The Heart

Organization of the Cardiovascular System

The cardiovascular system is divided into two circuits

•Pulmonary circuit

•blood to and from the lungs

•System circuit

•blood to and from the rest of the body

•Vessels carry the blood through the circuits

•Arteries carry blood away from the heart

•Veins carry blood to the heart

•Capillaries permit exchange

Anatomy of the Heart

The pericardia

•Visceral pericardium or epicardium

•Parietal pericardium

•Pericardial fluid

Superficial Anatomy of the Heart

•The heart consists of four chambers

•Two atria and two ventricles

•Major blood vessels of the heart include

•Inferior and superior vena cavae

•Aorta and pulmonary trunk

The Heart Wall

•Components of the heart wall include

•Epicardium

•Myocardium

•Endocardium

Internal Anatomy and Organization

•Atria

•Thin walled chambers that receive blood from the vena cavae

•Ventricles

•Thick walled chambers separated from the atria by AV valves

•Chordae tendineae

•Tendinous fibers attached to the AV valves

•Papillary muscle and trabeculae carneae

•Muscular projections on the inner wall of ventricles

Blood flow through the heart

•Right atria

•Tricuspid valve

•Right ventricle

•Pulmonary valve

•Pulmonary circuit

•Left atria

•Bicuspid valve

•Left ventricle

•Aortic valve

•Aorta and systemic circuit

Heart chambers and valves

•Structural Differences in heart chambers

•The left side of the heart is more muscular than the right side

•Functions of valves

•AV valves prevent backflow of blood from the ventricles to the atria

•Semilunar valves prevent backflow into the ventricles from the pulmonary trunk and aorta

Connective Tissues

•Connective tissue fibers of the heart

•Provide physical support and elasticity

•Distribute the force of contraction

•Prevent overexpansion

•The fibrous skeleton

•Stabilizes the heart valves

•Physically isolates atrial from ventricular cells

Blood Supply to the Heart

•Arteries include the right and left coronary arteries, marginal arteries, anterior and posterior interventricular arteries, and the circumflex artery

•Veins include the great cardiac vein, anterior and posterior cardiac veins, the middle cardiac vein, and the small cardiac vein

The Heartbeat

Cardiac Physiology

•Two classes of cardiac muscle cells

•Specialized muscle cells of the conducting system

•Contractile cells

The Conducting System

•The conducting system includes:

•Sinoatrial (SA) node

•Atrioventricular (AV) node

•Conducting cells

•Atrial conducting cells are found in internodal pathways

•Ventricular conducting cells consist of the AV bundle, bundle branches, and Purkinje fibers

Impulse Conduction through the heart

•SA node begins the action potential

•Stimulus spreads to the AV node

•Impulse is delayed at AV node

•Impulse then travels through ventricular conducting cells

•Then distributed by Purkinje fibers

The electrocardiogram (ECG)

•A recording of the electrical events occurring during the cardiac cycle

•The P wave accompanies the depolarization of the ventricles

•The QRS complex appears as the ventricles depolarize

•The T wave indicates ventricular repolarization

Contractile Cells

•Resting membrane potential of approximately –90mV

•Action potential

•Rapid depolarization

•A plateau phase unique to cardiac muscle

•Repolarization

•Refractory period follows the action potential

Calcium Ion and Cardiac contraction

•Cardiac action potentials cause an increase in Ca²⁺ around myofibrils

•Ca²⁺ enters the cell membranes during the plateau phase

•Additional Ca²⁺ is released from reserves in the sarcoplasmic reticulum

The cardiac cycle

•The period between the start of one heartbeat and the beginning of the next

•During a cardiac cycle

•Each heart chamber goes through systole and diastole

•Correct pressure relationships are dependent on careful timing of contractions

Pressure and volume changes: atrial systole

•rising atrial pressure pushes blood into the ventricle

•atrial systole

•the end-diastolic volume (EDV) of blood is in the ventricles

Pressure and volume changes: ventricular systole

•Isovolumetric contraction of the ventricles: ventricles are contracting but there is no blood flow

•Ventricular pressure increases forcing blood through the semilunar valves

Pressure and volume changes: ventricular diastole

•The period of isovolumetric relaxation when all heart valves are closed

•Atrial pressure forces the AV valves open

Heart sounds

•Auscultation – listening to heart sound via stethoscope

•Four heart sounds

•S₁ – “lubb” caused by the closing of the AV valves

•S₂ – “dupp” caused by the closing of the semilunar valves

•S₃ – a faint sound associated with blood flowing into the ventricles

•S₄ – another faint sound associated with atrial contraction

Cardiodynamics

Stroke Volume and Cardiac Output

•Cardiac output – the amount of blood pumped by each ventricle in one minute

•Cardiac output equals heart rate times stroke volume

Factors Affecting Heart Rate

•Autonomic innervation

•Cardiac reflexes

•Tone

•SA node

•Hormones

•Epinephrine (E), norepinephrine(NE), and thyroid hormone (T₃)

•Venous return

Medulla Oblongata centers affect autonomic innervation

•Cardioacceleratory center activates sympathetic neurons

•Cardioinhibitory center controls parasympathetic neurons

•Receives input from higher centers, monitoring blood pressure and dissolved gas concentrations

Basic heart rate established by pacemaker cells

•SA node establishes baseline

•Modified by ANS

•Atrial reflex

Factors Affecting stoke volume

•EDV

•Frank-Starling principle

•ESV

•Preload

•Contractility

•Afterload

Autonomic Activity

•Sympathetic stimulation

•Positive inotropic effect

•Releases NE

•Parasympathetic stimulation

•Negative inotropic effect

•Releases ACh

Exercise and Cardiac Output

•Heavy exercise can increase output by 300-500 percent

•Trained athletes may increase cardiac output by 700 percent

•Cardiac reserve

•The difference between resting and maximal cardiac output

Summary: Regulation of Heart Rate and Stroke Volume

•Sympathetic stimulation increases heart rate

•Parasympathetic stimulation decreases heart rate

•Circulating hormones, specifically E, NE, and T₃, accelerate heart rate

•Increased venous return increases heart rate

•EDV is determined by available filling time and rate of venous return

•ESV is determined by preload, degree of contractility, and afterload

The Heart and the Cardiovascular System

The heart is part of the cardiovascular system

•The goal of the cardiovascular system is to maintain adequate blood flow to all body tissues

•The heart works in conjunction with cardiovascular centers and peripheral blood vessels to achieve this goal