Chapter 17: The Cardiovascular System I – The Heart

Functions of the Heart

The heart is a muscular organ responsible for pumping blood throughout the body, ensuring the delivery of oxygen and nutrients and the removal of waste products.

• Pumping blood through the pulmonary and systemic circuits

• Maintaining blood pressure within vessels

• Ensuring one-way flow via valves

• Regulating blood supply based on metabolic needs

Location and Size of the Human Heart

The heart is located in the mediastinum, between the lungs, slightly left of the midline. It is roughly the size of a fist and weighs about 250-350 grams in adults.

• Base: Superior portion, where major vessels attach

• Apex: Inferior, pointed end, directed leftward

Layers of the Pericardium

The pericardium is a double-walled sac surrounding the heart, providing protection and anchoring.

• Fibrous pericardium: Tough, outer layer

• Serous pericardium: Inner layer, subdivided into:

  • Parietal layer: Lines the fibrous pericardium

  • Visceral layer (epicardium): Covers the heart surface

Layers of the Heart Wall

The heart wall consists of three layers, each with distinct structure and function.

• Epicardium: Outer layer, also the visceral pericardium

• Myocardium: Middle, muscular layer responsible for contraction

• Endocardium: Inner layer, lines chambers and valves

Chambers, Valves, and Great Vessels of the Heart

The heart has four chambers and several valves that ensure unidirectional blood flow.

• Chambers: Right atrium, right ventricle, left atrium, left ventricle

• Valves: Tricuspid, bicuspid (mitral), pulmonary semilunar, aortic semilunar

• Great vessels: Superior/inferior vena cava, pulmonary arteries/veins, aorta

Atrioventricular (A-V) Valves vs. Semilunar Valves

A-V valves and semilunar valves differ in structure and function.

• A-V valves: Located between atria and ventricles; prevent backflow into atria

• Semilunar valves: Located at bases of large arteries; prevent backflow into ventricles

• Physiology: A-V valves use chordae tendineae and papillary muscles; semilunar valves do not

Key Terms: Cardiac Structures

• Papillary muscles: Muscles in ventricles that anchor chordae tendineae

• Chordae tendineae: Tendinous cords connecting A-V valves to papillary muscles

• Pectinate muscles (musculi pectinati): Ridges in atrial walls

• Trabeculae carneae: Irregular muscular columns in ventricles

Pulmonary Circuit vs. Systemic Circuit

The heart pumps blood through two distinct circuits.

• Pulmonary circuit: Right side of heart; sends deoxygenated blood to lungs

• Systemic circuit: Left side of heart; sends oxygenated blood to body tissues

Blood Flow Through the Heart

Blood follows a specific pathway through the heart chambers and valves.

1. Deoxygenated blood enters right atrium via vena cava

2. Passes through tricuspid valve to right ventricle

3. Pumped through pulmonary valve to pulmonary arteries and lungs

4. Oxygenated blood returns via pulmonary veins to left atrium

5. Passes through mitral valve to left ventricle

6. Pumped through aortic valve to aorta and systemic circulation

Coronary Circulation

The myocardium receives blood via coronary arteries, which branch from the aorta.

• Coronary arteries: Supply oxygenated blood to heart tissue

• Coronary veins: Remove deoxygenated blood from myocardium

Structure and Function of Cardiac Muscle Cells

Cardiac muscle cells are striated, branched, and interconnected by intercalated discs, allowing synchronized contraction.

• Intercalated discs: Specialized junctions for electrical and mechanical coupling

• Involuntary contraction: Controlled by intrinsic and extrinsic mechanisms

Intrinsic Conducting System of the Heart

The heart's intrinsic conducting system coordinates contraction.

• Sinoatrial (SA) node: Pacemaker, initiates impulse

• Atrioventricular (AV) node: Delays impulse, allows atrial contraction

• AV bundle (Bundle of His): Conducts impulse to ventricles

• Bundle branches and Purkinje fibers: Distribute impulse throughout ventricles

Electrocardiogram (ECG/EKG)

An ECG records the electrical activity of the heart, showing depolarization and repolarization events.

• P wave: Atrial depolarization

• QRS complex: Ventricular depolarization

• T wave: Ventricular repolarization

Abnormal ECG Traces

Abnormal ECGs can indicate arrhythmias, myocardial infarction, or conduction blocks.

• Example: Absence of P wave may indicate atrial fibrillation

• Example: Elevated ST segment may indicate myocardial infarction

The Cardiac Cycle

The cardiac cycle consists of all events associated with blood flow through the heart during one heartbeat.

1. Atrial systole: Atria contract, blood moves to ventricles

2. Ventricular systole: Ventricles contract, blood moves to arteries

3. Diastole: Chambers relax, fill with blood

Heart Sounds

Heart sounds are produced by the closing of valves.

• "Lub" (S1): Closure of A-V valves

• "Dub" (S2): Closure of semilunar valves

Key Terms: Cardiac Physiology

• Cardiac output: Volume of blood pumped per minute ()

• Preload: Degree of stretch of cardiac muscle before contraction

• Afterload: Pressure the heart must overcome to eject blood

• Stroke volume: Volume of blood ejected per beat

• Systole: Contraction phase

• Diastole: Relaxation phase

• Heart rate: Beats per minute

• Venous return: Amount of blood returning to heart

Intrinsic vs. Extrinsic Controls of the Heart

Heart rate and force are regulated by intrinsic (within the heart) and extrinsic (external) mechanisms.

• Intrinsic: SA node, Starling Law

• Extrinsic: Autonomic nervous system (ANS), hormones (e.g., epinephrine)

Mechanisms Controlling Heart Rate

• ANS: Sympathetic increases, parasympathetic decreases heart rate

• Hormones: Epinephrine, thyroxine increase heart rate

• Other factors: Temperature, ions, fitness

Clinical Terms and Conditions

• Endocarditis: Inflammation of the endocardium

• Ectopic pacemaker: Abnormal pacemaker outside SA node

• Pericarditis: Inflammation of the pericardium

• Angina pectoris: Chest pain due to reduced blood flow

• Myocardial infarction: Heart attack, death of heart tissue

• Myocardial ischemia: Reduced blood supply to heart

• Cardiac tamponade: Compression of heart by fluid in pericardium

• Stenosis of a valve: Narrowing of heart valve

• Coronary thrombosis: Blood clot in coronary artery

• Valve insufficiency: Incomplete closure of valve

• Heart failure: Inability of heart to pump effectively

• Heart block: Impaired conduction between atria and ventricles

• Heart murmur: Abnormal heart sound

• Tachycardia: Rapid heart rate

• Bradycardia: Slow heart rate

• Fibrillation: Uncoordinated contraction

• Coronary angiography: Imaging of coronary arteries

• Coronary angioplasty: Procedure to open blocked arteries

• Calcium channel blockers: Drugs to reduce heart workload

• Starling Law: Greater stretch leads to greater contraction

• PVC (premature ventricular contraction): Early ventricular beat

• VT (ventricular tachycardia): Rapid ventricular rhythm

Clinical significance of multiple PVCs and VT: May indicate serious arrhythmias, risk of sudden cardiac death.