C H 9

THE MAMMALIAN HEART

HEART

• Made of cardiac muscle filled with blood• Coronary arteries deliver blood to the walls of the

heart itself

THE CARDIAC CYCLE

• Heart beats ~70x/min in continuous cycle• ‘Begin’ with heart filled

with blood and muscles in atrial walls contract: called atrial systole• Not very high in pressure b/c

walls of atria are thin• Forces blood through

atrioventricular valves into ventricles

THE CARDIAC CYCLE

• ~0.1 second after atrial systole is ventricular systole• Generates great pressure

(~120mmHg)• Pushed blood through

semilunar valves• Last for ~0.3 seconds

THE CARDIAC CYCLE

• When ventricles relax it is called ventricular diastole • As muscle relaxes, pressure in ventricles drops• Semilunar valves prevent backflow

http://www.youtube.com/watch?v=jLTdgrhpDCg

CARDIAC CYCLE

• Ventricle walls much thicker than atria walls• Generates higher

blood pressure in ventricles than atria

• Left ventricle thicker than right

CONTROL OF THE HEART BEAT

• Cardiac muscle is myogenic, meaning it naturally contracts and relaxes without nerve impulses• However, individual cells

do not (and cannot) contract on their own individual rhythm, it needs to be cyclical with all other heart cells

CONTROL OF THE CARDIAC CYCLE

• The heart has its own built-in controlling and coordinating system to regulate cardiac muscle contractions• Cardiac cycle begins in a specialized patch of

muscle in the right atrium called the sinoatrial node (SAN) aka pacemaker

CONTROL OF THE CARDIAC CYCLE

• Muscle cells of the SAN set rhythm for all other heart cells• Their natural rhythm is slightly faster than all

other muscle cells• They set up a wave of electrical activity which spreads

rapidly over the walls of the atria• Cardiac muscle responds to this electrical excitation by

contracting

CONTROL OF THE CARDIAC CYCLE

• Band of fibers between atria and ventricles prevent electrical signal from exciting ventricles to contract at the same time and atria• Electric signal must then be passed through conducting

fibers in the septum, known as the atrioventricular node (AVN)

CONTROL OF THE CARDIAC CYCLE

• The AVN picks up the electrical wave as it spreads out across the atria and passes it on to a bunch of conducting fibers known as the Purkyne tissues (Purkinjie fibers)• Transmits excitation to base of septum where it spreads

upwards through ventricle walls, causing ventricles to contract from the bottom up

IMPROPER CARDIAC COORDINATION

• If ventricles fail to contract from bottom up, the coordination of the contraction can go wrong and cause chaotic excitation• Small sections of cardiac muscle contract while other

relax, resulting in fibrillation, in which the heart wall simply flutters instead of contracting as a whole

• Almost always fatal unless treated instantly• http://www.youtube.com/watch?v=UAs6SDI7HZw

ELECTROCARDIOGRAMS (ECGS)

• Relatively easy to detect and record waves of excitation flowing through the heart muscle• Electrodes are placed on the skin over opposite

sides of the heart• Result is a graph of voltage over time • P= wave of excitation over atrial waves• Q, R, S = wave of excitation over ventricle walls• T = recovery of ventricle walls