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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