Chapter 9: Cardiac Muscle; The Heart As A Pump and Function of the Heart Valves Guyton and Hall,...
34
Unit Three: The Heart Chapter 9: Cardiac Muscle; The Heart As A Pump and Function of the Heart Valves Guyton and Hall, Textbook of Medical Physiology, 12 th edition
Chapter 9: Cardiac Muscle; The Heart As A Pump and Function of the Heart Valves Guyton and Hall, Textbook of Medical Physiology, 12 th edition
Chapter 9: Cardiac Muscle; The Heart As A Pump and Function of
the Heart Valves Guyton and Hall, Textbook of Medical Physiology,
12 th edition
Slide 2
Physiology of Cardiac Muscle Physiologic Anatomy a.Muscle
fibers arranged in a latticework b.Striated and Involuntary c.Actin
and myosin, typical myofibrils d.Sliding filament mechanism
Slide 3
Fig. 9.1 Structure of the heart and blood flow through it.
Slide 4
Physiology (cont.) Cardiac Muscle as a Synctium-cardiac muscle
fibers are made up of many individual cells connected in series and
in parallel (intercalated discs) Fig. 9.2 Synctium
Slide 5
Physiology (cont.) a.Composed of two synctiums: atrial and
ventricular b.Disks allow the action potential to travel easily
from cell to cell Action Potentials in Cardiac Muscle a.What causes
the long action potential and plateau? b.The AP is caused by the
opening of two channel types 1.The same fast Na + channels as in
skeletal muscle 2.Slow Ca ++ channels (slower to open and close)
3.Longer period of depolarization leads to plateau
Slide 6
Fig. 9.3 Rhythmic AP+s from Purkinje fibers and a ventricular
muscle.
Slide 7
Physiology (cont.) c.Permeability for K + decreases about
5-fold d.Prevents an early return to depolarized state Velocity of
Signal Conduction Refractory Period of Cardiac Muscle a.Refractory
to restimulation during the AP b.Ventricle; 0.25-0.30 sec. which is
the duration of the plateau c.There is an additional relative
refractory period
Slide 8
Fig. 9.4 Force of ventricular heart muscle contraction, showing
the duration of the refractory period and relative refractory
period, plus the effect of premature contraction. Note that there
is no summation as occurs in skeletal muscle.
Excitation-Contraction Coupling (cont.) a.Calcium enters from
extracellular fluid b.Triggers calcium release from SR c.At the end
of the plateau, calcium flow stops and calcium is reabsorbed by the
SR
Slide 11
Cardiac Cycle Cardiac Cycle-events from the beginning of one
heartbeat to the beginning of the next a.Cycle is initiated by the
spontaneous generation of an AP by the sinus node b.Delay of 0.1
sec from the atria to the ventricle which allows the atria to
contract before the ventricles Diastole-relaxation
Systole-contraction
Slide 12
Fig. 9.6 Events of the cardiac cycle
Slide 13
Cardiac Cycle (cont.) Effect of Heart Rate on the Duration of
the Cycle- As heart rate increases, the duration of the cycle
decreases Relationship of the ECG to the Cycle-electrical voltages
generated by the heart a.P wave-spread of depolarization through
the atria followed by atrial contraction b.QRS waves-electrical
depolarization of the ventricles c.T wave-repolarization of the
ventricles
Slide 14
Cardiac Cycle (cont.) Function of the Atria as a Primer
Pump-about 80% of the blood flows directly through the atria to the
ventricles; atrial contraction adds another 20% so it functions as
a primer pump Pressure Changes in the Atria-in Figure 9.6 there are
three minor pressure elevations a.a wave-caused by atrial
contraction b.c wave-occurs when the ventricles begin to contract
c.v wave-occurs at the end of ventricular contraction
Slide 15
Cardiac Cycle (cont.) Function of the Ventricles as
Pumps-period of of rapid filling; lasts about the first third of
diastole; a small amount fills during the second third and the
atrial contraction fills the last third
Slide 16
Emptying of the Ventricles During Systole Period of Isovolumic
Contraction a.Ventricular pressure rises as contraction begins
b.Initially the pressure is not sufficient to open the semilunar
valves c.Therefore, contraction is occurring but no emptying Period
of Ejection a.Pressure >80 mm Hg pushes the semilunar valves
open
Slide 17
b.Period of rapid ejection-first third (70%) c.Period of slow
ejection-second 2/3 (30%) Period of Isovolumic Relaxation a.At the
end of systole, ventricles relax quickly b.Blood in the arteries
push back and close the semilunar valves c.Ventricles continue to
relax but volume does not change
Slide 18
End Diastolic Volume-during diastole, volume of the ventricles
increases to about 110-120 ml Stroke Volume Output-as ventricles
empty during systole, the volume decreases about 70 ml End Systolic
Volume-remaining volume in the ventricle (40-50 ml) Ejection
Fraction-fraction of the end diastolic that is ejected (60%)
Slide 19
Function of the Valves Atrioventricular Valves (tricuspid and
mitral) a.Prevent backflow from the ventricles to the atria b.Close
and open passively Fig. 9.7
Slide 20
Valves (cont.) Function of the Papillary Muscles-attach to the
AV valves by the chordae tendineae; prevent the valves from bulging
back into the atria Aortic and Pulmonary Artery Valves a. Because
of smaller openings the velocity of flow is greater than with the
AV valves b.Subject to greater mechanical abrasion than AVs c.No
chordae tendineae
Slide 21
Relationship of Heart Sounds to Heart Beating a.First sound is
closure of the AV valves b.Second sound is closure of the SL valves
c.Lub-Dub Work Output of the Heart a.Stroke work output-amount of
energy the heart converts to work during each heartbeat b.Minute
work output-total amount of energy converted to work in one minute
c.Volume-pressure work and kinetic energy of blood flow
Slide 22
Fig. 9.8 Relationship between left ventricular volume and
intraventricular pressure during diastole and systole. Heavy red
lines indicate the volume pressure diagram (EW-external work).
Graphic Analysis of Ventricular Pumping
Slide 23
Volume-Pressure Diagram During the Cardiac Cycle Phase I:
Period of Filling Phase II: Period of Isovolumic Contraction Phase
III: Period of Ejection Phase IV: Period of Isovolumic
Relaxation
Slide 24
Fig. 9.9
Slide 25
Preload-the degree of tension on cardiac muscle when it begins
to contract; usually is the end- diastolic pressure when the
ventricle has filled Afterload-pressure in the aorta leading from
the ventricle;
Slide 26
Energy Requirements Oxygen Utilization By the Heart a.70-90%
from the oxidative metabolism of fatty acids b.10-30% from lactate
and glucose Efficiency of Cardiac Contracton a.Most of the chemical
energy is converted to heat b.20-25% max. efficiency of the normal
heart c.As low as 5-10% in heart failure
Slide 27
Regulation of Heart Pumping Intrinsic Regulation of Heart
PumpingFrank-Starling Mechanism-intrinsic ability of the heart to
adapt to increasing volumes of inflowing blood a.Within physiologic
limits, the heart pumps all the blood that returns to it by way of
the veins b.Muscle stretches and brought to optimal degree of
overlap for contraction c.Stretch of the right atrial muscle
increases the heart rate by 10-20%
Slide 28
Ventricular Function Curves : Another Way of Expressing the
Frank-Starling Mechanism 9.10 Left and right ventricular function
curvesFig. 9.11 Normal right and left ventricular output
Slide 29
Control by the ANS Excitation by the Sympathetic Nerves
a.Stimulation can increase heart rate from 70 bpm to 180-200 bpm
b.Increases the force of contraction two-fold c.Increases the
volume pumped and the ejection pressure d.Inhibition decreases the
heart rate and strength of contraction
Slide 30
ANS Control (cont.) Parasympathetic (Vagal) Stimulation
a.Strong stimulation will stop the heart for a few seconds; then
20-40 bpm b.Decrease the strength of contraction 20-30%
c.Distributed mainly to the atria
Slide 31
Fig. 9.12 Cardiac sympathetic and parasympathetic nerves
Slide 32
Fig. 9.13 Effect on the cardiac output curve of different
degrees of sympathetic and parasympathetic stimulation
Slide 33
Effects of Potassium and Calcium Ions on Heart Function
Potassium Ions a.Excess causes the heart to become flaccid and
dilated and slows heart rate b.Excess can block conduction through
the AV bundle c.Contraction becomes progressively weaker Calcium
Ions a.Excess causes the heart to go to spastic contractions
b.Deficiency causes flaccidity c.Regulated within a narrow range so
seldom important clinically
Slide 34
Effect of Temperature on Heart Function Effects of Temperature
a.Increase body temperature increases heart rate b.Decrease body
temp. decreases heart rate c.Contractile strength enhanced by
moderate increases in temperature d.Prolonged elevation eventually
causes weakness
