Anatomy and Physiology: The Heart

• Love is romanticized in literature and movies and has historically been linked to the heart, which was once believed to be the vessel of the soul and center of emotions.
• The human heart is a pump responsible for circulating blood throughout the body.
• The average heart beats 60-100 times per minute, totaling around 600 million beats by age 20.
• Blood acts as a transport medium, moving through pulmonary and systemic circuits.
• In the pulmonary circuit, blood exchanges oxygen for carbon dioxide.
• In the systemic circuit, blood delivers oxygen and collects waste from tissues.
• The heart's contraction drives blood movement, while valves ensure one-way flow.
• The heart is located in the mediastinum, a central area in the thoracic cavity, in front of the aorta and esophagus.
• Great vessels like the ascending aorta and superior vena cava are in the superior mediastinum.
• The heart's apex points to the left, causing a notch in the left lung.
• The heart is anchored to the posterior thoracic wall by the great vessels.
• The oblique pericardial sinus is an upside-down pocket formed by these connections.
• The heart, especially the apex and ventricles, floats freely in the mediastinum to allow strong contractions.
• During development, the heart becomes enclosed by a membrane, creating parietal and visceral layers.
• The pericardial sac is composed of fused fibrous pericardium and serous pericardium layers.
• The epicardium is the external or visceral serous membrane layer of the heart.
• The pericardial space between the parietal and visceral layers has serous fluid for frictionless movement.
• The myocardium is the muscular middle layer responsible for forceful circulation.
• Myocardial fibers are arranged spirally, causing a squeezing effect for blood ejection.
• The endocardium is the inner simple squamous epithelial lining, continuous with blood vessel linings.
• The endocardium lines the heart valves, a common site for endocarditis (inflammation).
• The atrioventricular septum provides structural support and electrical insulation.
• Heart valves are modifications of connective tissue that ensure separation of chambers.
• The right side of the heart receives blood from systemic circulation and sends it to the lungs via the pulmonary circuit.
• The left side receives blood from the pulmonary circuit and sends it back to the body via systemic circulation.
• Atria receive and collect blood and then contract to prime the ventricles.
• Ventricles pump blood into systemic and pulmonary circulation.
• The interatrial and interventricular septum separates the left and right sides of the heart.
• The right atrium receives blood from the superior vena cava, inferior vena cava, and coronary sinus.
• Pectinate muscle is a network of myocardial muscle on the anterior wall of the right atrium.
• The Oracle is a fold in the superior anterior wall of the right atrium, providing extra space.
• The crista terminalis divides the anterior and posterior walls inside the right atrium.
• The fossa ovalis is a thin spot in the interatrial septum, marking the fetal foramen ovale.
• The right ventricle is thicker than the atrium, featuring muscular networks called trabeculae carnae.
• Papillary muscles in the right ventricle anchor the tricuspid valve cusps via chordae tendineae.
• The infundibulum (or conus arteriosus) is a region in the right ventricle leading into the pulmonary trunk.
• The semilunar pulmonary valve prevents backflow from the pulmonary trunk.
• Reoxygenated blood returns from the lungs into the left atrium via pulmonary veins.
• The left atrium is similar to the right but has a thicker wall, lower chamber volume, and no crista terminalis.
• Trabeculae carneae, papillary muscles, and chordae tendineae are similar to those in the right ventricle.
• The left ventricle is thicker than the right, due to pumping against higher systemic pressure.
• The left atrioventricular valve (bicuspid or mitral valve) has two cusps.
• Atrioventricular valves (cuspid) prevent backflow from the ventricles to the atria.
• Semilunar valves at the base of the pulmonary trunk and aorta prevent backflow into the ventricles.
• Cuspid valves are extensions of the heart's fibrous skeleton, lined with endocardium.
• Chordae tendineae anchor cusps to papillary muscles on the ventricular walls.
• In diastole, blood flows through cuspid valves into the ventricles.
• During ventricular contraction, cusps tethered to the ventricular wall through tendinous cords prevent the cusps from collapsing back into the atria.
• Semilunar valves have a half-moon appearance and originate from the fibrous skeleton.
• Semilunar valves lack tendinous attachments but have nodules (thickenings) within the connective tissue.
• Semilunar valves have curved attachment with the convex surface towards the ventricles.
• Semilunar valves function as three pockets, preventing backflow by bulging outward when filled with blood.
• Both cuspid and semilunar valves function as one-way valves controlling blood flow.
• Cuspid valves open during diastole, allowing flow from atria to ventricles, and close during ventricular contraction.
• Semilunar valves open during ventricular contraction, allowing flow into the great arteries, and close during diastole.
• Endocarditis is an inflammation of the heart's inner lining that can be caused by bacterial infections or autoimmune disorders.
• Staphylococcus aureus and streptococcus both cause endocarditis, especially with microtrauma.
• Oral surgery and intravenous drug use increase the risk of endocarditis due to the possible introduction of infectious agents directly into the bloodstream.
• Treatment typically involves aggressive intravenous antibiotic treatment to limit the infection.
• Oral surgery and intravenous drug use both increase the risk of endocarditis due to the possible introduction of infectious agents directly into the bloodstream.
• Mitral valve prolapse is the misalignment of one of the valve cusps during closing of the mitral valve.
• Mitral valve prolapse is prevalent in the mitral valve, is the most common valvular condition affecting approximately three percent of the population.
• Valve prolapse can result in inefficient closure of the mitral valve and the passage of blood back into the left atrium, known as valvular regurgitation.
• Valve prolapse tends to produce a clicking sound upon auscultation as the valve snaps upward into the atrium.
• Regurgitation produces a swooshing sound, medically described as a heart murmur, requiring further evaluation.
• Stenosis of the pulmonary valve, referring to a thickening of valve cusps due to fibrosis and calcification, can diminish stroke volumes and cause fainting.
• Treatment of more severe cases involves a valve replacement surgery
• Coronary arteries (left and right) are the first arterial branches off the aorta.
• Blood flow is greater during diastole due to aortic valve closure creating positive pressure.
• Coronary vessels are the only arteries that receive greater blood flow during diastole.
• The left coronary artery splits into the anterior interventricular (LAD) and left circumflex branches.
• The right coronary artery gives off the marginal artery and travels laterally, ultimately passing posterior Li as the right circumflex branch.
• The circumflex branch continues along to give off the posterior interventricular branch
• Cardiac veins (great, middle) drain into the coronary sinus.
• Anterior cardiac veins drain independently into the right atrium.
• Angina pectoris - chest pain, oxygen deficiency in heart
• Vascular plaques - Protrusions into lumen of the vessel which decreases the vessel diameter and restricts blood flow.
• Angiography is done to figure out the location of the blockage or blockages.
• Angioplasty is a surgical intervention to correct the blockage where the balloon compresses the plaque creating the blockage and restores the diameter of the lumen.
• Coronary artery bypass grafts (CABG or cabbage surgeries) are a tissue graft is harvested and is sutured to the aorta at one end and to the disease to vessel path the site blockage on the other as a result blood can flow around the blockage to maintain perfusion of cardiac tissue.
• Myocardial infarction (heart attack) happens when plaque breaks off and completely blocks a downstream vessel where heart tissue becomes necrotic and dies.
• Left-sided heart failure - backup of blood and pulmonary circulation, pulmonary edema, fluid accumulation in the lungs which is known as congestive heart failure
• Right-sided heart failure - blood accumulates in the systemic venous system with right-sided heart failure, venous distention, accumulation of fluid in the peripheral tissues which is sometimes also called pitting edema.
• Right dominance accounts for 70% of population where the posterior interventricular artery branching from the right coronary artery.
• Left dominance accounts for 10% of the population where the posterior interventricular artery actually stems from the left coronary artery and
• Codominance accounts for 20% where both left and right arteries feed into the posterior interventricular artery to some extent.

Heart Conduction

• Specialized nerve cells initiate and propagate action potentials.
• Cardiac cycle: repeating process of contraction and relaxation
• Auto rhythm isset II - Nerve cells associated with heart have leaky channels which open and close intermittently allowing a constant stream of sodium to move down its concentration gradient into the cell.
• Fastest rate of depolarization seen in sinoatrial node (SA node) that is also commonly referred to as a pacemaker.
• Movement of action potential through heart creates electrical disturbances that can be detected with electrodes placed on subject's skin.
• Electrocardiography is where surface electrodes are placed at precise locations on both wrists on the left ankle and along the chest wall.
• Strong enough electrical impulses will deflect and register electrical potential between each of these leads providing different perspectives.
• 12-lead ECG is far beyond the scope of the present discussion a trained cardiologist will have spent years studying the ECG patterns in order to accurately identify abnormalities that could indicate cardiac arrhythmias and heart disease
• Simplified ECG pattern stages: P wave, PR segment, QRS complex, ST segment and T wave
• P wave represents atrial depolarization (contraction)
• PR segment is a pause in the movement of electrical potential from the atria to ventricles through the AV node before reaching the ventricles
• Bundle of hiss projects from the AV node and pierces the atrioventricular septum to enter the wall of the interventricular septum
• QRS complex is associated with depolarization of left and right ventricles beginning at the apex of the ventricles and progress upwards towards the vows.
• Repolarization of the SA node and atria but any pattern that this might make is masked by the depolarization taking place within the ventricles
• ST segment is another plateau period between depolarization and repolarization of ventricle
• T wave represents repolarization of the ventricles
• ECG abnormalities examples include elevation or depression of the ST segment and indication of an acute myocardial infarction.
• Cardiac cycle: diastole to P wave to QRS complex to T wave and back to diastole
• Diastole: atria and ventricles and atrioventrictular valves are relaxed where pressure is lowest in the ventricles which means the tricular valves stay open allowing blood to passively flow from the atria into the ventricles meaning semilunar valves are shut.
• P wave triggers atrial depolarization which results in filling the ventricles with the final 20% of blood for the volume to push blood out of the chambers.
• First heart sound: Closing of the atrioventricular valves creates the love
• Isovolumetric ventricular contraction: all four valves are closed and blood is neither entering or leaving the heart
• Ventricular ejection happens when the pressure and the ventricles finally surpasses that in the aorta and pulmonary trunk resulting in opening of the semilunar valves and the injection of from the ventricles into the great vessels
• Second heart sound or dub is when the pressure and the ventricles once again drops below that in the great vessels causing the semilunar valve to snap shut
• Isovolumetric ventricular relaxation ends after cycle goes back back to passive filling stage.
• Ectopic pacemakers - regions of hyper excitable cells
• Premature ventricular contraction - ventricular contraction that occurs at more or less the same time as atrial contraction.
• Ventricular fibrillation, coordinated activity is lost and spontaneous depolarization occurs at random points throughout the ventricular walls preventing the heart from generating enough pressure to force blood through the vascular system and creates a life-threatening situation where treatment is defibrillation
• Cardiac pacemakers - indwelling unit that is surgically implanted subcutaneously in the left thorax to re-establish sinus rhythm when arrhythmia is detected.
• Pressure is not high to push blood to through lungs so there are to shunts that bypass this action.
• The first shunt is an opening within the interatrial septum known as the foramen ovale which allows blood to pass from the right atrium directly into the left The second shunt is the ductus arteriosus which allows blood to pass from the pulmonary trunk directly into the aorta
• Patent foramen ovale happens when not all of the tissue fuses and doesn't close off access creating a whole in the heart to which treatment involves surgery.

Mediastinum

• Mediastinum is in-between the left and right pleural cavities and is bound superiorly by the thoracic Inlet and inferiorly by the middle portion of the diaphragm.
• The thoracic Inlet and the theoretical transverse thoracic plane makes up the superior mediastinum,
• The anterior region lies in front of the heart.
• The last segment can either be called the posterior or the inferior segment, found behind the diaphragm.
• Thymus - plays role in development of t-cells found in elderly.
• Great vessels - the aorta - left ventricle
• The arch itself gives rise to three vessels: brachiocephalic trunk, common carotid artery and subclavian arteries
• Superior vena cava - principal drainage system for upper body
• Trachea - portion of respiratory system that connects the larynx with primary bronchi.
• Has sequential cartilaginous rings to provide it with structural integrity from negative pressures in area
• Esophagus - muscular tube about 30 centimeters in length that extends from the laryngeal pharynx to the thoracic diaphragm which appears to enter the abdominal cavity.
• Thoracic lymph duct - sandwiched between above two structures that can cause significant edema of the lower limbs if there is a obstruction.
• Phrenic nerve - provides motor innervation to entire muscle of diaphragm
• Vagus nerve innervates and blends in with surface of esophagus
• Recurrent laryngeal nerves paralysis would paralyze the vocal box and affect an individual's ability to produce sound.
• Sympathetic chain supplies sympathetics by segmental outflow from spinal cord segments t1 through l2.
• Transaxial CT or MRI images the most used form for medical imaging of thorax
• Aortic valve, pulmonary valve, tricuspid and bicuspid points for listening to locations in routine check-up for screening purposes.

Additional Details on Valvular Disease, Coronary Circulation, and Fetal Heart Shunts:

• Mitral Valve Prolapse (MVP):
  • Results from the breakdown of connective tissue in one of the mitral valve leaflets, leading to elongation and thickening.
  • The affected leaflet protrudes into the left atrium during valve closure, potentially causing valvular regurgitation.
  • Auscultation may reveal a clicking sound as the valve snaps upwards or a heart murmur due to regurgitation.
  • Severe cases may require valve replacement to prevent complications like congestive heart failure.
• Stenosis:
  • Involves the thickening of valve cusps due to fibrosis and calcification, often following endothelial damage and inflammation.
  • Pulmonary valve stenosis can diminish stroke volumes and cause fainting.
  • Treatment for severe cases includes valve replacement surgery, with newer techniques using closed approaches with bovine or porcine valves.
• Coronary Circulation:
  • The left coronary artery splits into the anterior interventricular (left anterior descending or LAD) and left circumflex branches.
  • The right coronary artery gives off the marginal artery and travels laterally, ultimately becoming the right circumflex branch. The circumflex branch then gives off the posterior interventricular branch.
  • Coronary vessels are unique as they receive greater blood flow during diastole compared to systole.
  • The great cardiac vein runs with the anterior interventricular artery, and the middle cardiac vein runs with the posterior interventricular artery, all draining into the coronary sinus except for the anterior cardiac veins which drain independently into the right atrium.
• Angina Pectoris and Interventions:
  • Angina pectoris, or chest pain, is caused by oxygen deficiency in the heart due to restricted blood flow.
  • Angiography helps locate blockages, and angioplasty uses a balloon catheter to compress plaques, often with a stent to maintain vessel diameter.
  • Coronary artery bypass grafts (CABG or cabbage surgeries) involve harvesting a tissue graft to bypass the blocked vessel and restore blood flow.
• Myocardial Infarction and Heart Failure:
  • Myocardial infarction (MI) occurs when a plaque breaks off, completely blocking a downstream vessel and causing heart tissue necrosis.
  • Left-sided heart failure results in a backup of blood and pulmonary circulation, leading to pulmonary edema and congestive heart failure.
  • Right-sided heart failure causes blood to accumulate in the systemic venous system, leading to venous distention and peripheral tissue edema (pitting edema).
• Cardiac Dominance:
  • Right dominance (70% of the population) involves the posterior interventricular artery branching from the right coronary artery.
  • Left dominance (10%) involves the posterior interventricular artery stemming from the left coronary artery.
  • Codominance (20%) involves both left and right arteries feeding into the posterior interventricular artery.
• Fetal Heart Shunts:
  • Foramen ovale: An opening within the interatrial septum that allows blood to pass from the right atrium directly into the left atrium.
  • Ductus arteriosus: Allows blood to pass from the pulmonary trunk directly into the aorta.
  • Patent foramen ovale: An incomplete closure of the foramen ovale after birth, which can lead to reduced arterial oxygen saturation and cyanosis and may require surgical repair

Heart Conduction and Cardiac Cycle Details:

• Autorhythmicity:
  • Cardiac nerve cells have leaky sodium ion channels that allow spontaneous depolarization.
  • The sinoatrial (SA) node has the fastest rate of depolarization and acts as the heart's pacemaker.
• Electrocardiography (ECG):
  • Measures the electrical activity of the heart using surface electrodes.
  • 12-lead ECG provides multiple perspectives to identify cardiac abnormalities.
  • Simplified ECG pattern includes the P wave (atrial depolarization), PR segment (AV node pause), QRS complex (ventricular depolarization), ST segment, and T wave (ventricular repolarization).
• Cardiac Cycle Stages:
  • Diastole: Atria and ventricles are relaxed, AV valves are open, and blood passively flows into the ventricles where pressure is lowest in the ventricles.
  • P wave: Atrial depolarization and contraction fill the ventricles with the final 20% of blood volume.
  • QRS complex: Ventricular contraction begins, AV valves close (first heart sound or "lub"), isovolumetric ventricular contraction occurs.
  • Ventricular ejection: Semilunar valves open, and blood is ejected into the aorta and pulmonary trunk.
  • T wave: Ventricles begin to repolarize and relax.
  • Isovolumetric ventricular relaxation: Semilunar valves close (second heart sound or "dub")
• Arrhythmias:
  • Ectopic pacemakers: Hyper-excitable regions of the heart can cause premature ventricular contractions (PVCs) or inverted P waves.
  • Ventricular fibrillation (V-fib): Uncoordinated ventricular contractions can be life-threatening, requiring defibrillation.
  • Cardiac pacemakers: Implanted devices can re-establish sinus rhythm by delivering electrical pulses to the AV node or apex of the heart.

Mediastinum Structures and Imaging:

• Mediastinum Regions:
  • Superior mediastinum: Lies between the thoracic Inlet and the transverse thoracic plane.
  • Anterior mediastinum: Located in front of the heart and contains the thymus gland.
  • Middle mediastinum: Contains the heart.
  • Posterior (or inferior) mediastinum: Lies between the heart and the vertebral column.
• Great Vessels:
  • Aorta: Arches posterior and to the left, giving rise to the brachiocephalic trunk (which splits into the right common carotid and right subclavian arteries), left common carotid artery, and left subclavian artery.
  • Superior vena cava: Formed by the convergence of the left and right brachiocephalic veins.
• Other Structures:
  • Trachea: Connects the larynx to the primary bronchi, supported by cartilaginous rings.
  • Esophagus: Muscular tube extending from the laryngeal pharynx to the thoracic diaphragm.
  • Thoracic lymph duct: Principal lymphatic drainage for the left thorax, upper limb, abdomen, and lower limbs. Obstruction can cause edema.
  • Phrenic nerve: Innervates the diaphragm for motor function. "C3, 4, and 5 keep the diaphragm alive."
  • Vagus nerve: Provides parasympathetic innervation to the digestive tract and has branches that curve under the arch of the aorta (left recurrent laryngeal nerve) and the right subclavian artery (right recurrent laryngeal nerve), supplying the larynx.
  • Sympathetic chain: Supplies sympathetic innervation through segmental outflow from spinal cord segments T1-L2.
• Medical Imaging:
  • Transaxial CT or MRI images are viewed from a bottom-up perspective.
  • Surface anatomy is critical for stethoscope placement to auscultate heart sounds; the aortic valve is assessed on the right second intercostal space lateral of the sternum, the pulmonary valve on the left, and the tricuspid and bicuspid valves near the 5th intercostal space, for example.