The Cardiovascular System

Overview of the Cardiovascular System

The cardiovascular system is responsible for transporting blood throughout the body, delivering oxygen and nutrients, and removing waste products. It consists of the heart, blood, and blood vessels.

• Heart: Muscular organ that pumps blood; beats about 100,000 times per day, moving approximately 8,000 liters of blood.

• Blood: Fluid connective tissue that transports gases, nutrients, hormones, and wastes.

• Blood Vessels: Network of tubes (arteries, veins, capillaries) that carry blood throughout the body.

Essential Anatomy of the Heart

Pulmonary and Systemic Circuits

The heart pumps blood through two main circuits:

• Pulmonary Circuit: Carries blood to and from the gas exchange surfaces of the lungs.

• Systemic Circuit: Carries blood to and from the rest of the body.

• Each circuit begins and ends at the heart, and 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: Smallest vessels; sites of exchange of gases, nutrients, and wastes between blood and tissues.

Chambers of the Heart

• Right Atrium: Receives blood from the systemic circuit.

• Right Ventricle: Pumps blood into the pulmonary circuit.

• Left Atrium: Receives blood from the pulmonary circuit.

• Left Ventricle: Pumps blood into the systemic circuit.

Heart Location and Structure

• The heart is located in the mediastinum, between the two pleural cavities.

• The base is the superior part where great vessels connect; the apex is the pointed inferior tip.

Pericardium

• Pericardium: Double-walled sac surrounding the heart.

• Fibrous Pericardium: Outer tough layer.

• Serous Pericardium: Inner layer with parietal (outer) and visceral (inner, also called epicardium) layers.

• Pericardial Cavity: Space between parietal and visceral layers, containing pericardial fluid to reduce friction.

• Pericarditis: Inflammation of the pericardium, causing friction and possibly cardiac tamponade (restricted heart movement due to fluid accumulation).

Layers of the Heart Wall

• Epicardium: Outer layer (visceral pericardium).

• Myocardium: Middle, muscular layer composed of cardiac muscle tissue.

• Endocardium: Inner layer of simple squamous epithelium and areolar tissue.

Connective Tissues and Cardiac Skeleton

• Support cardiac muscle fibers, blood vessels, and nerves.

• Distribute contraction forces, add strength, prevent overexpansion, and provide elasticity.

• Cardiac Skeleton: Four dense bands of elastic tissue encircling valves and bases of pulmonary trunk and aorta; stabilize valves and electrically insulate atria from ventricles.

Internal Anatomy and Organization

• Chambers separated by septa: Interatrial septum (between atria) and interventricular septum (between ventricles; thicker).

• Atrioventricular (AV) Valves: Tricuspid (right) and mitral/bicuspid (left); prevent backflow from ventricles to atria.

• Semilunar Valves: Pulmonary and aortic; prevent backflow into ventricles.

Blood Flow Through the Heart

• Right atrium receives blood from superior and inferior vena cava.

• Blood flows through tricuspid valve to right ventricle, then through pulmonary valve to pulmonary trunk and arteries.

• Oxygenated blood returns via pulmonary veins to left atrium, passes through mitral valve to left ventricle, then through aortic valve to aorta.

Special Features

• Foramen Ovale: Fetal opening between atria; closes at birth to become fossa ovalis.

• Pectinate Muscles: Muscular ridges in atria.

• Trabeculae Carneae: Muscular ridges in ventricles.

• Moderator Band: Muscular ridge in right ventricle; conducts electrical signals.

Heart Valves and Disease

• Valves ensure one-way blood flow; AV valves have chordae tendineae and papillary muscles to prevent backflow.

• Valvular Heart Disease (VHD): Deterioration of valve function, often after carditis or rheumatic fever.

Coronary Circulation

Blood Supply to the Heart

• Coronary Arteries: Originate at aortic sinuses; right and left coronary arteries supply heart muscle.

• Right coronary artery supplies right atrium, parts of both ventricles, and conduction system; gives rise to marginal and posterior interventricular arteries.

• Left coronary artery supplies left atrium, left ventricle, and interventricular septum; gives rise to circumflex and anterior interventricular arteries.

• Arterial Anastomoses: Connections between arteries to maintain constant blood supply.

• Cardiac Veins: Great cardiac vein, middle cardiac vein, small cardiac vein, and others drain blood into coronary sinus and right atrium.

Coronary Artery Disease (CAD) and Myocardial Infarction (MI)

• CAD: Partial or complete blockage of coronary arteries, usually by atherosclerotic plaque or thrombus.

• Coronary Ischemia: Reduced blood supply to heart muscle.

• Angina Pectoris: Chest pain due to temporary ischemia, often triggered by exertion or stress.

• Myocardial Infarction (MI): Heart attack; death of cardiac muscle cells due to prolonged ischemia, often from coronary thrombosis.

• Diagnosis: ECG and blood tests for cardiac enzymes (troponin T, troponin I, CK-MB).

Treatment of CAD and MI

• Risk factor modification: stop smoking, control blood pressure, diet, exercise, reduce stress.

• Drug treatments: anticoagulants (aspirin, coumadin), beta-blockers, vasodilators (nitroglycerin), calcium channel blockers, pain relief, clot dissolution.

• Noninvasive surgery: atherectomy (removal of plaque), balloon angioplasty (balloon to compress plaque, often with stent placement).

• Coronary artery bypass graft (CABG): rerouting blood around blocked arteries using vessel grafts.

The Conducting System

Cardiac Muscle Cell Types

• Autorhythmic Cells: Control and coordinate heartbeat (pacemaker and conducting cells).

• Contractile Cells: Produce contractions that propel blood.

Components of the Conducting System

• Pacemaker Cells: Located in the sinoatrial (SA) node (right atrium wall) and atrioventricular (AV) node (junction of atria and ventricles).

• Conducting Cells: Found in internodal pathways, AV bundle, bundle branches, and Purkinje fibers.

Pacemaker Potential and Sinus Rhythm

• Pacemaker cells have unstable resting membrane potentials, leading to spontaneous depolarization.

• SA node: 60–100 action potentials/min; AV node: 40–60/min.

• SA node sets the pace (sinus rhythm); parasympathetic stimulation slows heart rate.

Impulse Conduction Through the Heart

1. SA node activity and atrial activation begin.

2. Stimulus spreads across atria to AV node.

3. Impulse delayed at AV node (100 ms); atrial contraction begins.

4. Impulse travels via AV bundle to bundle branches and Purkinje fibers; papillary muscles activated via moderator band.

5. Purkinje fibers distribute impulse to ventricular myocardium; ventricular contraction begins.

Disturbances in Heart Rhythm

• Bradycardia: Abnormally slow heart rate.

• Tachycardia: Abnormally fast heart rate.

• Ectopic Pacemaker: Abnormal cells generate rapid action potentials, disrupting normal rhythm.

Electrocardiogram (ECG/EKG)

• Records electrical events in the heart using surface electrodes.

• Key features:

  • P wave: Atrial depolarization.

  • QRS complex: Ventricular depolarization (ventricles contract after R wave).

  • T wave: Ventricular repolarization.

  • P–R interval: Start of atrial depolarization to start of QRS complex.

  • Q–T interval: Time for ventricles to depolarize and repolarize.

Cardiac Contractile Cells

• Form bulk of atrial and ventricular walls; receive stimulus from Purkinje fibers.

• Resting membrane potential: –90 mV (ventricular), –80 mV (atrial).

• Connected by intercalated discs (desmosomes and gap junctions) for force transmission and action potential propagation.

• Characteristics: small size, single central nucleus, branching, intercalated discs.

Action Potential in Cardiac Contractile Cells

1. Rapid Depolarization: Na+ influx through fast sodium channels.

2. Plateau: Ca2+ influx through slow calcium channels.

3. Repolarization: K+ efflux through slow potassium channels.

• Refractory Period: Absolute (200 ms, no response possible) and relative (50 ms, response only to strong stimuli).

• Total action potential duration: 250–300 ms (prevents tetany).

Role of Calcium Ions in Cardiac Contractions

• Extracellular Ca2+ entry during plateau provides ~20% of Ca2+ for contraction.

• Triggers additional Ca2+ release from sarcoplasmic reticulum (SR).

• Cardiac muscle is sensitive to extracellular Ca2+ levels.

• After contraction, Ca2+ is pumped back into SR or out of the cell.

Energy for Cardiac Contractions

• Primarily aerobic metabolism (mitochondrial breakdown of fatty acids and glucose).

• Oxygen delivered by blood; myoglobin stores oxygen in cells.

The Cardiac Cycle

Phases of the Cardiac Cycle

The cardiac cycle is the sequence of events from the start of one heartbeat to the next, including contraction (systole) and relaxation (diastole) of the heart chambers.

• Blood pressure rises during systole and falls during diastole.

• Blood flows from high to low pressure, controlled by timing of contractions and one-way valves.

• At 75 bpm, the cardiac cycle lasts about 800 ms; higher heart rates shorten all phases, especially diastole.

Detailed Phases

1. Atrial Systole: Atria contract, AV valves open, blood moves into ventricles.

2. Ventricular Systole (Atrial Diastole): Atria relax, ventricles contract, AV valves close (isovolumetric contraction), then semilunar valves open for ventricular ejection (stroke volume).

3. Ventricular Diastole: Semilunar valves close, ventricles relax (isovolumetric relaxation), AV valves open, ventricles fill passively.

• End-Diastolic Volume (EDV): Maximum blood in ventricles after filling.

• End-Systolic Volume (ESV): Blood remaining after contraction (~40% of EDV).

Heart Sounds

• S1: AV valves close ("lub").

• S2: Semilunar valves close ("dub").

• S3, S4: Softer sounds from blood flow and atrial contraction.

• Heart Murmur: Abnormal sounds from valve regurgitation.

Cardiac Output

Definition and Formula

• Cardiac Output (CO): Volume of blood pumped by the left ventricle per minute.

Formula:

• = cardiac output (mL/min)

• = heart rate (beats/min)

• = stroke volume (mL/beat)

Stroke Volume (SV)

• Stroke Volume: Amount of blood ejected by a ventricle in one beat.

Formula:

• Ejection Fraction: Percentage of EDV ejected during contraction.

Factors Affecting Heart Rate

• Autonomic Innervation: Sympathetic (increases HR) and parasympathetic (decreases HR) fibers via cardiac plexus and vagus nerves.

• Cardiac Centers: Medulla oblongata contains cardioacceleratory (sympathetic) and cardioinhibitory (parasympathetic) centers.

• Cardiac Reflexes: Baroreceptors (blood pressure) and chemoreceptors (O2/CO2) adjust cardiac activity.

• Autonomic Tone: Balance of sympathetic and parasympathetic activity; ACh decreases HR, NE increases HR.

• Bainbridge Reflex: Increased venous return stretches right atrium, increasing HR via sympathetic stimulation.

• Hormones: Epinephrine, norepinephrine, and thyroid hormone increase HR.

Factors Affecting Stroke Volume

• EDV (End-Diastolic Volume): Affected by filling time and venous return.

• Preload: Degree of ventricular stretch at end of diastole; directly proportional to EDV.

• Frank–Starling Principle: As EDV increases, stroke volume increases (within physical limits).

• ESV (End-Systolic Volume): Affected by preload, contractility, and afterload.

• Contractility: Force produced during contraction at a given preload; increased by sympathetic stimulation and hormones, decreased by parasympathetic stimulation.

• Afterload: Tension needed to open semilunar valves and eject blood; increased afterload decreases stroke volume.

Summary Table: Factors Affecting Cardiac Output

Cardiac Reserve

• Difference between resting and maximal cardiac output; indicates the heart's ability to increase output during increased activity or stress.

Cardiovascular Regulation

• Ensures adequate blood flow to tissues; cardiac centers adjust heart rate and vessel diameter in response to activity and emergencies.

Example: Application of Cardiac Output Formula

• If heart rate (HR) is 70 beats/min and stroke volume (SV) is 70 mL/beat:

mL/min (or 4.9 L/min)

Additional info: The above notes include expanded explanations, definitions, and examples for clarity and completeness, as would be expected in a mini-textbook study guide for college-level Anatomy & Physiology students.