Chapter 17: The Cardiovascular System I – The Heart (Study Notes)

Overview of the Heart

Introduction to the Cardiovascular System

• Cardiovascular system consists of the heart, blood vessels, and blood. The heart pumps blood, which carries oxygen and nutrients, into blood vessels that distribute it throughout the body.

• Location: The heart is located slightly to the left in the thoracic cavity, posterior to the sternum, in the mediastinum, and rests on the diaphragm.

• Structure: The heart is a cone-shaped organ with an apex pointing toward the left hip and a base facing the posterior rib cage.

• Chambers: Four hollow chambers: right and left atria (superior), right and left ventricles (inferior).

• Blood Flow: Atria receive blood from veins; ventricles pump blood into arteries.

Functions of the Heart

• The heart pumps blood through two circuits:

  • Pulmonary circuit: Right side pumps deoxygenated blood to the lungs for gas exchange.

  • Systemic circuit: Left side pumps oxygenated blood to the rest of the body.

• Gas Exchange: Occurs in alveoli (lungs) and systemic capillaries (tissues).

• Pressure: Pulmonary circuit is low-pressure; systemic circuit is high-pressure.

• Blood Pressure Regulation: The heart helps maintain blood pressure and organ perfusion.

• Hormone Production: The atria produce atrial natriuretic peptide (ANP), which lowers blood pressure by decreasing sodium and water retention in the kidneys.

Heart Anatomy and Blood Flow Pathway

The Pericardium, Heart Wall, and Heart Skeleton

• Pericardium: Membranous sac surrounding the heart, consisting of:

  • Fibrous pericardium: Tough outer layer attaching heart to surrounding structures.

  • Serous pericardium: Thin inner membrane producing serous fluid; includes parietal (outer) and visceral (inner/epicardium) layers.

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

• Heart Wall Layers:

  • Epicardium (visceral pericardium): Outermost layer.

  • Myocardium: Middle layer of cardiac muscle cells and fibrous skeleton (structural support and electrical insulation).

  • Endocardium: Innermost layer, composed of endothelium.

Great Vessels, Chambers, and Valves

• Great Vessels: Superior vena cava, inferior vena cava, pulmonary trunk and veins, aorta.

• Chambers:

  • Atria: Right atrium (large, thin-walled, anterior); left atrium (smaller, thicker-walled, posterior). Each has an auricle for extra blood capacity. Separated by interatrial septum (with fossa ovalis, a remnant of fetal foramen ovale).

  • Ventricles: Larger, thicker-walled than atria. Right ventricle (wider, thinner walls); left ventricle (thicker walls, pumps against higher resistance). Separated by interventricular septum.

• Valves:

  • Atrioventricular (AV) valves: Tricuspid (right, three cusps), bicuspid/mitral (left, two cusps). Prevent backflow into atria; anchored by chordae tendineae to papillary muscles.

  • Semilunar valves: Pulmonary valve (right ventricle to pulmonary trunk), aortic valve (left ventricle to aorta). Prevent backflow into ventricles.

Coronary Circulation

• Coronary arteries: Left and right coronary arteries branch from ascending aorta. Left coronary artery branches into anterior interventricular (LAD) and circumflex arteries.

• Coronary veins: Great, small, and middle cardiac veins drain into the coronary sinus, which empties into the right atrium.

• Coronary artery disease (CAD): Plaque buildup can lead to myocardial infarction (heart attack), causing tissue death. Symptoms include chest pain radiating to the left arm/neck, shortness of breath, sweating, anxiety, nausea, and vomiting.

Cardiac Muscle Tissue Anatomy and Electrophysiology

Cardiac Muscle Cell Properties

• Autorhythmicity: Cardiac muscle cells can set their own rhythm without nervous input.

• Types of cells: Pacemaker cells (1%) generate action potentials; contractile cells (99%) contract in response.

• Histology: Cardiac muscle cells are striated (alternating light/dark bands).

Cardiac Conduction System

• Pacemaker cells: Three populations:

  • Sinoatrial (SA) node: Upper right atrium; fastest rate (~60+ bpm); normal pacemaker (sinus rhythm).

  • Atrioventricular (AV) node: Posterior/medial to tricuspid valve; slower (~40 bpm); AV node delay allows ventricular filling.

  • Purkinje fiber system: Slowest (~20 bpm); includes AV bundle, bundle branches, and terminal branches.

• Conduction pathway: SA node → atrial cells → AV node (delay) → AV bundle → bundle branches → Purkinje fibers → ventricular muscle cells.

Action Potentials

• Pacemaker cell action potential phases:

  1. Slow initial depolarization (nonspecific cation channels open).

  2. Full depolarization (L-type Ca2+ channels open).

  3. Repolarization (K+ channels open).

  4. Minimum potential (cycle restarts).

• Contractile cell action potential phases:

  1. Rapid depolarization (Na+ influx).

  2. Initial repolarization (Na+ channels inactivate, brief K+ outflow).

  3. Plateau (slow Ca2+ influx).

  4. Repolarization (K+ outflow restores resting potential).

• Excitation-contraction coupling: Similar to skeletal muscle; contraction via sliding-filament mechanism.

Electrocardiogram (ECG/EKG)

• Measures: Electrical activity of all cardiac muscle cells over time.

• Waves:

  • P wave: Atrial depolarization.

  • QRS complex: Ventricular depolarization (Q: down, R: up, S: down).

  • T wave: Ventricular repolarization.

• Intervals/Segments: R-R, P-R, Q-T intervals; S-T segment.

• Dysrhythmia/Arrhythmia: Disturbance in electrical rhythm.

Mechanical Physiology of the Heart: The Cardiac Cycle

Cardiac Cycle Overview

• Cardiac cycle: Sequence of events from one heartbeat to the next; includes contraction (systole) and relaxation (diastole) of chambers.

• Blood flow: Driven by pressure gradients; valves prevent backflow.

• Heart sounds: S1 ("lub", AV valves close), S2 ("dub", semilunar valves close).

Phases of the Cardiac Cycle

1. Ventricular filling phase: Blood drains from atria to ventricles; AV valves open, semilunar valves closed. Atrial systole tops off ventricular volume (end-diastolic volume, EDV ~130 ml).

2. Isovolumetric contraction phase: Ventricles begin to contract; AV valves close (S1), semilunar valves still closed; volume unchanged.

3. Ventricular ejection phase: Ventricular pressure exceeds arterial pressure; semilunar valves open, blood ejected (~50 ml remains, end-systolic volume, ESV).

4. Isovolumetric relaxation phase: Ventricles relax; semilunar valves close (S2), AV valves still closed; volume unchanged.

• Wiggers diagram: Illustrates timing of electrical, mechanical, pressure, and volume changes in the cardiac cycle.

Cardiac Output and Regulation

Cardiac Output (CO)

• Definition: Amount of blood pumped by each ventricle per minute.

• Formula: $CO = HR \times SV$

• Example Calculation: $72\ \text{beats/min} \times 70\ \text{ml/beat} = 5040\ \text{ml/min} \approx 5\ \text{L/min}$

• Stroke Volume (SV): Amount of blood pumped per beat.

  • Calculated as: $SV = EDV - ESV$

  • Example: $120\ \text{ml} - 50\ \text{ml} = 70\ \text{ml}$

• Ejection Fraction: Percentage of EDV ejected per beat: $\text{Ejection Fraction} = \frac{SV}{EDV} \times 100\%$ (Normal: 50–65%)

Factors Influencing Stroke Volume

• Preload: Degree of stretch of ventricular cells before contraction; determined by EDV.

• Contractility: Heart's intrinsic ability to generate tension; increased contractility increases SV, decreases ESV.

• Afterload: Force ventricles must overcome to eject blood; determined by arterial blood pressure.

Factors Influencing Heart Rate

• Chronotropic agents: Factors that change SA node firing rate.

• Autonomic Nervous System (ANS):

  • Sympathetic: Norepinephrine increases HR and contractility (positive chronotropic and inotropic effects).

  • Parasympathetic: Vagus nerve (CN X) releases acetylcholine, decreasing HR (negative chronotropic effect).

• Endocrine Regulation: Hormones (thyroid hormone, glucagon, aldosterone, antidiuretic hormone, ANP) affect cardiac output.

• Other Factors: Electrolyte balance, temperature, age, fitness, exercise.

Heart Failure

• Definition: Any condition reducing the heart's pumping effectiveness; may affect left or right ventricle.

• Treatment: Lifestyle changes, medications, surgery to increase cardiac output.

Summary Table: Heart Valves and Their Locations

Valve	Location	Function
Tricuspid (Right AV)	Between right atrium and right ventricle	Prevents backflow into right atrium
Bicuspid/Mitral (Left AV)	Between left atrium and left ventricle	Prevents backflow into left atrium
Pulmonary Semilunar	Between right ventricle and pulmonary trunk	Prevents backflow into right ventricle
Aortic Semilunar	Between left ventricle and aorta	Prevents backflow into left ventricle

Key Equations

• Cardiac Output: $CO = HR \times SV$

• Stroke Volume: $SV = EDV - ESV$

• Ejection Fraction: $\text{Ejection Fraction} = \frac{SV}{EDV} \times 100\%$

Example Application

• If a patient has an EDV of 130 ml and an ESV of 60 ml, their stroke volume is $130 - 60 = 70$ ml. If their heart rate is 75 bpm, their cardiac output is $75 \times 70 = 5250$ ml/min or 5.25 L/min.

Additional info: This guide expands on the provided outline with definitions, examples, and formulas for clarity and exam preparation.