Chapter 14 – Cardiovascular System: The Heart (Anatomy & Physiology Study Notes)

Chapter 14 – Cardiovascular System: The Heart

Overview and Key Questions

This chapter introduces the structure and function of the heart as part of the cardiovascular system (CVS). It covers the anatomy of the heart, the properties of cardiac muscle cells, and the heart's role as a pump. Key questions include the heart's location, its ability to function independently, and its physiological limits.

• Where is your heart, exactly? – The heart is located in the thoracic cavity, slightly left of the midline, between the lungs.

• Can the heart function without external stimulation? – Yes, due to its autorhythmic cells.

• How much work does the heart do? – The heart beats about 100,000 times per day, pumping approximately 8,000 liters of blood daily.

Introduction to the Cardiovascular System

Functions and Components

The cardiovascular system is responsible for transporting materials throughout the body, including nutrients, gases, hormones, immune cells, and waste products.

• Major components: Heart, blood vessels (arteries, veins, capillaries), and blood.

• Heart: Divided by a septum into right and left halves; each half has an atrium (receives blood) and a ventricle (pumps blood out).

• Blood vessels: Arteries carry blood away from the heart; veins return blood to the heart; capillaries allow exchange between blood and tissues.

Pressure, Volume, Flow, and Resistance

Basic Principles

Blood flow in the cardiovascular system is driven by pressure gradients created by the heart's contractions.

• Pressure gradient (ΔP): Blood flows from regions of higher pressure to lower pressure.

• Driving pressure: Generated by ventricular contraction.

• Flow equation:

$\text{Flow} = \frac{\Delta P}{R}$

• R (Resistance): Opposition to flow, mainly determined by vessel diameter, length, and blood viscosity.

Anatomy of the Heart

External and Internal Structure

The heart is a muscular organ with four chambers and several layers of tissue.

• Pericardium: Double-walled sac surrounding the heart.

• Layers:

  • Fibrous pericardium (outermost)

  • Parietal pericardium (outer serous layer)

  • Visceral pericardium (epicardium) (inner serous layer)

  • Pericardial cavity: Space between parietal and visceral layers, contains pericardial fluid for lubrication.

• Heart wall layers:

  • Epicardium: Outer layer, also the visceral pericardium.

  • Myocardium: Middle, thick muscular layer responsible for contraction.

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

• Cardiac skeleton: Dense connective tissue supporting valves and insulating electrical impulses between atria and ventricles.

Valves of the Heart

Valves ensure unidirectional blood flow through the heart.

• Atrioventricular (AV) valves: Between atria and ventricles (tricuspid on right, bicuspid/mitral on left).

• Semi-lunar valves: At exits of ventricles (pulmonary and aortic valves).

• Chordae tendineae and papillary muscles: Prevent AV valve prolapse during ventricular contraction.

Coronary Circulation

The heart muscle (myocardium) receives its own blood supply via the coronary arteries.

• Coronary arteries: Originate from the aorta; include right and left coronary arteries, circumflex artery, and anterior interventricular (LAD) artery.

• Coronary veins: Collect deoxygenated blood and return it to the right atrium via the coronary sinus.

Clinical Application: Coronary Artery Disease (CAD)

• CAD: Caused by atherosclerotic plaque buildup, leading to reduced blood flow (ischemia).

• Angina pectoris: Chest pain due to temporary ischemia.

• Myocardial infarction (MI): Heart attack caused by prolonged blockage and tissue death.

• Treatments: Lifestyle modification, medications (anticoagulants, beta blockers, vasodilators), angioplasty, and coronary artery bypass graft (CABG) surgery.

Cardiac Muscle Cells

Types and Structure

• Autorhythmic cells: Pacemaker cells that generate and conduct electrical impulses; do not contract.

• Contractile cells: Make up most of the myocardium; responsible for contraction.

• Key features: Single nucleus, branched, connected by intercalated discs (desmosomes and gap junctions), abundant mitochondria.

Excitation-Contraction Coupling

• Calcium-induced calcium release: Influx of Ca2+ through L-type channels triggers further Ca2+ release from the sarcoplasmic reticulum via ryanodine receptors.

• Relaxation: Ca2+ is pumped back into the SR and out of the cell via the Na+/Ca2+ exchanger (NCX).

• Force of contraction: Depends on the number of active crossbridges (amount of Ca2+ bound to troponin) and sarcomere length.

Electrical Activity of the Heart

Action Potentials in Cardiac Cells

• Contractile cells:

  • Phase 4: Resting membrane potential

  • Phase 0: Rapid depolarization (Na+ influx)

  • Phase 1: Initial repolarization (Na+ channels close)

  • Phase 2: Plateau (Ca2+ influx balances K+ efflux)

  • Phase 3: Rapid repolarization (K+ efflux)

• Autorhythmic cells:

  • No stable resting potential; gradual depolarization (pacemaker potential) due to open If (funny) channels (Na+ influx).

  • Depolarization: Ca2+ influx through voltage-gated channels.

  • Repolarization: K+ efflux.

Conduction System of the Heart

• Sinoatrial (SA) node: Primary pacemaker, initiates each heartbeat.

• Atrioventricular (AV) node: Delays impulse, allowing atrial contraction before ventricular contraction.

• AV bundle (Bundle of His), bundle branches, Purkinje fibers: Rapidly conduct impulses through ventricles.

• Autorhythmicity: Heart can contract without neural or hormonal input.

Electrocardiogram (ECG/EKG)

• P wave: Atrial depolarization

• QRS complex: Ventricular depolarization

• T wave: Ventricular repolarization

• P-R interval: Start of atrial depolarization to start of QRS

• Q-T interval: Duration of ventricular depolarization and repolarization

The Heart as a Pump

Cardiac Cycle

The cardiac cycle consists of alternating periods of contraction (systole) and relaxation (diastole) in the atria and ventricles.

• Systole: Contraction phase; pressure rises, blood is ejected.

• Diastole: Relaxation phase; pressure falls, chambers fill with blood.

• Phases: Atrial systole, atrial diastole, ventricular systole, ventricular diastole.

Pressure and Volume Changes

• Blood flows from high to low pressure.

• Valves: Ensure one-way flow; open and close in response to pressure changes.

• Elastic arteries: Act as pressure reservoirs, maintaining flow during diastole.

Cardiac Output

• Definition: Volume of blood pumped by the left ventricle per minute.

• Equation:

$\text{CO} = \text{HR} \times \text{SV}$

• CO: Cardiac output (mL/min)

• HR: Heart rate (beats/min)

• SV: Stroke volume (mL/beat)

• Stroke volume equation: $\text{SV} = \text{EDV} - \text{ESV}$

• EDV: End-diastolic volume (volume in ventricle at end of diastole)

• ESV: End-systolic volume (volume in ventricle at end of systole)

• Ejection fraction: Percentage of EDV ejected per beat

Regulation of Heart Function

Autonomic Control of Heart Rate

• Parasympathetic (vagus nerve): Decreases heart rate by increasing K+ permeability and decreasing Ca2+ permeability in pacemaker cells.

• Sympathetic: Increases heart rate by increasing Na+ and Ca2+ permeability.

• Resting heart rate: Normally dominated by parasympathetic activity.

Factors Affecting Stroke Volume

• Preload: Degree of stretch of cardiac muscle fibers at end of diastole (proportional to EDV).

• Frank-Starling Law: Stroke volume increases with increased EDV.

• Contractility: Strength of contraction at a given fiber length; increased by positive inotropic agents (e.g., epinephrine, norepinephrine).

• Afterload: Resistance the ventricle must overcome to eject blood (mainly arterial blood pressure).

Summary Table: Factors Affecting Cardiac Output

Factors Affecting Heart Rate (HR)	Factors Affecting Stroke Volume (SV)
Autonomic innervation Hormones Fitness level Age	End-diastolic volume (EDV) End-systolic volume (ESV) Preload Contractility Afterload

Key Terms

• Autorhythmicity: The heart's ability to generate its own rhythm.

• Ischemia: Reduced blood supply to tissues.

• Inotropic agent: Substance that affects contractility of the heart.

• Pacemaker potential: Gradual depolarization in autorhythmic cells leading to action potential.

Additional info: Some diagrams and tables referenced in the original notes have been summarized or described in text for clarity. For a more detailed understanding, students should refer to textbook figures and clinical case studies.