MC

Cardiovascular System & Blood Flow

Pulmonary and Systemic Circulation

  • The cardiovascular system supports two main circuits: Pulmonary Circulation (lungs) and Systemic Circulation (body).
  • Blood flow pathway overview:
    • Right side of the heart pumps deoxygenated blood to the lungs via the pulmonary trunk; lungs exchange gases; oxygenated blood returns to the left side through the pulmonary veins.
    • Left side of the heart pumps oxygenated blood to the body via the aorta; systemic arteries distribute to tissues; systemic venous blood returns to the right atrium via the superior and inferior venae cavae.
  • Cardiac output (CO) concept: CO is the volume of blood pumped by the heart per unit time.
    • Fundamental relation (often used): CO = HR \times SV where HR is heart rate and SV is stroke volume.
  • Heart orientation and dimensions:
    • Size roughly that of a fist; about 10 ounces; wide at the base; apex tilts to the left.
  • The pericardium:
    • A double-walled protective sac that surrounds the heart.
    • Provides lubrication, allows room to expand, and anchors to the sternum and diaphragm.
    • Layers include:
    • Parietal fibrous layer (outer, dense connective tissue)
    • Visceral serous membrane (epicardium) covering the heart
    • The pericardial cavity contains 5–30 mL of pericardial fluid.
    • Pericarditis: painful inflammation of the pericardial membranes; can affect epicardium, myocardium, and endocardium.
  • Cardiac anatomy reference points (general orientation):
    • Right side: receives deoxygenated blood from the body; right atrium and right ventricle.
    • Left side: receives oxygenated blood from the lungs; left atrium and left ventricle.
    • Major vessels: aorta, superior and inferior vena cavae, pulmonary trunk, pulmonary veins.

Layers of the Heart (Heart Wall)

  • The heart wall is composed of three tunic-like layers: epicardium, myocardium, and endocardium.
  • Epicardium (visceral pericardium): serous membrane; contains adipose tissue in thick layer; coronary blood vessels travel through.
  • Endocardium: smooth inner lining; covers valve surfaces; continuous with the endothelium of blood vessels.
  • Myocardium: muscular layer; muscle fibers arranged in spirals around the heart; contains the fibrous skeleton which:
    • Provides a framework of collagenous and elastic fibers
    • Attaches to valve tissue
    • Provides electrical insulation between atria and ventricles
    • Plays a key role in contractile activity

The Chambers of the Heart

  • Four chambers:
    • Right atrium (receives returning blood)
    • Right ventricle (pumps blood to the lungs via the pulmonary trunk)
    • Left atrium (receives oxygenated blood from the lungs via the pulmonary veins)
    • Left ventricle (pumps blood to the body via the aorta)
  • Auricles (appendages) enlarge the atrial chambers to increase capacity.
  • Interatrial septum: wall separating the right and left atria.
  • Interventricular septum: wall separating the right and left ventricles.
  • Internal muscular ridges and features:
    • Pectinate muscles: internal ridges in the right atrium and auricles.
    • Trabeculae carneae: internal ridges in the ventricles; help with contraction and may reduce suction effects.
    • Papillary muscles: anchor the chordae tendineae of AV valves.
  • Sulci (grooves) on the heart surface:
    • Atrioventricular (AV) sulcus: separates atria from ventricles; contains coronary arteries.
    • Interventricular sulcus: overlies the septum and divides the right and left ventricles.
  • Key vessels and structures nearby:
    • Aorta, right pulmonary artery, superior vena cava, right pulmonary veins, interatrial septum, fossa ovalis, pectinate muscles, right AV valve, tendinous cords, trabeculae carneae, right ventricle, inferior vena cava, left pulmonary artery, pulmonary trunk, left pulmonary veins, pulmonary valve, left atrium, aortic valve, left AV valve, left ventricle, papillary muscle, intervertebral septum, endocardium, myocardium, epicardium.

The Valves and Blood Flow

  • Valves regulate unidirectional blood flow and maintain proper pressure gradients:
    • Semilunar valves control flow into the arteries:
    • Pulmonary valve opens between the right ventricle and the pulmonary trunk.
    • Aortic valve opens between the left ventricle and the aorta.
    • Right AV valve = tricuspid valve; Left AV valve = mitral valve.
    • Coronary arteries branch off near the base of the aorta; openings are adjacent to the AV valves.
  • Cardiac cycle phases (valves open/close changes in response to pressure):
    • Ventricles relax: atrial pressure is higher than ventricular pressure; AV valves open; semilunar valves close; blood flows from atria to ventricles.
    • Ventricles contract: ventricular pressure rises; AV valves close; semilunar valves open; blood flows from ventricles into the aorta and pulmonary trunk.
  • Coronary circulation:
    • Supplies coronary vessels with blood; generally about 5% of blood flow (resting coronary flow).
    • Resting coronary blood flow is approximately 4,250 mL/min (as stated in the transcript).
  • Arterial supply to the heart muscle:
    • Left coronary artery branches off the ascending aorta:
    • Anterior interventricular (left) branch: supplies ventricles and about 2/3 of the interventricular septum.
    • Circumflex branch: courses in the left coronary sulcus; supplies left atrium and the posterior wall of the left ventricle.
    • Right coronary artery branches off the ascending aorta:
    • Right marginal branch: supplies lateral right atrium and ventricle.
    • Posterior interventricular branch: supplies posterior ventricular walls.

Defects of the Heart

  • Coronary artery disease (CAD): constriction of arteries due to lipid deposits leading to atherosclerosis.
    • Endothelial damage from hypertension, diabetes, etc., initiates monocyte entry and macrophage formation; fats and cholesterol accumulate; platelets bind and release growth factors promoting smooth muscle proliferation and collagen deposition; plaques bulge and narrow the lumen.
    • Angina pectoris: chest pain due to reduced blood flow and oxygen supply to the heart.
    • Plaques can rupture, causing clots or fat emboli; vasospasms may occur with nitroglycerin secretion relief.
  • Risk factors and treatment:
    • Risk factors: high LDL (low-density lipoprotein) cholesterol, dysfunctional receptors, aging, genetics, male sex, obesity, smoking, stress, physical inactivity.
    • Treatments: bypass surgery, angioplasty, stents.

Heart Attack & Drainage

  • Myocardial infarction (heart attack): death of myocardial tissue due to prolonged ischemia; a major cause of mortality.
  • Venous drainage of the heart:
    • Great cardiac vein drains anterior surface; empties into the coronary sinus.
    • Posterior interventricular vein drains posterior surface; empties into the coronary sinus.
    • Left marginal vein drains into the coronary sinus.
    • Coronary sinus: large vein in the coronary sulcus; drains into the right atrium.

Cardiac Muscle and Metabolism

  • Cardiocytes (cardiac muscle cells):
    • Short, thick cells.
    • Repair of damage occurs by fibrosis (scarring).
    • Intercalated discs join cardiomyocytes and contain mechanical and electrical junctions.
  • Cardiac metabolism:
    • Fatty acids, glucose, lactic acid, and amino acids serve as fuels; the heart is highly aerobic and capable of using various substrates.

Conduction & Nerve Supply

  • The conduction system coordinates heartbeat via intrinsic rhythm and specialized pathways:
    • Sinoatrial (SA) node: natural pacemaker; initiates and determines heart rate.
    • Atrioventricular (AV) node: delays impulse to allow atrial contraction to complete before ventricular contraction.
    • AV bundle (bundle of His): transmits impulses from AV node into the ventricles; splits into left and right bundle branches.
    • Purkinje fibers: distribute impulses through the ventricular myocardium.
  • Signals travel as follows:
    • SA node fires -> excitation spreads through atrial myocardium -> AV node fires -> signal passes down AV bundle -> Purkinje fibers distribute excitation through ventricular myocardium.
  • Autonomic nervous system regulation:
    • Sympathetic nerves: increase heart rate (HR) and contraction strength.
    • Parasympathetic nerves (vagal): decrease HR; fiber distribution:
    • Right vagus nerve primarily to SA node.
    • Left vagus nerve primarily to AV node.
    • Little to no vagal stimulation of myocardial contractility.
  • Cardiac innervation pathway:
    • Parasympathetic: nuclei in medulla oblongata -> cardiac plexus -> cardiac nerves to the heart.
    • Sympathetic: origin in the lower cervical to upper thoracic spinal segments -> sympathetic chain ganglia -> cervical ganglia -> cardiac nerves to SA/AV nodes and myocardium.

Electrical Activity of the Heart

  • Systole and diastole definitions:
    • Systole: contraction.
    • Diastole: relaxation.
  • Normal rhythm:
    • Sinus rhythm: normal heartbeat generated by the SA node; adult average ~ 70-80\ \mathrm{bpm} (often attributed to vagal tone).
  • Ectopic focus: spontaneous firing from sites other than the SA node; HR may be 40-50 bpm or lower, which may not sustain life.
  • Pacemaker physiology (spontaneous depolarization):
    • No stable resting membrane potential; starts at about -60\ \mathrm{mV}; slow Na+ influx; when threshold reached, Na+ and Ca^{2+} channels open; peaks at action potential; K+ channels open, K+ leaves the cell; cycle repeats.
  • Impulse conduction timeline:
    • SA node fires, atrial myocardium depolarizes and contracts (atrial systole).
    • AV node fires and conduction slows (delay ~50 ms) to allow ventricular filling (ventricular diastole).
    • Fast conduction through AV bundle and Purkinje fibers leads to rapid ventricular depolarization and contraction (ventricular systole).
    • Ventricular systole: apex contracts first and ejects blood; rapid pressure rise closes AV valves; semilunar valves open.
  • Cardiac arrhythmias:
    • Ventricular fibrillation: random electrical activity; heart cannot pump blood; immediate defibrillation required.
    • Defibrillation: electrical shock to restore normal rhythm.
    • Premature ventricular contraction (PVC): extra beats from ventricular ectopic focus; may occur in healthy individuals or with disease.

Myocardial Electricity & Electrocardiogram (ECG)

  • Cardiomyocyte resting potential and action potentials:
    • Resting potential around -90\ \mathrm{mV}; depolarization occurs with stimulus; plateau phase lasts 200-250\ \mathrm{ms} to sustain contraction (Ca^{2+} influx); Ca^{2+} binds to troponin for cross-bridge cycling; repolarization occurs when Ca^{2+} channels close and K^{+} channels open; refractory period ~ 250\ \mathrm{ms}.
  • The electrocardiogram (ECG) components:
    • P wave: atrial depolarization and atrial systole.
    • QRS complex: ventricular depolarization and onset of ventricular systole.
    • S segment: corresponds to plateau phase of ventricular action potential.
    • T wave: ventricular repolarization and relaxation.
  • Cardiac cycle phases (sequence):
    • Ventricular filling: ventricles expand; AV valves open; passive filling occurs; end-diastolic volume (EDV) ~ 130\ \mathrm{mL}.
    • Atrial systole: atria contract to contribute to ventricular filling.
    • Isovolumetric contraction: ventricles depolarize; AV valves close; S1 heart sound occurs.
    • Ventricular ejection: semilunar valves open; rapid ejection followed by reduced ejection; end-systolic volume (ESV) ~ 60\ \mathrm{mL}; S2 heart sound.
    • Isovolumetric relaxation: ventricles expand; semilunar valves close; S2 occurs; cardiac cycle completes.
  • Cardiac cycle duration:
    • Approximately 0.8\ \mathrm{s} per cycle at about 75\ \mathrm{bpm}.
  • Cardiac arrhythmias and interventions (summary):
    • Ventricular fibrillation requires immediate defibrillation to restore rhythm.
    • Premature ventricular contractions (PVCs) reflect ectopic ventricular activity.

Pressure Gradients, Valvular Issues

  • Valve function is dictated by pressure gradients:
    • AV valves open when ventricular pressure is low during ventricular diastole; closes when ventricular pressure rises during systole.
    • Semilunar valves open when ventricular pressure exceeds arterial pressure; closed when arterial pressure exceeds ventricular pressure.
  • Valvular pathologies:
    • Valvular insufficiency (regurgitation): failure of valve to prevent reflux of blood.
    • Valvular stenosis: stiff cusps and fibrous tissue causing constricted opening.
    • Mitral valve prolapse: mitral valve cusps bulge into the left atrium; can cause chest pain and dyspnea.

The Cardiac Cycle & Heart Sounds

  • Auscultation basics:
    • First heart sound (S1): louder and longer; AV valves close; turbulence and wall movement.
    • Second heart sound (S2): softer and sharper; semilunar valves close; turbulence and wall movement.
    • S3: third heart sound; rare after age 30.
  • Phases of the cardiac cycle in order:
    • Ventricular filling: AV valves open; ventricles fill; end-diastolic volume ~ 130\ \mathrm{mL}.
    • Isovolumetric contraction (early systole): AV valves close; S1 occurs; no change in volume.
    • Ventricular ejection: semilunar valves open; rapid ejection, then reduced ejection; EDV changes to ESV (~60\ \mathrm{mL}).
    • Isovolumetric relaxation: ventricles relax; semilunar valves close; S2 occurs; ventricles fill again.
  • Cardiac cycle duration around 0.8\ \mathrm{s} at ~75\ \mathrm{bpm}.

Cardiac Cycle Overview and Some Clinical Concepts

  • Congestive heart failure (CHF): failure of one or both ventricles to eject blood effectively.
    • Left ventricular failure leads to pulmonary congestion and SOB.
    • Right ventricular failure leads to systemic venous congestion, hepatomegaly, edema.
  • Cardiac output (CO): amount ejected per minute; resting CO is typically about 4-6\ \mathrm{L/min}.
  • Cardiac reserve: difference between maximum possible CO and resting CO.
  • Pulse and heart rate norms:
    • Infants: around 120\ \mathrm{bpm}.
    • Young females: 72-80\ \mathrm{bpm}.
    • Young males: 64-72\ \mathrm{bpm}.
    • In elderly, resting rate tends to increase with age.
  • Tachycardia: heart rate > 100\ \mathrm{bpm} in adults.
  • Bradycardia: heart rate < 60\ \mathrm{bpm} in adults.
  • Chronotropic factors: factors that influence heart rate.

Autonomic Nervous System Regulation of the Heart

  • Intrinsic firing rate of the heart is around 100\ \mathrm{bpm}.
  • Vagal tone maintains resting HR around 70-80\ \mathrm{bpm}.
  • Baroreceptors (pressure sensors) in the aorta detect changes in blood pressure and adjust HR accordingly.
  • Chemoreceptors respond to changes in pH, CO2, and O2; they are more critical for respiration but influence heart rate as part of reflexes.
  • Stroke volume regulation involves:
    • Preload: the initial stretch of the cardiac muscle prior to contraction (related to venous return).
    • Contractility: strength of myocardial contraction; positive inotropes increase contractility.
    • Afterload: the pressure the heart must work against to eject blood; primarily in the aorta and pulmonary trunk.
  • Frank-Starling mechanism: the relationship between EDV and SV; commonly summarized as SV = EDV in the transcript context, with the standard formulation often written as SV = EDV - ESV.
  • Effects of afterload:
    • Higher afterload (e.g., hypertension) reduces the amount of blood ejected per beat and can impair ejection.
  • Exercise adaptations:
    • Regular exercise can lead to ventricular hypertrophy and changes in regulation to support increased cardiac output.