anatomical structures, as illustrated below.

• Right Atrium

• Right Ventricle

• Left Atrium

• Left Ventricle

• Superior/Inferior Vena Cava

• Pulmonary Arteries and Veins

• Aorta

• Valves: Tricuspid, Pulmonary, Mitral, Aortic

Structure of Cardiac Muscle Cells

Cardiac muscle cells, or cardiomyocytes, are specialized for contraction and electrical conduction.

• Striated appearance due to organized sarcomeres.

• Intercalated discs connect cells, allowing rapid electrical communication.

• Single central nucleus per cell (occasionally two).

• Abundant mitochondria for high energy demand.

Depolarization and Repolarization in Heart Electrophysiology

Depolarization and repolarization are key processes in the cardiac action potential.

• Depolarization: Rapid influx of Na+ ions, causing the cell membrane potential to become more positive.

• Repolarization: Efflux of K+ ions, restoring the negative resting membrane potential.

• Role: These processes enable the propagation of electrical impulses and coordinated contraction.

Types and Functions of Pacemaker Cells

Pacemaker cells initiate and regulate the heart's rhythmic contractions.

• Sinoatrial (SA) Node: Primary pacemaker, sets the heart rate.

• Atrioventricular (AV) Node: Delays impulse, allowing atrial contraction before ventricular contraction.

• Bundle of His, Bundle Branches, Purkinje Fibers: Conduct impulses rapidly through ventricles.

Electrical Conduction System of the Heart

The heart's conduction system coordinates the sequence of contraction.

• Impulse originates at the SA node.

• Spreads through atria to AV node.

• Travels down the Bundle of His, bundle branches, and Purkinje fibers to ventricles.

Waves and Intervals of an ECG

An electrocardiogram (ECG) records the electrical activity of the heart.

• P wave: Atrial depolarization.

• QRS complex: Ventricular depolarization.

• T wave: Ventricular repolarization.

• PR interval: Time from atrial to ventricular depolarization.

• QT interval: Duration of ventricular activity.

Pressure Gradients and Valve Function

Pressure differences across heart chambers drive valve opening and closing.

• Valves open when upstream pressure exceeds downstream pressure.

• Valves close when downstream pressure exceeds upstream pressure.

• Ensures unidirectional blood flow.

Cardiac Cycle

The cardiac cycle describes the sequence of events in one heartbeat.

• Systole: Contraction phase, blood ejected from chambers.

• Diastole: Relaxation phase, chambers fill with blood.

Stroke Volume and Cardiac Output

Stroke volume and cardiac output are key measures of heart function.

• Stroke Volume (SV): Amount of blood ejected by a ventricle per beat.

• Average SV: Approximately 70 mL per beat.

• Cardiac Output (CO): Volume of blood pumped by the heart per minute.

• Formula:

• Average CO: About 5 L/min in adults.

Frank-Starling Law

The Frank-Starling law describes the relationship between ventricular filling and contraction strength.

• Increased preload (ventricular filling) leads to stronger contraction and higher stroke volume.

• Ensures the heart pumps out all the blood it receives.

Factors Influencing Stroke Volume

Three main factors affect stroke volume:

• Preload: Degree of stretch of cardiac muscle before contraction.

• Contractility: Strength of contraction at a given preload.

• Afterload: Resistance the heart must overcome to eject blood.

Inotropic and Chronotropic Agents

These agents modify heart function.

• Inotropic agents: Affect contractility (e.g., epinephrine increases contractility).

• Chronotropic agents: Affect heart rate (e.g., acetylcholine decreases heart rate).

Regulation of the Cardiovascular System

The autonomic nervous system regulates heart function.

• Sympathetic Nervous System (SNS): Increases heart rate and contractility.

• Parasympathetic Nervous System (PNS): Decreases heart rate.

Hormonal Regulation of Cardiac Output

Several hormones influence cardiac output.

• Aldosterone: Increases blood volume and pressure.

• Antidiuretic Hormone (ADH): Promotes water retention, increasing blood volume.

• Atrial Natriuretic Hormone (ANH): Reduces blood volume and pressure.

Summary Table: Heart Valves and Blood Flow

Additional info: Academic context and definitions have been expanded for clarity and completeness.