Anatomy

97

The Heart

Objectives • Describe the organization of the cardiovascular system. • Describe the location and general features of the heart, including the pericardium. • Discuss the differences between nodal cells and conducting cells, and describe the

components and functions of the conducting system of the heart. • Identify the electrical events associated with a normal electrocardiogram. • Explain the events of the cardiac cycle including atrial and ventricular systole and

diastole, and relate the heart sounds to specific events in the cycle. • Define cardiac output, heart rate, and stroke volume and describe the factors that

influence these variables. • Explain how adjustments in stroke volume and cardiac output are coordinated at

different levels of activity.

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 between the above two • Superficial fibrous pericardium is anchored to mediastinum

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

98

• Epicardium – visceral pericardium • Myocardium – cardiac muscle • Endocardium – simple squamous epithelium

Internal anatomy and organization • Atria

• Thin-walled chambers that receive blood from the veins (VC & PV) • Pump blood into ventricles

• Ventricles • Thick-walled chambers separated from the atria by AV valves • Pump blood into arteries (PA & Aorta)

• Chordae tendineae • Tendinous fibers attached to the AV valves and papillary muscles

Blood flow through the heart • Right atrium

• Tricuspid valve • Right ventricle

• Pulmonary valve – pulmonary trunk – pulmonary arteries • Pulmonary circuit – pulmonary veins • Left atrium

• Bicuspid valve(mitral valve) • Left ventricle

• Aortic valve • Aorta and systemic circuit – vena cava – right atrium

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

Blood supply to the heart • Coronary arteries include the right and left coronary arteries, marginal arteries,

anterior interventricular artery (anterior descending), and the circumflex artery • Coronary veins include the great cardiac vein, anterior and posterior cardiac veins,

the middle cardiac vein, and the small cardiac vein, and the coronary sinus

99

Cardiac physiology • Two classes of cardiac muscle cells

• Specialized muscle cells of the conducting system (control heartbeat) • Contractile cells (produces powerful contractions that propels blood)

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 (AV bundle, bundle

branches, and Purkinje fibers) • 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 (masks atrial repolarization) • 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

100

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 exiting chamber • 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

CO cardiac output

(ml/min) =

Hr heart rate

(beats/min) X

SV stroke volume

(ml/beat)

Normal Resting Values

6000 ml/min or 6 l/min = 75/beats/min X 80 ml/beat

Moderate Exercise Values 13.44 l/min = 120/beats/min X 112 ml/beeat

Heavy exercise can increase CO to 18-30 l/min Elite athletes can increase CO ≈ 700% to 40 l/min!!!!

101

SV ⇑ from ⇑ in End-Diastolic Volume (120ml resting) venous pressure ⇑ ventricular volume

This stretches the LV myocardium = ⇑ force of contraction Starling's Law *Why does RHR ⇓ for athletes?

The proportion of blood ejected from the LV compared to total volume is termed ejection fraction (65%)

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

Heart Disease mostly from poor coronary circulation Ischemia decreased O2 supply

Angina Pectoris chest pain from ischemia w/o death to myocardium.

Caused by: stress (Ψ or phys.), atherosclerosis, ↑BP, nicotine...

Myocardial Infarction (MI) Heart attack - #1 cause of death in USA Necrosis of myocardium.

Congenital Heart Disease #1 cause of full term newborn fatality

Atrial Septal Defect (ASD)

Foramen ovale remains open after birth

Ventricular Septal Defect (VSD)

Hole in interventricular septum

Patent Ductus Arteriosus (PDA)

Patent fetal Ductus Arteriosus

Tetrolagy of Fallot

VSD, aorta arising from RV or VSD, pulmonic RV hypertrophy

Largest contributor to "blue baby"

Transposition of the Great Vessels

Aorta arises from the RV

Pulmonary artery arises from the LV

Other Heart Diseases

Rheumatic heart disease, congestive heart disease, arrhythmic heart disease