Cardiac Physiology

Anatomy and Physiology II

BIO 232

The Circulatory System

• The heart

• Pumps the blood to the tissues

• The blood vessels

• Passageways through which blood is directed from the heart to the tissues and back

• The blood

• Transport medium with in which materials are dissolved or suspended

The Heart

• Size of a human fist

• Tilted toward the left

• Right and left halves constitute separate pumps

• Common to refer to right heart and left heart

• Heart Wall

• Epicardium) - outer layer

• Myocardium - cardiac muscle

• Endocardium - inner layer continuous with the endothelium of blood vessels

• Parietal pericardium - sac that surrounds the heart

Myocardium of the Heart

• Interlacing bundles of cardiac muscle arranged spirally around the circumference of the heart

• Contraction occurs from the apex & blood is wrung out of the heart

• Individual cardiac cells are joined by intercalated discs

• Junctional areas that allow for electrical communication

Organization of Cardiac Muscle Fibers

Chambers of the Heart

• 4 chambers

• 2 upper - atria

ØEach has an auricle - expandable ear shaped appendage

• 2 lower - ventricles

• Both hold same blood volume

Gross Anatomy of the Heart

Heart Valves

• Atrioventricular valves - (AV)

• Supported by fibrous strings - chordae tendinae

• AV right - tricuspid valve

• AV left - bicuspid valve - Mitral

Heart Valves

• Semilunar valves

• Pulmonary

• Aortic

Blood Flow through the Heart

The Cardiac Cycle

• The repeating pattern of contraction and relaxation of the heart

• Systole = contraction of the heart

• Diastole = relaxation of the heart

The Cardiac Cycle Begins

• Two atria relaxed (atrial diastole) - note ventricles also relaxed

• Venous return fills the atria

Cardiac Cycle Continues

• The build up of pressure in atria opens AV valves

• The ventricles are 80% filled before atria contract

• Atrial systole

• Contraction of atria adds the final 20%

• End diastolic volume - The total volume of blood in the ventricles at the end of diastole

• Next atria relax (atrial diastole)

Cardiac Cycle Continues

• Next the ventricles contract = ventricular systole

• This snaps the AV valves shut

• Isovolumetric contraction

• As pressure rises in ventricles - semilunars open (at about 120mm Hg)

• This begins the ejection phase

• About 2/3 the volume leaves (stroke volume)

• The 1/3 left - end-systolic volume

• Ventricular volume decreases - back pressure closes semilunars

Cardiac Cycle Ends

• Next isovolumetric relaxation

• AV and semilunar valves are closed

• This lasts until the pressure in the ventricles falls below that in the atria

• Cycle starts again

• Average cycle - 0.85 sec each - 70 times a min

• Range - 45-100 /sec

Heart Sounds

• Lub-dub sound

• Due to valve closure

• Lub sound - 1st sound

• vibrations when AV valves close - during isovolumetric contraction of the ventricles

• Dub sound - 2nd sound

• When semilunar valves close - pressure in the ventricles falls below the pressure in the arteries

Heart Murmurs

• Abnormal heart sounds - via abnormal blood flow patterns typically due to turbulence

• A stenotic valve - stiff, narrowed valve that does not open completely

• Blood is pushed through at high velocity - whistle sound

• Lub-whistle-dub =stenotic semilunar valve

• Systolic murmur

• Lub-dub-whistle - stenotic AV valve

• Diastolic murmur

Stenotic Mitral Valve

Heart Murmurs

• Insufficient valve - valve that cannot close completely - valve edges scarred

• Turbulence caused when blood flows backward through the insufficient valve colliding with blood moving in the opposite direction

• Swishing or gurgling sound occurs

• Lub-swish-dub - insufficient AV valve

Ø Systolic murmur

• Lub-dub-swish - insufficient semilunar valve

Ø Diastolic murmur

Electrical Activity of the Heart

• 99% of the cardiac cells are contractile

• Remainder - autorhythmic cells (nodal cells)

• Sinoatrial node - SA node

• Atrioventricular node - AV node

• Bundle of His

• Purkinje Fibers

Sinoatrial Node - SA Node

• Pacemaker of the heart

• Exhibits spontaneous electrical activity

• 70-80 APs/min

• Located in the right atrium near the opening of the superior vena cava

Electrical Activity of the Heart: The Pacemaker Potential

• During diastole - SA node exhibits slow depolarization

• Pacemaker potential

• Starts at -60mv - gradually depolarizes to -40mV

ØSpontaneous depolarization due to Ca⁺⁺ current - via slow calcium channels

Ø-40 is threshold for an action potential

The Pacemaker Potential

• Pacemaker Potential (continued)

• At threshold - Fast Ca⁺⁺ channels opened

Ø Also voltage sensitive Na ⁺ channels involved

• Repolarization - K ⁺ channels

• Once repolarized pacemaker potential begins again

Pacemaker Potentials & APs in the SA node

Conduction in Myocardial Cells

• Cells surrounding SA node also capable of spontaneous activity

• These cells exhibit slower depolarizations that the SA node

• A pacemaker other than the SA node - called ectopic pacemaker or focus

• Normally these cells are activated by conduction from SA node

Conduction in Myocardial Cells

• Myocardial cells stimulated by APs from pacemaker

• Depolarize to threshold - then Na⁺ gates open

• AP shows rapid reversal to -15mV

• This level of depolarization maintained for 200 - 300 mS

• Plateau phase - via slow Ca⁺⁺ current

• Repolarization via K⁺ channels

Cardiac Action Potential

Conducting System of the Heart

• Depolarization from SA node arrives AV node

• Inferior portion of the interatrial septum

• Impulse from AV node travel through AV bundle

• Bundle of His

Conduction System of the Heart

• Continue down interventricular septum as right and left bundle branches

• These terminate in Purkinje fibers in wall of ventricle

Conduction System of the Heart

ECG - Electrocardiogram

• Recording of the electrical activity of the heart from surface of the body.

• Originally EKG - K comes from Kardia = heart in German

ECG Waves

• P Wave

• Depolarization of the atria

• QRS Wave (Complex)

• Depolarization of ventricles

• T wave

• Repolarization of the ventricles

Sequence of Excitation of the Heart

ECG Electrodes

• Bipolar limb leads

• Lead I - right arm to left arm

• Lead II - right arm to left leg

• Lead III - left arm to left leg

• Unipolar leads - allows recording of voltages from heart at various surface sites

Abnormalities in Rate

• Tachycardia - rapid heart rate above 100 beats per minute

• Bradycardia - slow heart rate - fewer than 60 beats per minute

Abnormalities in Rhythm

• Any variation from normal rhythm and sequence of excitation of the heart is termed an arrhythmia

• Extrasystole (premature beat) - originates from and ectopic focus

Abnormalities in Rhythm

• Ventricular fibrillation - uncoordinated, chaotic contractions of the ventricles - can lead to brain damage or death - ventricles are ineffective pumps

• Heart block - defects in cardiac conduction system

• Complete heart block - complete dissociation of atrial & ventricular activity

Cardiac Output (CO) & Its Control

• Volume of blood pumped by each ventricle per minute (not total amount of blood pumped by the heart)

• The volume of blood in the pulmonary & systemic circulation is typically identical

• Determinants of CO

• Heart rate (75 BPM)

• Stroke volume (70 ml/beat)

• CO = 75 BMP x 70 ml/beat = 5250 ml/min

• Exercise - CO = 20 -25 liters/min

• 35 liter/min peak in athletes

Autonomic Influences on Heart Rate

• Parasympathetic input - primarily to the atrium (SA & AV nodes) - sparse to ventricles

Decreases heart rate - CO decreases

• Sympathetic input - atria (SA & AV nodes) & richly to the ventricles

• Controls heart action during emergencies or exercise - when there is greater need for blood

• Increases \rate and strength of contraction - CO increases