The Cardiovascular System: The Heart (Chapter 18 Study Notes)

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The Cardiovascular System: The Heart

Overview

The heart is a muscular organ that pumps blood throughout the body via the pulmonary and systemic circuits. This chapter covers the anatomy of the heart, the flow of blood, the structure and function of cardiac muscle, and the physiological events that regulate heart function.

Anatomy of the Heart

Location and Structure

Location: The heart is located in the mediastinum, between the lungs, behind the sternum, and above the diaphragm.
Size: Roughly the size of a fist.
Orientation: The apex points down and to the left; the base is superior and to the right.

External Anatomy

Major Vessels: Superior vena cava, inferior vena cava, aorta, pulmonary trunk, pulmonary veins.
Coronary Vessels: Right and left coronary arteries, great cardiac vein, coronary sinus.
Chambers: Right atrium, left atrium, right ventricle, left ventricle.

Heart Wall Layers

Epicardium: Outer layer; also called the visceral layer of the serous pericardium.
Myocardium: Middle layer; composed of cardiac muscle responsible for contraction.
Endocardium: Inner layer; lines the heart chambers and covers the valves.

Pericardium

Fibrous Pericardium: Dense connective tissue that protects, anchors, and prevents overfilling.
Serous Pericardium: Double-layered membrane (parietal and visceral layers) with a pericardial cavity filled with serous fluid to reduce friction.

Heart Chambers and Valves

Atria (Receiving Chambers)

Function: Receive blood returning to the heart and push it into the ventricles.
Structure: Small, thin-walled; contain auricles to increase volume.
Right Atrium: Receives blood from the superior vena cava, inferior vena cava, and coronary sinus.
Left Atrium: Receives blood from four pulmonary veins.

Ventricles (Discharging Chambers)

Function: Pump blood out of the heart.
Structure: Thick-walled, especially the left ventricle.
Right Ventricle: Pumps blood to the pulmonary circuit via the pulmonary trunk.
Left Ventricle: Pumps blood to the systemic circuit via the aorta.

Heart Valves

Atrioventricular (AV) Valves: Prevent backflow into atria when ventricles contract.
- Tricuspid Valve: Right AV valve (three flaps).
- Mitral (Bicuspid) Valve: Left AV valve (two flaps).
- Chordae Tendineae: Collagen cords anchoring valves to papillary muscles.
Semilunar (SL) Valves: Prevent backflow into ventricles.
- Pulmonary Valve: Right ventricle to pulmonary trunk.
- Aortic Valve: Left ventricle to aorta.

Blood Flow Through the Heart

Pulmonary and Systemic Circuits

Pulmonary Circuit: Right side of heart pumps oxygen-poor blood to lungs for gas exchange.
Systemic Circuit: Left side of heart pumps oxygen-rich blood to all body tissues.

Pathway of Blood

Blood enters right atrium from superior/inferior vena cava and coronary sinus.
Passes through tricuspid valve into right ventricle.
Pumped through pulmonary valve into pulmonary trunk and arteries to lungs.
Returns via pulmonary veins to left atrium.
Passes through mitral valve into left ventricle.
Pumped through aortic valve into aorta and systemic circulation.

Coronary Circulation

Arterial Supply

Right and Left Coronary Arteries: Branch from the aorta and supply the heart muscle.
Left Coronary Artery: Divides into anterior interventricular artery and circumflex artery.
Right Coronary Artery: Divides into right marginal artery and posterior interventricular artery.

Venous Drainage

Coronary Sinus: Collects blood from cardiac veins and empties into right atrium.
Great Cardiac Vein: Runs along anterior interventricular sulcus.
Middle Cardiac Vein: Runs along posterior interventricular sulcus.
Small Cardiac Vein: Runs along right inferior margin.
Anterior Cardiac Veins: Drain directly into right atrium.

Cardiac Muscle and Contraction

Structure of Cardiac Muscle

Striated, involuntary muscle with intercalated discs containing gap junctions and desmosomes.
Autorhythmicity: Cardiac cells can self-initiate depolarization and contraction.
Functional Syncytium: All cardiac fibers contract together due to gap junctions.

Heart Physiology

Intrinsic Cardiac Conduction System

Noncontractile cells initiate and distribute impulses to coordinate heart contraction.
Pacemaker Potential: Unstable resting membrane potential; depolarization due to Ca2+ influx.
Sequence: SA node → AV node → AV bundle → bundle branches → Purkinje fibers.

Extrinsic Innervation

Autonomic Nervous System (ANS): Modifies heart rate and force.
Sympathetic: Increases heart rate and contractility.
Parasympathetic: Decreases heart rate via the vagus nerve.
Cardiac Centers: Located in the medulla oblongata (cardioacceleratory and cardioinhibitory centers).

Electrocardiography (ECG/EKG)

P wave: Atrial depolarization.
QRS complex: Ventricular depolarization.
T wave: Ventricular repolarization.
P-R interval: Start of atrial excitation to start of ventricular excitation.
ST segment: Ventricular myocardium depolarized.
Q-T interval: Start of ventricular depolarization to end of repolarization.

Cardiac Cycle

Phases of the Cardiac Cycle

Systole: Contraction phase (ventricles contract, atria relax).
Diastole: Relaxation phase (ventricles fill, atria contract).
Duration: At 75 beats/min, one cycle lasts 0.8 seconds (atrial systole 0.1s, ventricular systole 0.3s, quiescent period 0.4s).

Heart Sounds

"Lub-dup" sounds: "Lub" is closure of AV valves; "dup" is closure of semilunar valves.
Valve Auscultation: Specific intercostal spaces for each valve (e.g., mitral valve at apex, tricuspid at right sternal margin).

Cardiac Output and Regulation

Cardiac Output (CO)

Definition: Amount of blood pumped by each ventricle per minute.
Formula:
Stroke Volume (SV): Volume of blood pumped per beat.
- Example:
- Normal CO:

Regulation of Stroke Volume

Preload: Degree of stretch of cardiac muscle before contraction (Frank-Starling law: increased preload increases SV).
Contractility: Strength of contraction at a given muscle length (increased contractility increases SV).
Afterload: Pressure the ventricles must overcome to eject blood (higher afterload decreases SV).

Regulation of Heart Rate

Sympathetic Stimulation: Increases HR and contractility (norepinephrine on adrenergic receptors).
Parasympathetic Stimulation: Decreases HR (acetylcholine causes hyperpolarization).
Baroreceptors: Respond to pressure changes, affecting HR via reflex arcs.
Chemical Regulation:
- Epinephrine: Increases HR and contractility.
- Thyroxine: Increases HR and enhances epinephrine effects.
- Ions: Ca2+ and K+ levels critically affect HR.
- Temperature, Age, Gender: Also influence HR.

Homeostatic Imbalances

Common Disorders

Pericarditis: Inflammation of pericardium causing chest pain.
Incompetent Valve: Valve does not close properly, causing backflow.
Stenotic Valve: Valve does not open completely, restricting blood flow.
Angina Pectoris: Temporary chest pain due to reduced blood flow.
Myocardial Infarction: Heart attack; cardiac cell death from lack of oxygen.
Arrhythmias: Abnormal heart rhythms due to conduction system problems.
Fibrillation: Rapid, irregular contractions; heart becomes nonfunctional.
Congestive Heart Failure: Heart cannot meet tissue demands; may cause pulmonary or peripheral congestion.
Atherosclerosis: Fatty buildup in arteries impairs nutrient and oxygen delivery.

Table: Comparison of Pulmonary and Systemic Circuits

Feature	Pulmonary Circuit	Systemic Circuit
Origin	Right ventricle	Left ventricle
Destination	Lungs	Body tissues
Blood Oxygen Level	Oxygen-poor to oxygen-rich	Oxygen-rich to oxygen-poor
Main Artery	Pulmonary trunk/arteries	Aorta
Main Vein	Pulmonary veins	Venae cavae

Example: Cardiac Output Calculation

If heart rate is 80 beats/min and stroke volume is 65 mL/beat:

Additional info: Some details, such as the exact anatomical locations for auscultation and the full list of homeostatic imbalances, were expanded for academic completeness.