Cardiovascular System: Pulmonary & Systemic Circuits

Overview of Cardiovascular System

• The cardiovascular system comprises the heart and blood vessels.
• The heart circulates blood through about 75,000 miles of blood vessels.
• Cardiology is the study of the normal heart and its associated diseases.
• The heart pumps 7,000 liters of blood daily.
• The heart contracts approximately 2.5 billion times in a lifetime.
• Two major divisions of the cardiovascular system are the pulmonary circuit and the systemic circuit.

Pulmonary Circuit

• The right side of the heart supplies blood to the pulmonary circuit, which carries blood to the lungs and back.
• Receives blood with high CO2 content and wastes from the body.
• Blood is pumped into the pulmonary trunk, which divides into the right and left pulmonary arteries.
• CO2 is unloaded and O2 is picked up in the air sacs (alveoli) of the lungs.
• Oxygen-rich blood flows through the pulmonary veins to the left side of the heart.

Systemic Circuit

• The left side of the heart supplies the systemic circuit, carrying blood to the body's tissues and back.
• It receives blood from the pulmonary veins.
• Blood is pumped into the aorta, which makes a U-turn at the aortic arch to supply the head, neck, and upper limbs, then passes downward.
• The aorta travels through the thoracic and abdominal cavities, issuing arteries to organs before branching into the lower limbs.
• Deoxygenated systemic blood returns to the right side of the heart via the superior and inferior vena cava.

Size and Location of the Heart

• The heart lies in the thoracic cavity within the mediastinum, medial to the lungs and deep to the sternum.
• Heart size varies with body size, averaging 14 cm long, 9 cm wide, and weighing approximately 300 g.
• About two-thirds of the heart lies to the left of the median plane due to its tilt.
• The broad superior portion beneath the 2nd rib is called the base (attachment point for great vessels).
• The inferior end tapers to a blunt point at the 5th intercostal space, known as the apex.

Coverings of the Heart

• The Pericardium is a double-walled sac enclosing the heart.
• The outer wall is the pericardial sac (parietal pericardium) that has a tough, fibrous layer and a thin, serous layer.
• The serous layer turns inward at the base to form the epicardium (visceral pericardium) that covers the heart surface.
• Ligaments anchor the pericardial sac to the diaphragm and sternum and fibrous tissue to the mediastinal tissue.
• The pericardial cavity, filled with pericardial fluid (serous fluid), lies between the parietal and visceral layers of the serous pericardium, reducing friction.
• Pericarditis, an inflammation of the pericardium, may cause a painful friction rub with each heartbeat.
• The pericardium isolates the heart from other thoracic organs and allows for expansion while resisting excessive distension.

Structure of the Heart

• The heart wall consists of three layers: epicardium, myocardium, and endocardium.
• The epicardium (visceral pericardium) is a serous membrane where major coronary blood vessels travel.
• The Myocardium (middle layer) is the thickest layer and consists of cardiac muscle responsible for the heart's work.
• Cardiac muscle fibers are shorter, larger in diameter, and more square-ish than skeletal muscle fibers, with branching forming networks.
• Cardiac muscle contains actin, myosin, striated - zones, and Z discs.
• Two functional networks exist: atrial and ventricular networks (syncytium).
• Intercalated discs (desmosomes and gap junctions) connect fibers within the networks, allowing synchronized function.
• Muscle spirals form a myocardial vortex, creating a twisting motion upon contraction.
• Cardiac muscle has less sarcoplasmic reticulum than skeletal muscle and requires Ca+2 from extracellular fluid.
• Cardiac muscle cells have a longer refractory period and more mitochondria than skeletal muscle.
• Contractions last 10-15 times longer than skeletal muscle twitch due to prolonged Ca+2 delivery.
• Cardiac muscle cells contract when stimulated by autorhythmic fibers.
• Damaged cardiac muscle is repaired by fibrosis (scarring), as cardiac muscle lacks satellite cells for division.
• Cardiac muscle depends almost exclusively on aerobic respiration for ATP production in terms of metabolism.
• The Endocardium (inner layer) is simple squamous epithelium lining the heart chambers, covering valve surfaces, and continuous with the endothelium of blood vessels.
• The heart contains a fibrous skeleton of collagenous and elastic fibers, concentrated in walls between chambers, around valves, and in interconnecting sheets.
• The skeleton provides structural support around valves/openings, anchors cardiocytes, and serves as electrical insulation between atria and ventricles.
• The heart is a cone-shaped, muscular pump with four chambers, separated into right and left sides to keep blood flow entirely separate.
• The two superior chambers are the right and left atria.
• The atria receive blood from great veins and are thin-walled.
• The chambers are separated via the interatrial septum.
• Each atrium features an auricle, a small earlike extension that slightly increases volume in the anterior view.

Heart Chambers

• The Right Atrium receives blood from the superior and inferior vena cava and the coronary sinus.
• The fossa ovalis, an oval depression in the septum between the atria, is a remnant of the foramen ovale.
• Blood passes from the right atrium to the right ventricle via the right atrioventricular (AV) valve (tricuspid valve).
• The Left Atrium receives blood from the pulmonary veins.
• Blood passes from the left atrium to the left ventricle via the left AV valve, also known as the bicuspid (mitral) valve.
• The Inferior chambers, right and left ventricles, pump blood into arteries, separated by the interventricular septum.
• The Right Ventricle forms most of the heart's anterior surface and has thinner walls than the left because it pumps blood into the nearby lungs with low resistance.
• Blood passes from the right ventricle to the pulmonary trunk via the pulmonary semilunar valve.
• The Left Ventricle forms the apex of the heart and has walls two to four times thicker.
• Blood travels through the entire body.
• Blood passes from the left ventricle through the aortic semilunar valve into the aorta.
• Heart valves, featuring two or three flaps (cusps or leaflets) covered with endocardium, ensure one-way blood flow.
• Stringlike tendinous cords (chordae tendineae) connect AV valve cusps to papillary muscles on the ventricle floor to prevent flipping.
• Semilunar valves (pulmonary and aortic) regulate blood flow from ventricles into great arteries.
• Valves open and close via blood pressure changes during chamber contraction and relaxation without muscular effort.
• Sulci (grooves) mark chamber boundaries on the heart's surface, filled with fat and coronary blood vessels.

Blood Supply to the Heart - Coronary Circulation

• Coronary (cardiac) circulation involves the flow of blood through vessels piercing the myocardium, delivering oxygenated blood and nutrients while removing CO2 and waste.
• A right and left coronary artery originate from the aorta with openings deep in the pockets formed by the aortic valve cusps..
• The left coronary artery (LCA), approximately 3.5 cm long, runs through the coronary sulcus under the left auricle and splits into two.
• The anterior interventricular branch (left anterior descending [LAD]artery) runs to the apex then joints the posterior interventricular branch.
• The circumflex branch continues around the left side giving off the left marginal branch to the left ventricle, then ends by supplying the left atrium/ posterior wall..
• The right coronary artery (RCA) supplies the right atrium and sinoatrial node (pacemaker), extends to the right auricle and branches.
• The right marginal branch goes to the apex, supplying the lateral right atrium and ventricle.
• The RCA continues around the right margin, branches to the atrioventricular node, then typically gives off a posterior interventricular branch (supplies walls of both ventricles, joins LAD).
• The coronary sinus, a large transverse vein in the coronary sulcus on the heart's posterior, drains blood from veins to right atrium.
• Blood flow in coronary arteries increases during ventricular relaxation, unlike most organs.
• Myocardium contraction compresses and obstructs arteries.
• The aortic valve is forced open, and cusps block coronary artery openings during ventricular contraction.
• Blood surges back during ventricular relaxation causing a dicrotic notch.
• Vessels can be 90% blocked with collateral circulation.
• Heart muscle can survive even with 10-15% blood supply.
• Low myocardial oxygen levels can weaken heart cells, causing myocardial ischemia, or myocardial infarction (MI)/(heart attack) in permanent instances.

Heart Attack

• A heart attack (myocardial infarction) results from the death of cardiac muscle tissue due to interrupted blood supply in coronary arteries . Thrombus or embolus.
• Tissue beyond the obstruction dies and is replaced by non-contractile scar tissue.
• Treatments include clot dissolving drugs, angioplasty, and stents (1-inch stainless steel tube mesh). Angioplasty involves threading a tube to plaque, expanding a balloon, but vessel re-closure or collapse can occur.
• Bypass surgery uses a leg vein section attached to the aorta and coronary artery to bypass an obstruction, providing blood flow to heart muscle again

The Heartbeat

• A cardiac conduction system coordinates the heartbeat via an internal pacemaker and nerve-like conduction pathways through the myocardium.
• Conduction fibers initiate and distribute impulses without contracting.
• The rhythmic electrical signal order is SA node, AV node, Bundle of His, then Purkinje fibers.
• The SA Node (sinoatrial node) in the right atrium initiates heartbeat, determines heart rate, and is the pacemaker firing at 90-100 x/min.
• The sinus rhythm is a cycle of 70-80 bpm in adults, which can go up to 60-100 bpm.
• The vagus nerve (CN X) via PSNS brakes keeps HR to 72.
• Sinus bradycardia is < 60bpm from PSNS activation, athletes, post MI, and hypotension.
• Sinus tachycardia is >100bpm from SNS, exercise, CHF, and MI.
• Sinus arrest is when the SA node stops firing and pacemaker takes over.
• Signals from SA node that spread reach AV Node.
• The AV Node (atrioventricular node) (40-60 x/min) near the right AV valve is an electrical gateway to the ventricles, but the fibrous skeleton prevents currents in other routes.
• The AV node is activated by APs from the SA node.
• The Bundle of His (30-40 x/min) carries signals from the AV node, forking into right and left branches for interventricular septum and apex.
• Purkinje Fibers (30-40 x/min) spread throughout the ventricular myocardium and excite cardiocytes in heart wall muscles of ventricles.
• Cardiocytes continue the electrical activity themselves, by flowing ions through gap junctions after Purkinje fibers give signals.
• The heart receives sympathetic and parasympathetic nerves affecting heart rate/contraction.
• Sympathetic stimulation raises heart rate up to 230 bpm and stimulates heart contraction.
• Stimulation increases heart rate and contraction strength and dilates coronary arteries.
• The parasympathetic pathway from the vagus nerves in the medulla oblongata leads to the SA/AV nodes, reducing heart rate.
• Electrolyte imbalances, caffeine, hypoxia, nicotine, and other drugs can cause other conduction parts to fire before the SA node.

Electrocardiogram

• An electrocardiogram or ECG records electrical changes on the myocardium during the cardiac cycle, indicating impulse conduction.
• it determines if theres and abnormal conduction pathway, an enlarged heart, or damages.
• The SA node impulses generate an ECG with these waves:
• A P wave signals atrial depolarization.
• Atrial systole follows approximately 100 ms after the P wave starts.
• A QRS complex indicates ventricular depolarization.
• Atrial repolarization and diastole also occur during the QRS interval, but the signal is obscured by ventricular activity.
• Ventricular systole starts during the ST segment after the QRS complex.
• The ST segment corresponds to the plateau in the myocardial action potential.
• The heart contracts and ejects blood during the ST segment.
• A T Wave signals generated ventricular repolarization immediately before diastole.
  • Ventricles require more time to repolarize than depolarize.
• P-Q(PR) interval represents conduction time from atrial excitation to ventricular excitation.
• The S-T segment represents when contractile fibers are fully depolarized/plateau.

Blood Flow, Sounds, and the Cardiac Cycle

• The cardiac cycle is one heart chamber contraction and relaxation lasting 0.8 sec at 72/bpm.
• Systole is the heart's contraction phase, and diastole is the heart's relaxation phase.
• Atria contract during atrial systole while ventricles relax, followed by ventricles contracting during systole while atria relax (atrial diastole).
• The pressure in the heart chambers rises and falls during each cardiac cycle.

Ventricular Diastole and Atrial Systole

• Ventricles relax/expand during diastole creating lower pressure and the AV valves opening as blood flow in.
• AV valve cusps hang low.
• As fluid fills, the cusps float up towards closer position.
• Atria add the remaining 30% of blood to ventricles
• End-diastolic volume (EDV) volume of 130 mL of blood filled in each vetricle.

Ventricular Systole and Atrial Diastole

• Atria repolarize, relax, and start diastole in cycle.
• Ventricles begin depolarizing generating signals, contract, and pressure increases.
• When ventricular pressure rises, the AV valves push together for complete seal.
• Papillary muscles contract slightly before ventricular myocardium, preventing valve bulging.
• This isovolumetric phase with valves closed stops blood from going anywhere, so there is no change in volume.
• At this point, the pressures in the aorta (80 mm Hg) and pulmonary trunk (10 mm Hg) surpass those in the ventricles. Ejection starts when ventricular pressure beats arterial pressure, opening semilunar valves..
• The pressure in the arteries oppose opening, but when pressure beats arterial pressure, the valves will respond.
• Ejection takes 200 to 250 ms along the plateau of action potential with slight trailing.
• The T wave occurs late here.
• Ventricles only expel 54% volume = 70ml = stroke volume The remaining blood of roughly, 60ml, is the end systolic volume (ESV) with EDV(130ml)-SV(70ml) = ESV(60ml). More exercise lets SV be 90%.
• At diastole, blood rushes back semilunar, filling cusps/valves for close and slight dicrotic notch pressure.
• As valve closes, the S2 heart beat is heard with the ventricles expanding.
• The cycle restarts.
• During diastole, blood briefly flows backward through the semilunar valves, filling the cusps and closing the valves, creating the dicrotic notch.

Heart Sounds

• Two audible heart sounds occur during cycle during cardiac cycle: S1 and S2 both described as lubb-dupp.
• S1 (lubb) first happens during ventricular systole by flowing turbulent when the A-V valves close- S1 is louder and longer
• S2 (dupp) second sound happens during ventricular diastole by flowing turbulent when the pulmonary and aortic.
• A murmur (abnormal heart sound) from the cusps not completely closing

Cardiac Output

• The heart’s output is variable to maintain oxygen.
• Cardiac output (CO) blood ejected each minute.
• Stroke volume is how much volume/beat is in the ventricle and heart rate is number of beats. - CO equals SV times HR.
• 4-6 L of blood passes every minute with 75 HR by 70ml of beat = 5,250 mL/min = 5.25 L/min.
• Exercises help for people to have 21 L/min versus athletes at 35L/min

Regulation of Stroke Volume

• Three factors regulate stroke volume that are preload, degree of stretch and the heart is about to beat as contractility ( individual ventricular muscle fibers, afterload (pressure for ejection).
• Stroke Volume(SV)is calculated with Preload*Contractility/Afterload
• Increases in the first two increase output while increase of afterload will reduce stroke volume.
• Preload stretches tension, heart stretching before compression. More stretching with increase preload equals increase SV.
• Myocardial contractibility involves how acting and myosin interactions increases. Strength increases by calcium.
• Increase extra calcium in cells to create strong prolong contractions causing arrest.
• Semilunar valves must overcome atrial pressure blood. Volume limited by preload.
• Increase constriction equals increase resistance, equals decrease volume.

Regulation of Heart Rate

• Changing heart rate can control output and pressure.
• Resting hearts are 120 bmp in newborns and decline to 72-80 females, 64-72 in males.
• If rate go up that’s tachycardia is persistently high with 100 bpm due to stress, anxiety, etc.
• Heart rates can drop volume Bradycardia is at 60 bpm due to hypothermia or surgery deliberately.
• Autonomous regulation is a control of the heart system from medulla
• Sympathetic impulses increase heart rate and contraction.
• Impulses are adrenergic related and release epinephrine in fibers.
• Second messengers open calcium which depolarizes Sa Node/speeds up heart.
• Parasympathetic vagus slows down hearts rate.

Heart/Rate Factors

• Chemical increase heart rates with hormone (epinephrine ,norepinephrine).
• Nicotine and caffeine stimulate heart rates by adrengenic. Ions impact rate. Potassium impacts greatest
• Less potassium means the RMP is threshold to the cell depolarize for less and less repolarize to decrease HR.
• other factors like physical fitness, temperature, gender impact rates too.

Exercise and the Heart

• Exercise causes the cardiac output and oxygen to increase.
• Proprioceptors activate and muscle activity increases.
• Exercise improves venous return.
• Athlete adaptation includes low resting heart rate (40-60bpm) through ventricle growth.
• Lance armstrong is known to have 32 to 34 bpm naturally.
• With exercise, heart reserves help work harder and toleration more than others.
• Aerobic exercise allows HDL increases, triglyceride decreases, lung improvement, weight control, decreased blood pressure.

Explore the cardiovascular system, focusing on the heart and blood vessels. Learn about the pulmonary circuit, where blood is oxygenated in the lungs, and the systemic circuit, which distributes oxygen-rich blood throughout the body. Understand how these circuits work together to maintain life.