Introduction to the Heart and Cardiovascular System

Introduction to the Cardiovascular System

  • Cardiovascular System Components:

    • The heart.

    • Blood.

    • Blood vessels.

  • The Heart - General Statistics:

    • Beats approximately 100,000100,000 times each day.

    • Pumping approximately 8000 dm38000\,dm^3 of blood per day.

  • Circulatory Pathways:

    • Pulmonary Circuit:

      • Moves blood between the heart and lungs.

      • Function: To remove carbon dioxide and pick up oxygen from the lungs.

    • Systemic Circuit:

      • Moves blood between the heart and all of the body tissues.

      • Function: Delivers oxygen and nutrients and removes wastes.

  • Types of Blood Vessels:

    • Arteries: Carry blood away from the heart.

    • Veins: Return blood to the heart.

    • Capillaries (Exchange Vessels):

      • Interconnect the smallest arteries and the smallest veins.

      • Function: Facilitate the exchange of dissolved gases, nutrients, and wastes between the blood and surrounding tissues.

  • The Four Chambers of the Heart:

    • Right Atrium: Receives deoxygenated blood from the systemic circuit.

    • Right Ventricle: Pumps deoxygenated blood into the pulmonary circuit.

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

    • Left Ventricle: Pumps oxygenated blood into the systemic circuit.

External Anatomy and the Heart Wall

  • Location of the Heart:

    • The heart is shaped like an upside-down triangle.

    • It is positioned posterior to the sternum.

    • The base (superior portion) is where the great vessels connect.

    • The apex (inferior pointed tip) is located at the level of the 5th intercostal space.

    • The heart sits between the two pleural cavities in the mediastinum.

  • The Pericardium:

    • The membrane which encloses the heart.

    • Fibrous Pericardium: Tough connective tissue that attaches the heart to adjacent structures in the chest.

    • Serous Pericardium: A thin serous membrane forming a double-layered sac.

      • Visceral Pericardium (Epicardium): The serous lining directly on the heart.

      • Parietal Pericardium: The serous membrane lining the pericardial cavity.

    • Pericardial Cavity: The space between the visceral and parietal pericardium; it contains serous fluid to reduce friction.

  • The Heart Wall Layers:

    • Epicardium (Outer Layer): Is the visceral layer of the serous pericardium that covers the surface of the heart.

    • Myocardium (Middle Layer): The muscular wall of the heart, consisting of concentric layers of cardiac muscle tissue.

    • Endocardium (Inner Layer): Lined by simple squamous epithelium (endothelium).

  • Superficial Anatomy:

    • Base: Superior aspect of the heart; the location of the "Great Vessels":

      • Superior vena cava.

      • Inferior vena cava.

      • Aorta.

      • Pulmonary trunk.

      • Pulmonary arteries.

      • Pulmonary veins.

    • Apex: The tip of the heart located between the 4th and 5th rib space.

    • Atria: Two upper, thin-walled chambers. Features the Auricle (atrial appendage), an expandable pouch located medially.

    • Ventricles: Two lower chambers.

    • Sulci (Grooves): These contain fat and blood vessels.

      • Coronary Sulcus: Divides the atria and ventricles.

      • Anterior and Posterior Interventricular Sulci: Separate the left and right ventricles and contain blood vessels supplying the cardiac muscle.

Internal Anatomy of the Heart

  • Internal Septa:

    • Interatrial Septum: Consists of fibrous tissue and separates the atria.

    • Interventricular Septum: Consists of cardiac muscle and separates the ventricles.

  • Right Atrium Inflow:

    • Superior Vena Cava: Receives blood from the head, neck, upper limbs, and chest.

    • Inferior Vena Cava: Carries blood from the trunk, viscera, and lower limbs.

    • Coronary Sinus: Cardiac veins return blood to this sinus, which opens into the right atrium.

  • Specific Structures of the Right Atrium:

    • Foramen Ovale: An opening through the interatrial septum before birth connecting the fetal atria; it closes at birth to form the fossa ovalis.

    • Pectinate Muscles: Prominent muscular ridges on the anterior atrial wall and inner surface of the auricle.

  • Right Ventricle Construction:

    • Tricuspid Valve (Right AV Valve): Located between the right atrium and right ventricle, featuring three cusps. Free edges attach to chordae tendineae, which are anchors from the papillary muscles.

    • Function: Prevents backflow from the right ventricle into the right atrium.

    • Trabeculae Carneae: Muscular ridges on the internal surface of both ventricles.

    • Moderator Band: A muscular ridge that delivers electrical signals to the papillary muscles.

  • Pulmonary Circuit Flow (from RA):

    • Deoxygenated blood moves through the conus arteriosus (superior end of the right ventricle).

    • Passes through the Pulmonary (semilunar) valve.

    • Enters the Pulmonary trunk.

    • Divides into the Pulmonary arteries, delivering blood to the lungs for gas exchange with alveoli.

  • Left Atrium and Left Ventricle:

    • Blood enters the Left Atrium via the left and right Pulmonary veins.

    • Mitral Valve (Left AV Valve/Bicuspid Valve): Located between the left atrium and left ventricle; has two cusps attached to chordae tendineae and papillary muscles. Prevents backflow into the left atrium.

    • Aortic (semilunar) Valve: Oxygenated blood leaves the left ventricle through this valve to enter the Aorta.

Coronary Circulation

  • Overview: Supplies blood to the heart's muscle tissue; includes arteries and veins emerging from the ascending aorta.

  • Right Coronary Artery (RCA):

    • Supplies the right atrium, portions of both ventricles, and the electrical conducting system.

    • Gives rise to Marginal arteries and the Posterior interventricular artery.

  • Left Coronary Artery (LCA):

    • Supplies the left ventricle, left atrium, and interventricular septum.

    • Gives rise to the Circumflex artery and the Anterior interventricular artery (also known as the Left Anterior Descending or LAD).

  • Arterial Anastomoses: Interconnect the anterior and posterior interventricular arteries to maintain blood supply to cardiac muscle.

  • Cardiac Veins:

    • Anterior Veins: Great cardiac vein, anterior cardiac veins, and small cardiac vein.

    • Posterior Veins: Posterior cardiac vein, middle cardiac vein, and small cardiac vein.

    • Coronary Sinus: Most cardiac veins drain here; it empties into the right atrium.

    • Exception: Anterior cardiac veins drain directly into the right atrium.

Cardiac Conduction System

  • Definition: A specialized network of modified cardiac muscle cells responsible for generating and conducting electrical impulses.

  • Primary Purposes:

    • Ensure atria contract first and ventricles contract second.

    • Maintain regular, rhythmic heartbeat and cardiac output.

  • Specific Components and Flow:

    • Sinoatrial (SA) Node: Located in the superior portion of the right atrium. Known as the primary pacemaker. Fires at 60–10060\text{--}100 action potentials per minute at rest.

    • Internodal Pathways: Carry impulses from SA to AV node. Includes the Bachmann’s Bundle (branch of the anterior pathway) which conducts to the left atrium for simultaneous contraction.

    • Atrioventricular (AV) Node: Located in the posterior-inferior right atrium. It delays the impulse by approximately 0.10 sec0.10\,sec to allow for complete atrial contraction and ventricular filling. If the SA node fails, the AV node can generate impulses at 40–60 beats/min40\text{--}60\,beats/min.

    • Atrioventricular Bundle (Bundle of His): Passes through the fibrous skeleton into the interventricular septum.

    • Right and Left Bundle Branches: Extend through the interventricular septum to their respective ventricles.

    • Purkinje Fibers: Rapidly distribute impulses throughout the ventricular myocardium for efficient systole. These conduct impulses faster than any other cardiac tissue.

Cardiac histology and Electrocardiography (ECG)

  • Cardiac Muscle Characteristics: Striated, involuntary, branched cells. They are connected by intercalated discs.

    • Desmosomes: Hold cells together.

    • Gap Junctions: Allow electrical communication for coordinated contraction.

  • Cell Types:

    • Autorhythmic Cells (Pacemaker Cells): SA node, AV node, and conducting system; set the rhythm.

    • Contractile Cells: Make up the majority of the myocardium; produce force.

  • ECG Waves:

    • P Wave: Atria depolarize; atrial contraction follows.

    • QRS Complex: Ventricles depolarize (strong signal); ventricular contraction follows.

    • T Wave: Ventricles repolarize.

    • Intervals/Segments: P-R interval, S-T segment, Q-T interval.

Cardiac Action Potential and Contraction

  • Automaticity: Cardiac muscle contracts automatically without neural or hormonal control.

  • Pacemaker Potential (Prepotential): SA and AV nodes do not have a stable resting potential. A slow influx of Na+Na^+ without K+K^+ outflow causes spontaneous firing. SA node defaults to 80–10080\text{--}100 times per minute.

  • Ventricular Contractile Cell Action Potential:

    • Duration is approximately 200–300 ms200\text{--}300\,ms.

    • Phase 1: Rapid Depolarization: Rapid Na+Na^+ influx (3–5 ms3\text{--}5\,ms).

    • Phase 2: The Plateau: Partial repolarization (175 ms175\,ms). Caused by Ca2+Ca^{2+} entry and some K+K^+ loss. Values plateau at approximately 0 mV0\,mV.

    • Phase 3: Repolarization: K+K^+ rapidly leaves the cell (75 ms75\,ms).

  • Refractory Periods:

    • Absolute Refractory Period: Long period where cells cannot respond; ensures heart relaxes to fill. Tetanic contraction would be fatal.

    • Relative Refractory Period: Short period where response depends on the stimulus degree.

  • Steps of Contraction:

    1. Action potential travels along sarcolemma.

    2. Ca2+Ca^{2+} channels open; extracellular Ca2+Ca^{2+} enters.

    3. Calcium-Induced Calcium Release (CICR): Extracellular Ca2+Ca^{2+} triggers the sarcoplasmic reticulum (SR) to release more Ca2+Ca^{2+}.

    4. Contraction occurs.

    5. Relaxation occurs when intracellular Ca2+Ca^{2+} is pumped back into the SR or out of the cell.

  • Energy: Aerobic breakdown of fatty acids and glucose; uses oxygen stored in myoglobin.

The Cardiac Cycle

  • Definition: All events associated with one complete heartbeat, lasting approximately 0.8 sec0.8\,sec at 75 beats/min75\,beats/min.

  • Phases:

    • Systole: Contraction phase; pressure rises.

    • Diastole: Relaxation phase; pressure falls.

  • Step-by-Step Sequence:

    1. Atrial Systole: Atria eject blood into ventricles; ends with End-Diastolic Volume (EDV) (maximum blood volume).

    2. Ventricular Systole (First Phase): Isovolumetric contraction; pressure closes AV valves.

    3. Ventricular Systole (Second Phase): Ventricular ejection; pressure opens semilunar valves. Stroke Volume (SV) (approx. 60%60\% of EDV) is ejected.

    4. Ventricular Diastole (Early): Pressure drops, semilunar valves close. Ventricles contain End-Systolic Volume (ESV) (approx. 40%40\% of EDV).

    5. Ventricular Diastole (Late): Isovolumetric relaxation followed by passive filling as all chambers relax.

  • Heart Sounds:

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

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

Cardiac Output (CO) and Dynamics

  • Formula: CO=HR×SVCO = HR \times SV

    • COCO = cardiac output (cm3 min−1cm^3\,min^{-1}).

    • HRHR = heart rate (beats/minbeats/min).

    • SVSV = stroke volume (cm3/beatcm^3/beat).

  • Regulation of Heart Rate:

    • Autonomic Innervation:

      • Sympathetic: Medulla's Cardioacceleratory center. Releases Norepinephrine at the SA node to increase heart rate.

      • Parasympathetic: Medulla's Cardioinhibitory center. Vagus Nerve (CN X) releases Acetylcholine to decrease heart rate.

    • Atrial (Bainbridge) Reflex: Increased venous return stretches the right atrium, triggering sympathetic activity to increase heart rate.

    • Hormones: Heart rate is increased by Epinephrine (E), Norepinephrine (NE), and Thyroid hormone (T3T_3).

  • Regulation of Stroke Volume (SV=EDV−ESVSV = EDV - ESV):

    1. Preload: Degree of ventricular stretching during diastole. Governed by the Frank-Starling Law ("more in = more out"): increased EDV leads to increased SV.

    2. Contractility: Strength of contraction at a given preload.

      • Positive Inotropic Factors: Increase contractility (Sympathetic stim, Epinephrine, high Ca2+Ca^{2+}, Digoxin). Result: Decreased ESV, Increased SV.

      • Negative Inotropic Factors: Decrease contractility (Parasympathetic stim, Beta blockers, Ca2+Ca^{2+} channel blockers). Result: Increased ESV, Decreased SV.

    3. Afterload: Pressure the ventricles must overcome (arterial pressure). Increased afterload (e.g., hypertension) leads to increased ESV and reduced SV.

  • Exercise Effects:

    • Resting CO: Approximately 5 dm3 min−15\,dm^3\,min^{-1}.

    • Intense exercise: Up to 20–35 dm3 min−120\text{--}35\,dm^3\,min^{-1}.

    • Athletic conditioning: Results in larger SV and resting bradycardia (slow heart rate) because the high SV maintains efficient CO at lower rates.