Introduction to the Human Heart

Introduction to the Heart

• The cardiovascular system consists of the heart, blood, and blood vessels
• Approximately 100,000 heartbeats occur each day
• About 8000 liters of blood are pumped daily
• Blood pumps through both the systemic and pulmonary circuits from the four-chambered heart itself
• The pulmonary circuit carries oxygen-poor blood to the lungs
• The systemic circuit carries oxygen-rich blood throughout the body

Heart Functions: Pulmonary and Systemic Circuits

• The pulmonary circuit transports blood to/from the gas exchange surfaces of the lungs
• The systemic circuit transports blood to/from the rest of the body
• Each circuit begins and ends at the heart
• Blood travels through these circuits in sequence

Types of Blood Vessels

• Arteries carry blood away from the heart
• Veins return blood to the heart
• Capillaries (exchange vessels) connect smallest arteries and veins, allowing gas, nutrient, and waste exchange

Chambers of the Heart

• The right atrium receives blood from the systemic circuit
• The right ventricle pumps blood into the pulmonary circuit
• The left atrium receives blood from the pulmonary circuit
• The left ventricle pumps blood into the systemic circuit

Heart Location and Position

• Great vessels connect at the base (superior)
• The pointed tip is the apex (inferior)
• The heart is located between two pleural cavities in the mediastinum
• The Pericardium surrounds the heart including an outer fibrous layer and an inner serous layer consisting of the parietal and visceral layers
• The pericardial cavity exists between the parietal and visceral layers and contains pericardial fluid

Pericarditis and Cardiac Tamponade

• Pericarditis is caused by pathogens in the pericardium
• Inflamed pericardial surfaces rub, making a distinctive scratching sound
• Cardiac tamponade restricts heart movement due to excess fluid in the pericardial cavity

Heart Anatomy: Superficial, Walls and Cardiac Skeleton

• Atria are the two thin-walled superior chambers.
• Each atrium has an expandable outer auricle
• Sulci are surface grooves, containing fat and blood vessels
• The coronary sulcus marks the border between atria and ventricles
• The anterior and posterior interventricular sulci mark the boundary between the left and right ventricles
• The heart wall consists of three layers: epicardium, myocardium, endocardium
• The epicardium is the outer surface (also known as the visceral layer of serous pericardium)
• The parietal layer of serous pericardium covers the visceral layer
• The myocardium is the middle layer of cardiac muscle tissue
• The endocardium covers the inner surface

Connective Tissues and Heart Chambers/Septa

• Connective tissues support muscle fibers, blood vessels, and nerves of the myocardium
• These tissues distribute force, add strength, prevent overexpansion and provide elasticity
• The cardiac skeleton consists of four dense bands of elastic tissue that encircle heart valves/vessel bases and stabilize valve/muscle positions
• Cardiac skeleton electrically insulates ventricular cells from atrial cells
• Septa are muscular partitions separating the heart chambers
• The interatrial septum separates the atria
• The interventricular septum separates the ventricles, and is much thicker than the interatrial septum

Heart Valves

• Atrioventricular (AV) valves separate the atria from the ventricles
• These valves include the tricuspid and mitral valves
• Fibrous tissue folds extend into openings between atria and ventricles, permitting one-way blood flow
• Semilunar valves (aortic and pulmonary) prevent backflow into the ventricles

Vena Cavae and Right Atrium

• The right atrium receives blood from the superior and inferior vena cavae
• The superior vena cava carries blood from the head, neck, upper limbs, and chest
• The inferior vena cava carries blood from the trunk, viscera, and lower limbs
• The foramen ovale is an opening in the interatrial septum that exists before birth which connects the two atria of the fetal heart, eventually closes to become the fossa ovalis
• Pectinate muscles are prominent muscular ridges on the anterior atrial wall and inner auricle surface
• Blood flows from the right atrium to the right ventricle through the tricuspid valve

Right Ventricle & Pulmonary Trunk

• The tricuspid valve has three cusps that prevent backflow
• Free valve edges attach to chordae tendineae from papillary muscles of the ventricle
• Trabeculae carneae are muscular ridges on the internal surfaces of both ventricles
• The moderator band is a muscular ridge that delivers stimulus for contraction to papillary muscles
• The conus arteriosus is at the superior end of the right ventricle, ending at the pulmonary valve
• The pulmonary valve has three semilunar cusps and leads to the pulmonary trunk
• The pulmonary circuit starts here which then divides into the left and right pulmonary arteries

Left Atrium & Mitral Valve

• The left atrium receives blood from the left and right pulmonary veins
• Blood flows from the left atrium to the left ventricle through the mitral valve
• The mitral valve (or bicuspid valve) consists of two cusps

Left Ventricle & Aorta

• The left ventricle, unlike the right ventricle, does not have a moderator band
• Blood leaves the left ventricle through the aortic valve into the ascending aorta
• Aortic sinuses are saclike expansions at the base of the ascending aorta
• The ascending aorta turns into the aortic arch and subsequently becomes the descending aorta
• Compared to the left ventricle, the right ventricle holds and pumps the same amount of blood, has thinner walls, develops less pressure, and is more pouch-shaped
• The heart valves prevent backflow of blood

Atrioventricular and Semilunar Valves

• Atrioventricular (AV) valves exist between atria and ventricles
• When ventricles contract, blood pressure closes the valves
• Papillary muscles contract and tense the chordae tendineae, thus preventing backflow into the atria
• Regurgitation is the backflow of blood
• Semilunar valves prevent backflow into the ventricles
• Valvular heart disease (VHD) is the deterioration of heart valves that can develop after carditis (inflammation of heart) and can result from rheumatic fever

Coronary Arteries and Veins

• Coronary circulation ensures the heart muscle receives blood first
• Coronary arteries originate at the aortic sinuses
• Elevated aortic blood pressure and elastic rebound maintain coronary blood flow
• The right coronary artery (RCA) supplies the right atrium, portions of both ventricles, and the electrical conducting system
• It gives rise to the marginal arteries and posterior interventricular artery
• The left coronary artery (LCA) supplies the left atrium, left ventricle, and interventricular septum and gives rise to the circumflex artery and anterior interventricular artery
• Arterial anastomoses create interconnections by connecting the anterior and the posterior interventricular arteries which maintains constant blood supply to muscle
• The great cardiac vein drains blood from the region supplied by the anterior interventricular artery and returns blood to the coronary sinus to open into right atrium
• The posterior, middle, and small cardiac veins empty into the great cardiac/coronary sinus
• Anterior cardiac veins empty directly into the right atrium

Coronary Artery Disease

• Coronary artery disease (CAD) involves partial or complete blockage of coronary circulation, which prevents heart muscle cells from receiving adequate oxygen and nutrients
• Reduced blood flow decreases cardiac performance
• Coronary ischemia results from reduced circulatory supply due to partial or complete coronary artery blockage
• The disease’s usual cause is a fatty deposit or atherosclerotic plaque
• The plaque/associated thrombus narrows the passageway
• Spasms in smooth muscle can further reduce or stop blood flow
• Angina pectoris (chest pain) is a common first symptom of CAD
• Temporary ischemia can develop with increased heart workload
• An MI occurs when part of the coronary circulation becomes blocked, causing cardiac muscle cells to die from lack of oxygen
• Most MIs result from severe CAD and are commonly caused by coronary thrombosis (a thrombus in the plaque)
• Death of affected tissue creates a nonfunctional area known as an infarct
• ECGs and blood tests can diagnose MIs
• Damaged myocardial cells release cardiac troponin T/I and creatinine phosphokinase (CK-MB) into circulation

Interventions and Risk Factor Modification

• Treatment includes stopping smoking, treating high blood pressure, adjusting diet, reducing stress, and increasing physical activity
• Atherectomy involves inserting a catheter to remove plaque
• Balloon angioplasty involves using an inflatable catheter balloon to press plaque against vessel walls
• Stents may be inserted to maintain vessel opening
• Coronary artery bypass graft (CABG) involves using a section of another vessel
• A CABG may involve up to four coronary arteries routed during a single operation

Cardiac Contraction: Physiology and Cells

• Heartbeat describes a single cardiac contraction, where the heart chambers contract in a series
• First the atria contract and then the ventricles
• Autorhythmic cells control/coordinate heartbeat
• Contractile cells produce contractions that propel blood
• The conducting system consists of specialized cardiac muscle cells that initiate/distribute electrical impulses and demonstrate autorhythmicity
• Pacemaker cells in the Sinoatrial (SA) initiate impulses, and the Atrioventricular (AV) nodes exist at the junction between atria and ventricles
• Conducting cells include internodal pathways of the atria, the atrioventricular (AV) bundle, bundle branches, and Purkinje fibers
• Pacemaker potential gradually depolarizes pacemaker cells with no stable resting membrane potential

Heart Rhythms and Impulses

• The SA node spontaneously depolarizes at a rate of 60-100 action potentials/minute
• The AV node spontaneously depolarizes at a rate of 40-60 action potentials/minute
• The SA node establishes sinus rhythm
• SA node activity and atrial activation begin the impulse conduction
• Stimulus spreads across atria to the AV node with a 100 msec delay.
• The atrial contraction happens as the impulse travels in the AV bundle to the left and right bundle branches to the Purkinje fibers and the papillary muscles
• Purkinje fibers distribute the impulse to the ventricular myocardium and ventricular contraction begins
• Arrythmias are disturbances in heart rhythm
• Bradycardia describes abnormally slow heart rate
• Tachycardia describes abnormally fast heart rate
• An ectopic pacemaker contains abnormal cells and disrupt timing of ventricular contractions
• The electrocardiogram (ECG or EKG) records electrical events in the heart using electrodes and diagnosing damage

ECG Features, Potentials, and Contraction

• An ECG includes the detection of the P wave (atrial depolarization), QRS complex (ventricular depolarization), and T wave (ventricular repolarization)
• Ventricles begin contracting after the R wave
• The P-R interval records atrial depolarization to the start of the QRS complex
• The Q-T interval records the time required for ventricles to undergo a single cycle of depolarization and repolarization
• Cardiac contractile cells form the bulk of atrial and ventricular walls and respond to stimuli from Purkinje fibers
• Ventricular and atrial cell resting membrane potentials are -90 mV and -80 mV
• Intercalated discs interconnect cardiac contractile cells held by desmosomes and linked by gap junctions, allowing transfer of contractile force
• Rapid depolarization is caused by Na+ influx through fast sodium channels
• Plateau results from extracellular Ca2+ influx through slow calcium channels
• Repolarization is caused by K+ efflux through slow potassium channels
• During the absolute refractory period (200 msec), cardiac contractile cannot respond
• The relative refractory period (50 msec) only responds to strong stimuli
• Action potentials in ventricular contractile cells last 250-300 msec, preventing summation and tetany
• Extracellular Ca2+ crosses plasma membrane during plateau providing roughly 20% for contraction, triggering release of additional Ca2+ from sarcoplasmic reticulum (SR)

Energy for Cardiac Contractions

• Cardiac muscle is sensitive to extracellular Ca2+ concentrations, and intracellular Ca2+ is pumped back to regulate muscle function
• Cardiac contractions rely on aerobic energy which uses the mitochondrial breakdown of fatty acids/glucose and myoglobin to store oxygen for contractile cells

Cardiac Cycle

• The cardiac cycle is from one heartbeat to the next
• Systole contraction and diastole relaxation alternate
• Blood pressure rises during and falls during diastole in each chamber
• Contractions control blood flow from high to low pressure and one-way valves direct flow
• A cardiac cycle lasts 800 msec with a rate of 75 bpm
• The cycle phases shorten, particularly during diastole, when increases in heart rate occur
• Four phases existing include atrial systole, Atrial diastole, Ventricular systole, and Ventricular diastole

Cardiac Phases, Heart Sounds, and Output

• Atrial systole is the beginning phase with open left/right atrioventricular valves ejects blood into the ventricles while simultaneously initiating atrial diastole and the expansion of blood volume in the ventricles (end-diastolic volume (EDV))
• Ventricles build isovolumetric contraction pressure by closing the AV valves during ventricular systole
• Semilunar valves open, allowing blood ejection and stroke volume (SV)
• The semilunar valves close as ventricular pressure falls as the ventricles meet end-systolic volume (ESV) (40% of end-diastolic volume)
• Isovolumetric relaxation occurs, closing all heart valves with high ventricular pressure
• Atrial pressure is greater than ventricular pressure during ventricular diastole, passively filling when the cycle begins again.
• Severe atrial damage is survivable while severe ventricular damage can cause heart failure
• Heart sounds are detected by stethoscope
• S1 and S2 are described as '"Lubb"' from loud AV valve closing and '"Dupp"'
• S3 and S4 exist due to soft sounds as blood flows into the ventricles
• Heart murmurs describe regurgitation through valves
• Cardiac output (CO) is the amount of blood pumped by the left ventricle in one minute, Cardiac Output is measured by CO = HR × SV where the HR is heart rate and the SV is the stroke volume

Cardiac Measurement and Function

• HR = heart rate (beats/min)
• SV = stroke volume (mL/beat)
• SV = EDV – ESV
• End-diastolic volum (EDV) is the amount of blood in each ventricle at the end of diastole.
• End-systolic volume (ESV) volume is the amount of blood remaining in each ventricle at end of systole
• Ejection fraction (EF) describes the percentage of EDV is ejected during contraction
• Autonomic activity and circulating hormones factor the process

Autonomic Activity and Control Factors

• Cardiac plexus stimulation occurs through the vagus nerves (CN X)
• Medulla oblongata cardiac centers (cardioacceleratory and cardioinhibitory) exert sympathetic influence
• Baroreceptors and chemoreceptors adjust cardiac activity through autonomic tone which maintains dual control through released ACh and NE for fine adjustments
• Closer membrane potentials of pacemaker cells are altered by changes caused by autonomic tone through Ach released to parasympathetic neurons which alters heart rate
• Increases in heart rate is increased by venous return through Bainbridge reflex (atrial reflex) and sympathic neurons with released NE, Hormones, and Venous return.

Stroke Volume and Control Factors

• Stroke volume depends on EDV, factors affecting include filling time, venous return, and preload
• Filling time is the duration of diastole.
• Preload describes the amount of ventricular stretching as stroke volume
• As Stroke volume increases through an increase in end disatollic volume causing the ventricular expansion to be limited by myocardial tissye elasticity and Cardiac tissue. Factors. ESV-preload, contraction and ater load

Control Factors

Overview of the cardiovascular system with a focus on the heart's function, its role in pulmonary and systemic circuits. Exploration of blood vessels and their roles, including arteries, veins, and capillaries. Discussion of the heart's chambers and functionality.