The Cardiovascular System: The Heart – Comprehensive Study Notes

The Cardiovascular System: The Heart

Heart Anatomy

The heart is a muscular organ responsible for pumping blood throughout the body via two main circuits: the pulmonary and systemic circuits. It is divided into four chambers and is protected by several layers of tissue.

• Pulmonary Circuit: Carries blood to the lungs to exchange carbon dioxide for oxygen.

• Systemic Circuit: Delivers oxygenated blood to body tissues and returns deoxygenated blood to the heart.

• Receiving Chambers: The right atrium (receives blood from systemic circuit) and left atrium (receives blood from pulmonary circuit).

• Pumping Chambers: The right ventricle (pumps blood to pulmonary circuit) and left ventricle (pumps blood to systemic circuit).

Size and Location of the Heart

• Approximately the size of a fist and weighs less than 1 pound.

• Located in the mediastinum between the second rib and fifth intercostal space, on the superior surface of the diaphragm, with two-thirds to the left of the midsternal line.

• Anterior to the vertebral column and posterior to the sternum.

Coverings of the Heart

• Pericardium: Double-walled sac surrounding the heart.

• Fibrous Pericardium: Superficial layer that protects, anchors, and prevents overfilling.

• Serous Pericardium: Deep two-layered membrane:

  • Parietal Layer: Lines internal surface of fibrous pericardium.

  • Visceral Layer (Epicardium): On external surface of heart.

  • Layers separated by the pericardial cavity filled with serous fluid to reduce friction.

Three Layers of the Heart Wall

• Epicardium: Visceral layer of serous pericardium.

• Myocardium: Middle layer composed of cardiac muscle cells arranged in circular or spiral bundles. Contains the cardiac skeleton (connective tissue) that supports vessels and valves.

• Endocardium: Innermost layer; continuous with endothelial lining of blood vessels, lines heart chambers, and covers cardiac skeleton of valves.

Chambers and Associated Great Vessels

• The heart has four chambers: two atria (upper) and two ventricles (lower).

• Interatrial Septum: Separates the atria; contains the fossa ovalis (remnant of fetal foramen ovale).

• Interventricular Septum: Separates the ventricles.

• Surface Features: Coronary sulcus (encircles junction of atria and ventricles), anterior and posterior interventricular sulci (landmarks for septum position).

• Atria: Small, thin-walled chambers; auricles increase atrial volume.

  • Right Atrium: Receives deoxygenated blood from the body via three veins:

    1. Superior vena cava (from regions above diaphragm)

    2. Inferior vena cava (from regions below diaphragm)

    3. Coronary sinus (from coronary circulation)

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

• Ventricles: Thick-walled chambers that discharge blood.

  • Right Ventricle: Pumps blood into the pulmonary trunk.

  • Left Ventricle: Pumps blood into the aorta (largest artery in the body).

  • Papillary Muscles: Project into ventricular cavity and anchor chordae tendineae attached to atrioventricular valves.

Heart Valves

Valves ensure unidirectional blood flow through the heart, opening and closing in response to pressure changes.

• Atrioventricular (AV) Valves: Located between atria and ventricles; prevent backflow into atria when ventricles contract.

  • Tricuspid valve (right AV valve)

  • Bicuspid (mitral) valve (left AV valve)

  • Chordae Tendineae: Anchor valve cusps to papillary muscles, holding flaps closed during contraction.

• Semilunar (SL) Valves: Located between ventricles and major arteries; prevent backflow into ventricles.

  • Pulmonary semilunar valve (right ventricle to pulmonary trunk)

  • Aortic semilunar valve (left ventricle to aorta)

  • Each has three half-moon-shaped cusps.

• No valves between major veins and atria; backflow is prevented by inertia and heart contraction compressing venous openings.

Table: Types of Heart Valves

Valve Type	Location	Function
Atrioventricular (AV)	Between atria and ventricles	Prevent backflow into atria
Semilunar (SL)	Between ventricles and arteries	Prevent backflow into ventricles

Homeostatic Imbalances of Valves

• Incompetent Valve: Blood backflows, causing the heart to repump the same blood repeatedly.

• Valvular Stenosis: Stiff flaps constrict opening, requiring the heart to exert more force to pump blood.

• Defective valves can be replaced with mechanical, animal, or cadaver valves.

Pathway of Blood Through the Heart

Blood flows through the heart in a specific sequence, ensuring oxygenation and nutrient delivery to tissues.

• Right Side:

  1. Superior vena cava, inferior vena cava, and coronary sinus

  2. Right atrium

  3. Tricuspid valve

  4. Right ventricle

  5. Pulmonary semilunar valve

  6. Pulmonary trunk

  7. Pulmonary arteries

  8. Lungs (gas exchange)

• Left Side:

  1. Four pulmonary veins

  2. Left atrium

  3. Bicuspid (mitral) valve

  4. Left ventricle

  5. Aortic semilunar valve

  6. Aorta

  7. Systemic circulation

• Equal volumes of blood are pumped to both circuits.

• Pulmonary circuit is short, low-pressure; systemic circuit is long, high-pressure.

• Left ventricle walls are three times thicker than right, pumping with greater pressure.

Coronary Circulation

The heart's own blood supply is provided by the coronary circulation, which is the shortest in the body and is delivered when the heart is relaxed.

• Coronary Arteries: Left and right coronary arteries arise from the base of the aorta and supply arterial blood to the heart, encircling it in the coronary sulcus.

• Coronary Veins: Cardiac veins collect blood from capillary beds and empty into the coronary sinus, which returns blood to the right atrium.

Homeostatic Imbalances of Coronary Circulation

• Angina Pectoris: Thoracic pain due to fleeting deficiency in blood delivery to myocardium; cells are weakened.

• Myocardial Infarction (Heart Attack): Prolonged coronary blockage; areas of cell death are replaced with noncontractile scar tissue.

Cardiac Muscle Fibers

Cardiac muscle cells are specialized for continuous, rhythmic contraction and are interconnected for coordinated function.

• Striated, short, branched, and interconnected with one (occasionally two) central nuclei.

• Contain many large mitochondria (25–35% of cell volume) for resistance to fatigue.

• Connected by intercalated discs containing:

  • Desmosomes: Hold cells together and prevent separation during contraction.

  • Gap Junctions: Allow ions to pass between cells, enabling electrical coupling and functional syncytium (single coordinated unit).

Electrical Events of the Heart

The heart can depolarize and contract without nervous system stimulation due to its intrinsic conduction system.

The Intrinsic Conduction System

• Consists of specialized pacemaker cells that initiate and distribute impulses to coordinate depolarization and contraction.

Sequence of Excitation

1. Sinoatrial (SA) Node: Pacemaker in right atrial wall; depolarizes faster than other myocardium, generating impulses at about 75 beats/minute (sinus rhythm).

2. Atrioventricular (AV) Node: In inferior interatrial septum; delays impulses by ~0.1 second due to smaller fibers and fewer gap junctions.

3. Atrioventricular (AV) Bundle (Bundle of His): In superior interventricular septum; only electrical connection between atria and ventricles.

4. Right and Left Bundle Branches: Two pathways in interventricular septum carrying impulses toward apex.

5. Subendocardial Conducting Network (Purkinje Fibers): More elaborate on left side; ventricular contraction follows from apex toward atria. Entire process takes ~0.22 seconds.

Homeostatic Imbalances of Conduction

• Arrhythmias: Irregular heart rhythms.

• Fibrillation: Rapid, irregular contractions; heart becomes ineffective as a pump, risking brain death. Treated by defibrillation (electrical shock to reset rhythm).

• Heart Block: Defective AV node prevents impulses from reaching ventricles; treated with artificial pacemaker.

Electrocardiography (ECG/EKG)

Electrocardiography detects electrical currents generated by the heart, producing a composite recording of all action potentials (ECG/EKG).

• Electrodes placed on the body measure voltage differences; a 12-lead ECG is standard.

• Main features:

  • P wave: Depolarization of SA node and atria.

  • QRS complex: Ventricular depolarization and atrial repolarization.

  • T wave: Ventricular repolarization.

  • PR interval: Beginning of atrial excitation to beginning of ventricular excitation.

  • ST segment: Entire ventricular myocardium depolarized.

  • QT interval: Beginning of ventricular depolarization through ventricular repolarization.

Table: Main Features of ECG

Feature	Event
P wave	Atrial depolarization
QRS complex	Ventricular depolarization, atrial repolarization
T wave	Ventricular repolarization
PR interval	Atrial to ventricular excitation
ST segment	Ventricular myocardium depolarized
QT interval	Ventricular depolarization to repolarization

Homeostatic Imbalances Detected by ECG

• Enlarged R wave may indicate enlarged ventricles.

• Elevated or depressed ST segment indicates cardiac ischemia.

• Prolonged QT interval suggests repolarization abnormality, increasing risk of arrhythmias.

• Junctional blocks, flutters, and fibrillations are also detectable.

Mechanical Events of the Heart

• Systole: Period of heart contraction.

• Diastole: Period of heart relaxation.

• Cardiac Cycle: Blood flow through the heart during one complete heartbeat, including atrial and ventricular systole and diastole.

• Mechanical events follow electrical events seen on ECG.

Phases of the Cardiac Cycle (Left Side)

1. Ventricular Filling: Pressure is low; 80% of blood flows passively from atria to ventricles (AV valves open, SL valves closed). Atrial systole (P wave) pushes remaining 20% into ventricle.

2. Ventricular Systole: Atria relax, ventricles contract. Rising ventricular pressure closes AV valves; when pressure exceeds that in arteries, SL valves open and blood is ejected.

3. Isovolumetric Relaxation: Following ventricular repolarization (T wave), ventricles relax. All valves are closed; when atrial pressure exceeds ventricular pressure, AV valves open and the cycle repeats.

Regulation of Heart Rate

Heart rate is regulated by autonomic nervous system, chemical factors, and other influences.

• Autonomic Nervous System:

  • Sympathetic Activation: Emotional or physical stressors activate sympathetic nerves, releasing norepinephrine, which increases heart rate and contractility.

  • Parasympathetic Activation: Acetylcholine hyperpolarizes pacemaker cells, decreasing heart rate.

• Chemical Regulation:

  • Hormones: Epinephrine (from adrenal medulla) and thyroxine increase heart rate and contractility.

  • Ions: Proper intra- and extracellular concentrations of Ca2+ and K+ are essential; imbalances can be dangerous.

• Other Factors:

  • Age: Fetus has fastest HR; declines with age.

  • Gender: Females have faster HR than males.

  • Exercise: Increases HR; trained athletes may have lower resting HR.

  • Body Temperature: HR increases with increased temperature.

Key Equations

• Cardiac Output (CO): The amount of blood pumped by each ventricle in one minute.

  • $CO = HR \times SV$

  • Where $HR$ is heart rate (beats per minute) and $SV$ is stroke volume (mL per beat).

Example: If a person has a heart rate of 70 bpm and a stroke volume of 70 mL/beat, their cardiac output is $CO = 70 \times 70 = 4900$ mL/min or 4.9 L/min.

Additional info: The above notes expand on the original outline by providing definitions, context, and examples for each major concept, ensuring a self-contained and comprehensive study guide for exam preparation.