the heart

Heart Anatomy

  • Size: Approximately the size of a fist.

  • Location: Situated within the mediastinum, specifically between the second rib and the fifth intercostal space.

  • Position: Located on the superior surface of the diaphragm.

  • Orientation:

    • Two-thirds of the heart lies to the left of the midsternal line.

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

  • Base: The posterior surface of the heart, which leans toward the right shoulder.

  • Apex: Points toward the left hip.

  • Apical Impulse: Can be palpated between the fifth and sixth ribs.

Layers of the Heart Wall

  • Pericardium: A double-walled sac surrounding the heart.

    • Superficial Fibrous Pericardium: Protects the heart, anchors it to surrounding structures, and prevents overfilling with blood.

    • Deep Two-Layered Serous Pericardium:

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

      • Visceral Layer (Epicardium): Located on the external surface of the heart.

      • Pericardial Cavity: The fluid-filled space between the parietal and visceral layers that decreases friction during rhythmic movement.

  • Endocardium: Lines the heart chambers and covers the cardiac skeleton of the valves.

  • Myocardium: Composed of spiral bundles of contractile cardiac muscle cells.

  • Cardiac Skeleton: A crisscrossing, interlacing layer of connective tissue that:

    • Anchors cardiac muscle fibers.

    • Supports the great vessels and valves.

Atria: The Receiving Chambers

  • Auricles: Appendages that serve to increase atrial volume.

  • Right Atrium: Receives blood from three specific veins:

    1. Superior vena cava.

    2. Inferior vena cava.

    3. Coronary sinus.

  • Left Atrium: Receives blood from four pulmonary veins.

  • Interatrial Septum: The wall that separates the right and left atria.

  • Fossa Ovalis: A shallow depression in the interatrial septum; it is the remnant of the foramen ovale of the fetal heart.

Ventricles: The Discharging Chambers

  • Function: Act as the actual pumps of the heart.

  • Right Ventricle: Comprises most of the anterior surface and pumps blood into the pulmonary trunk.

  • Left Ventricle: Comprises the posteroinferior surface and pumps blood into the aorta, the largest artery in the body.

  • Interventricular Septum: The wall that separates the ventricles.

  • Structural Details:

    • Trabeculae Carneae: Irregular ridges of muscle located on the internal walls of the ventricles.

    • Papillary Muscles: Project into the ventricular cavity and anchor the chordae tendineae.

Heart Valves

  • General Function: Ensure unidirectional blood flow through the heart, opening and closing in response to pressure changes.

  • Atrioventricular (AV) Valves: Prevent backflow into the atria when ventricles contract.

    • Tricuspid Valve: The right AV valve.

    • Mitral Valve (Bicuspid Valve): The left AV valve.

    • Chordae Tendineae: Cords that anchor valve cusps to the papillary muscles, holding valve flaps in the closed position.

  • Semilunar (SL) Valves: Prevent backflow into the ventricles from the great arteries.

    • Aortic Semilunar Valve.

    • Pulmonary Semilunar Valve.

Pathway of Blood Through the Heart

  • Pulmonary Circuit:

    • Right atrium $\rightarrow$ tricuspid valve $\rightarrow$ right ventricle.

    • Right ventricle $\rightarrow$ pulmonary semilunar valve $\rightarrow$ pulmonary trunk $\rightarrow$ pulmonary arteries $\rightarrow$ lungs.

  • Systemic Circuit:

    • Lungs $\rightarrow$ pulmonary veins $\rightarrow$ left atrium.

    • Left atrium $\rightarrow$ mitral valve $\rightarrow$ left ventricle.

    • Left ventricle $\rightarrow$ aortic semilunar valve $\rightarrow$ aorta.

    • Aorta $\rightarrow$ systemic circulation.

Coronary Circulation

  • Coronary Arteries: Arise from the base of the aorta.

    • Left Coronary Artery (LCA): Branches into the anterior interventricular artery and the circumflex artery. Supplies the interventricular septum, anterior ventricular walls, left atrium, and posterior wall of the left ventricle.

    • Right Coronary Artery (RCA): Branches into the right marginal artery and the posterior interventricular artery. Supplies the right atrium and most of the right ventricle.

  • Coronary Veins: Collect blood from capillary beds.

    • Coronary Sinus: Empties into the right atrium; it is formed by merging cardiac veins including:

      • Great cardiac vein (in the anterior interventricular sulcus).

      • Middle cardiac vein (in the posterior interventricular sulcus).

      • Small cardiac vein (from the inferior margin).

    • Anterior Cardiac Veins: Several veins that empty directly into the right atrium anteriorly.

Microscopic Anatomy of Cardiac Muscle

  • Intercalated Discs: Junctions between cells that anchor cardiac cells together.

  • Desmosomes: Prevent cells from separating during the force of contraction.

  • Gap Junctions: Allow ions to pass from cell to cell, electrically coupling adjacent cells.

  • Functional Syncytium: The arrangement allows the heart to behave as a single coordinated unit.

Cardiac Muscle Contraction

  • Similarities with Skeletal Muscle:

    • Depolarization opens voltage-gated fast Na+Na^+ channels in the sarcolemma.

    • Membrane potential reverses from −90 mV-90\,mV to +30 mV+30\,mV. These channels close rapidly.

    • The depolarization wave travels down T tubules to the Sarcoplasmic Reticulum (SR), releasing Ca2+Ca^{2+}.

    • Excitation-contraction coupling occurs; Ca2+Ca^{2+} binds to troponin, causing filaments to slide.

  • Differences from Skeletal Muscle:

    • Cardiac muscle does not require nervous system stimulation to contract.

    • It can depolarize the entire heart; cardiomyocytes contract as a unit or not at all.

    • It has a long absolute refractory period of 250 ms250\,ms to prevent tetanic contractions.

Pacemaker (Autorhythmic) Cells

  • Electrical Properties: Possess unstable resting membrane potentials (known as pacemaker potentials or prepotentials) due to the opening of slow Na+Na^+ channels.

  • Action Potential Initiation:

    1. Pacemaker Potential: Repolarization closes K+K^+ channels and opens slow Na+Na^+ channels, leading to an ion imbalance.

    2. Depolarization: At threshold, Ca2+Ca^{2+} channels open, resulting in an explosive influx of Ca2+Ca^{2+} that produces the rising phase of the action potential.

    3. Repolarization: Inactivation of Ca2+Ca^{2+} channels and opening of voltage-gated K+K^+ channels, leading to an efflux of K+K^+.

Sequence of Excitation

  • Overview: Pacemaker cells pass impulses across the heart in approximately 220 ms220\,ms.

  • 1. Sinoatrial (SA) Node:

    • The pacemaker of the heart, located in the right atrial wall.

    • Depolarizes faster than the rest of the myocardium.

    • Generates impulses at roughly 75/minute75/\text{minute} (sinus rhythm).

    • Inherent rate is 100/minute100/\text{minute}, but tempered by extrinsic factors.

  • 2. Atrioventricular (AV) Node:

    • Located in the inferior interatrial septum.

    • Delays impulses by approximately 0.1 second0.1\,\text{second} due to smaller fiber diameters and fewer gap junctions.

    • Delay allow atria to contract before the ventricles.

    • Inherent rate of 50/minute50/\text{minute} without SA node input.

  • 3. Atrioventricular (AV) Bundle (Bundle of His):

    • Located in the superior interventricular septum.

    • The only electrical connection between the atria and ventricles (they are not connected by gap junctions).

  • 4. Right and Left Bundle Branches: Extend through the interventricular septum.

  • 5. Subendocardial Conducting Network (Purkinje Fibers):

    • Completes the pathway into the apex and ventricular walls.

    • More elaborate on the left side of the heart.

    • In the absence of AV node input, the bundle and network depolarize at 30/minute30/\text{minute}.

  • Result: Ventricular contraction immediately follows, proceeding from the apex toward the atria.

Extrinsic Innervation of the Heart

  • Regulation: The heartbeat is modified by the Autonomic Nervous System (ANS) via cardiac centers in the medulla oblongata.

  • Sympathetic Stimulation: Increases heart rate and force of contraction.

  • Parasympathetic Stimulation: Decreases heart rate.

  • Cardioacceleratory Center: Sympathetic; affects SA and AV nodes, heart muscle, and coronary arteries.

  • Cardioinhibitory Center: Parasympathetic; inhibits SA and AV nodes via the vagus nerves.

Electrocardiography (ECG/EKG)

  • Definition: A composite of all action potentials generated by nodal and contractile cells at a given time.

  • Waves:

    • P wave: Atrial depolarization initiated by the SA node.

    • QRS complex: Ventricular depolarization and atrial repolarization.

    • T wave: Ventricular repolarization.

Heart Sounds

  • Lub-Dupp: Two distinct sounds associated with valve closure.

    • First Sound (Lubb): Occurs as AV valves close; signifies the beginning of systole.

    • Second Sound (Dupp): Occurs as SL valves close; signifies the beginning of ventricular diastole.

  • Pause: Indicates period of heart relaxation.

  • Heart Murmurs: Abnormal sounds indicating incompetent or stenotic valves.

  • Auscultation Regions:

    • Aortic Valve: Second intercostal space at the right sternal margin.

    • Pulmonary Valve: Second intercostal space at the left sternal margin.

    • Mitral Valve: Over the apex (fifth intercostal space) in line with the middle of the clavicle.

    • Tricuspid Valve: Right sternal margin of the fifth intercostal space.

Mechanical Events: The Cardiac Cycle

  • Definitions:

    • Systole: Contraction.

    • Diastole: Relaxation.

  • Phases of the Cycle:

    1. Ventricular Filling (mid-to-late diastole): Pressure is low; AV valves are open. 80%80\% of blood flows passively into ventricles. Atrial systole occurs, delivering the remaining 20%20\%.

      • End Diastolic Volume (EDV): Volume of blood in each ventricle at the end of ventricular diastole.

    2. Ventricular Systole: Atria relax; ventricles contract.

      • Isovolumetric Contraction Phase: Rising pressure causes AV valves to close; for a brief moment, all valves are closed.

      • Ejection Phase: Ventricular pressure exceeds pressure in large arteries, forcing SL valves open.

      • End Systolic Volume (ESV): Volume of blood remaining in each ventricle after systole.

    3. Isovolumetric Relaxation (early diastole): Ventricles and atria relax.

      • Backflow in the aorta and pulmonary trunk closes SL valves.

      • Dicrotic Notch: A brief rise in aortic pressure as blood rebounds off the closed SL valve.

      • When atrial pressure eventually exceeds ventricular pressure, AV valves open, and the cycle repeats.

Cardiac Output (CO)

  • Definition: Volume of blood pumped by each ventricle in one minute.

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

    • HR (Heart Rate): Number of beats per minute.

    • SV (Stroke Volume): Volume of blood pumped out by one ventricle with each beat.

  • Normal Resting Values:

    • CO=75 beats/min×70 mL/beat=5.25 L/minCO = 75\,\text{beats/min} \times 70\,mL/\text{beat} = 5.25\,L/\text{min}

  • Cardiac Reserve: The difference between resting CO and maximal CO.

Regulation of Stroke Volume

  • Formula: SV=EDV−ESVSV = EDV - ESV

  • Influencing Factors:

    • Preload: The degree of stretch of cardiac muscle cells before they contract.

      • Frank-Starling Law of the Heart: The force of heart contraction is directly proportional to the initial length of the muscle fiber. Greater stretch (preload) increases the force of contraction, thereby increasing SV.

      • Venous Return: Most important factor in stretching muscle; increased by slow heartbeat and exercise.

    • Contractility: Contractile strength at a given muscle length.

    • Afterload: The pressure the ventricles must overcome to eject blood (arterial BP).

      • Hypertension increases afterload, leading to increased ESV and reduced SV.

Autonomic and Physical Influences

  • Sympathetic Nervous System: Activated by stressors. Norepinephrine binds to β1-adrenergic\beta_1\text{-adrenergic} receptors, increasing HR and contractility.

  • Parasympathetic Nervous System: Opposes sympathetic effects. Acetylcholine hyperpolarizes pacemaker cells by opening K+K^+ channels, slowing HR. The heart at rest exhibits vagal tone.

  • Hormones: Epinephrine and thyroxine increase heart rate.

  • Demographics and Environment:

    • Age: Fetuses have the fastest HR.

    • Gender: Females generally have faster HR than males.

    • Exercise: Increases HR.

    • Temperature: HR increases with increased body temperature.

Homeostatic Imbalances

  • Electrolyte Imbalances:

    • Hypocalcemia: Depresses the heart.

    • Hypercalcemia: Increases HR and contractility.

    • Hyperkalemia: Alters electrical activity; potential for heart block and cardiac arrest.

    • Hypokalemia: Results in a feeble heartbeat and arrhythmias.

  • Heart Rate Imbalances:

    • Tachycardia: Abnormally fast HR (>100\,\text{beats/min}); can lead to fibrillation.

    • Bradycardia: HR slower than 60 beats/min60\,\text{beats/min}; may cause inadequate circulation in non-athletes, but is common in endurance athletes.

  • Congestive Heart Failure (CHF): CO is so low that blood circulation is inadequate for tissue needs. Underlying causes include:

    • Coronary atherosclerosis (clogged arteries).

    • Persistent high blood pressure.

    • Multiple myocardial infarcts.

    • Dilated cardiomyopathy (DCM).