The Heart

An Introduction to the Heart and Cardiovascular System

• The Cardiovascular System Components:

  • Heart.

  • Blood.

  • Blood vessels.

• Heart Performance Statistics:

  • The heart beats approximately $100,000$ times each day.

  • It pumps about $8000$ liters of blood per day.

Anatomy and Circuits of the Heart

• Pulmonary Circuit:

  • Function: Carries blood to and from gas exchange surfaces of the lungs.

  • Origin: Begins at the Right Ventricle.

  • Termination: Ends at the Left Atrium.

• Systemic Circuit:

  • Function: Carries blood to and from the rest of the body.

  • Origin: Begins at the Left Ventricle.

  • Termination: Ends at the Right Atrium.

• Sequential Travel: Blood travels through these circuits in sequence; each circuit begins and ends at the heart.

• Four Chambers of the Heart:

  • Right Atrium: Receives blood from the systemic circuit.

  • Right Ventricle: Pumps blood into the pulmonary circuit.

  • Left Atrium: Receives blood from the pulmonary circuit.

  • Left Ventricle: Pumps blood into the systemic circuit.

• Gross Structural Features:

  • Base: The superior portion where the great vessels connect.

  • Apex: The pointed inferior tip.

  • Mediastinum: The region between the two pleural cavities where the heart sits.

The Heart Wall and Pericardium

• The Pericardium:

  • Fibrous Pericardium: The outer dense fibrous layer.

  • Serous Pericardium: The inner double-layered membrane.

    • Parietal Layer: The outer layer of the serous pericardium.

    • Visceral Layer (Epicardium): The inner layer that covers the heart surface.

  • Pericardial Cavity: Located between the parietal and visceral layers; contains pericardial fluid.

• Layers of the Heart Wall:

  • Epicardium (Visceral layer of serous pericardium): Consists of mesothelium and areolar tissue.

  • Myocardium: The middle layer containing cardiac muscle cells, connective tissues, blood vessels, and nerves.

  • Endocardium: The inner lining of the heart, consisting of endothelium and areolar tissue.

• Clinical Conditions of the Pericardium:

  • Pericarditis: Caused by pathogens in the pericardium; inflamed surfaces rub together, producing a distinctive scratching sound.

  • Cardiac Tamponade: Restricted heart movement due to excess fluid in the pericardial cavity.

Internal Anatomy and Heart Valves

• Muscular Septa:

  • Interatrial Septum: Separates the two atria.

  • Interventricular Septum: Separates the two ventricles; significantly thicker than the interatrial septum.

• Atrioventricular (AV) Valves:

  • Located between the atria and ventricles (Tricuspid and Mitral valves).

  • Permit blood flow in one direction only.

  • When ventricles contract, blood pressure closes the valves.

  • Papillary Muscles: Contract to tense the chordae tendineae, preventing regurgitation (backflow) into the atria.

  • Mitral Valve Prolapse (MVP): Stretchy valve leaflets bulge backward into the left atrium during contraction.

• Semilunar Valves:

  • Includes Pulmonary and Aortic valves.

  • Prevent backflow of blood into the ventricles.

  • Do not have muscular braces (no papillary muscles/chordae tendineae).

  • Valvular Heart Disease (VHD): Deterioration of valve function; may follow carditis or rheumatic fever (autoimmune response to streptococcal bacteria).

Detailed Blood Flow and Chamber Characteristics

• Right Atrium Specifics:

  • Receives blood from the Superior Vena Cava (head, neck, upper limbs, chest), Inferior Vena Cava (trunk, viscera, lower limbs), and the Coronary Sinus (venous blood from the heart).

  • Foramen Ovale: An opening in the interatrial septum before birth; connects fetal atria; closes at birth to become the fossa ovalis.

  • Pectinate Muscles: Muscular ridges on the anterior atrial wall and auricle.

• Right Ventricle Specifics:

  • Blood flows through the Tricuspid Valve (three cusps).

  • Contains the Moderator Band.

  • The Conus Arteriosus is the superior end which leads to the Pulmonary Valve (three semilunar cusps).

  • The pulmonary valve leads to the Pulmonary Trunk, which divides into left and right pulmonary arteries.

• Left Atrium Specifics:

  • Receives blood from left and right pulmonary veins.

  • Blood flows through order the Mitral Valve (Bicuspid valve, two cusps) into the left ventricle.

• Left Ventricle Specifics:

  • Lacks a moderator band.

  • Pumps blood through the Aortic Valve into the Ascending Aorta.

  • Aortic Sinuses: Saclike expansions at the base of the ascending aorta.

  • The ascending aorta transitions into the Aortic Arch and then the Descending Aorta.

• Comparison of Ventricles:

  • The Right Ventricle has thinner walls, develops less pressure, and is pouch-shaped.

  • The Left Ventricle has thicker walls and is round in cross-section to generate high pressure.

  • Both ventricles hold and pump the same amount of blood.

Coronary Circulation and Heart Disease

• Coronary Arteries:

  • Originate at aortic sinuses.

  • Flow is maintained by elevated blood pressure and the elastic rebound of the aorta.

  • Right Coronary Artery Branches: Atrial arteries, Marginal arteries, Posterior interventricular artery.

  • Left Coronary Artery Branches: Circumflex artery, Anterior interventricular artery.

• Coronary Veins:

  • Great cardiac vein, Small cardiac vein, Middle cardiac vein, Posterior vein of left ventricle.

  • All drain into the Coronary Sinus.

• Coronary Artery Disease (CAD):

  • Areas of partial or complete blockage of coronary circulation.

  • Coronary Ischemia: Reduced circulatory supply due to blockage.

  • Angina Pectoris: Temporary ischemia during exertion; causes chest constriction and pain radiating to arms, back, and neck.

• Myocardial Infarction (MI):

  • Heart attack; blockage leads to cell death from lack of oxygen.

  • Creates a nonfunctional area known as an infarct.

  • Coronary Thrombosis: Thrombus formation at a plaque; the most common cause of MI.

The Conducting System and Electrical Activity

• Cardiac Cells:

  • Autorhythmic Cells: Control and coordinate the heartbeat.

  • Contractile Cells: Produce the force to propel blood.

• Components of the Conducting System:

  • Sinoatrial (SA) Node: Located in the wall of the right atrium; the primary pacemaker.

  • Atrioventricular (AV) Node: Located at the junction of atria and ventricles.

  • Conducting Cells: Internodal pathways, AV bundle, bundle branches, and Purkinje fibers.

• Pacemaker Potentials:

  • Gradual depolarization; no stable resting membrane potential.

  • SA Node Rate: $60-100$ action potentials per minute.

  • AV Node Rate: $40-60$ action potentials per minute.

  • Sinus Rhythm: Established by the SA node.

• Electrocardiogram (ECG/EKG):

  • P Wave: Atrial depolarization.

  • P-R Interval: Period including conduction through AV node and AV bundle ($Elapsed time = 150\,msec$).

  • Q Wave: Beginning of ventricular depolarization ($Elapsed time = 175\,msec$).

  • QRS Complex: Completion of ventricular depolarization ($Elapsed time = 225\,msec$).

• Heart Rhythm Disturbances:

  • Bradycardia: Abnormally slow heart rate.

  • Tachycardia: Abnormally fast heart rate.

  • Ectopic Pacemaker: Abnormal cells generate high rates of action potentials, bypassing the normal system.

Cardiac Contractile Cells and Action Potentials

• Cell Characteristics:

  • Small size, single central nucleus.

  • Intercalated Discs: Linked by desmosomes (force transfer) and gap junctions (electrical propagation).

  • Resting Membrane Potentials: Ventricular cells are approximately $-90\,mV$; atrial cells are approximately $-80\,mV$.

• Action Potential Phases (Ventricular):

  1. Rapid Depolarization: Caused by $Na^+$ entry; lasts $3-5\,msec$; ends with closure of voltage-gated fast sodium channels.

  2. The Plateau: Caused by $Ca^{2+}$ entry; lasts approximately $175\,msec$; ends with closure of slow calcium channels.

  3. Repolarization: Caused by $K^+$ loss; lasts $75\,msec$; ends with closure of slow potassium channels.

• Refractory Periods:

  • Absolute Refractory Period: $200\,msec$; no response permitted.

  • Relative Refractory Period: $50\,msec$; response only to strong stimuli.

  • Total duration is $250-300\,msec$, which prevents summation and tetany.

The Cardiac Cycle

• Definition: From the start of one heartbeat to the beginning of the next.

• Phases:

  • Systole: Contraction phase; pressure rises.

  • Diastole: Relaxation phase; pressure falls.

• Cycle Timing: At $75\,bpm$, the cycle lasts $800\,msec$. Increased heart rate shortens all phases, primarily diastole.

• Step-by-Step Events:

  1. Atrial Systole: Atria contract, forcing blood into ventricles.

  2. Atrial Diastole: Begins as atrial systole ends.

  3. Ventricular Systole (First Phase): Isovolumetric contraction; AV valves close, but pressure is insufficient to open semilunar valves.

  4. Ventricular Systole (Second Phase): Ventricular ejection; semilunar valves open.

  5. Ventricular Diastole (Early): Isovolumetric relaxation; semilunar valves close.

  6. Ventricular Diastole (Late): Passively filling ventricles as AV valves open.

• Pressure and Volume:

  • End-Diastolic Volume (EDV): Maximum blood volume in ventricles at the end of atrial systole.

  • End-Systolic Volume (ESV): Blood remaining in ventricles after ejection; approx. $40\%$ of EDV.

  • Stroke Volume (SV): Amount of blood ejected ($SV = EDV - ESV$).

• Heart Sounds:

  • S1 (Lubb): Produced by closing AV valves.

  • S2 (Dupp): Produced by closing semilunar valves.

  • S3 & S4: Soft sounds associated with blood flow into ventricles and atrial contraction.

  • Murmur: Sound produced by valve regurgitation.

Cardiac Output and Regulation

• Formula: $CO = HR \times SV$

  • $CO$: Cardiac output ($mL/min$).

  • $HR$: Heart rate ($beats/min$).

  • $SV$: Stroke volume ($mL/beat$).

• Factors Affecting Heart Rate:

  • Autonomic Innervation:

    • Cardioacceleratory Center: (Medulla) Sympathetic neurons increase HR.

    • Cardioinhibitory Center: (Medulla) Parasympathetic neurons (Vagus nerve, CN X) decrease HR.

  • Hormones: Epinephrine (E), Norepinephrine (NE), and Thyroid hormone ($T_3$).

  • Bainbridge Reflex (Atrial Reflex): Increase in venous return triggers stretch receptors in the right atrium, increasing HR via sympathetic activity.

• Factors Affecting Stroke Volume:

  • Preload: Degree of ventricular stretching during diastole; directly proportional to EDV.

  • Frank-Starling Principle: As EDV increases, stroke volume increases.

  • Contractility: Force produced during contraction at a given preload. Increased by sympathetic stimulation (NE, E) and glucagon; decreased by parasympathetic stimulation (ACh).

  • Afterload: Tension the ventricle must produce to open semilunar valves. Increased by factors restricting blood flow. As afterload increases, stroke volume decreases.

• Clinical Ranges: Average resting cardiac output is around $5\,L/min$. Trained athletes can reach a maximum of $35-40\,L/min$ during peak exercise.