Chapter 9 - Guyton Flashcards
The ventricles then supply the main
pumping force that propels the blood either:
1) through the pulmonary circulation by the right ventricle or 2) through the peripheral circulation by the left ventricle
three major types of cardiac muscle:
atrial muscle, ventricular muscle, and specialized excitatory and conductive muscle fibers
cell membranes that separate individual cardiac muscle cells from one another
intercalated discs
syncytium
cardiac cells are so interconnected that when one
of these cells becomes excited, the action potential spreads to all of them, spreading from cell to cell (atrial and ventricular
Resting membrane potential of cardiac muscle is
-85 to -95 mV
After the initial spike, the membrane remains depolarized for about 0.2 second, exhibiting a plateau as shown in the figure, followed at the end of the plateau by abrupt repolarization. What is the significance?
The presence of this plateau (0.2-0.3 s) in the action potential causes ventricular contraction to last as much as 15 times as long in cardiac muscle as in skeletal muscle.
Why does the plateau in cardiac muscle occur?
prolonged period of depolarization through influx of both sodium and calcium, decreased outflow of potassium prevents return to normal resting level
What are the fastest conducting fibers in the heart called?
Purkinje fibers
The normal refractory period of the ventricle is ______ second, which is about the duration of the prolonged plateau action potential.
0.25 to 0.30
Where does the calcium come from that contributes to the strength of the cardiac muscle contraction?
SR and T-tubules (with only SR calcium the contraction would not be strong enough)
What is the significance of mucopolysaccharides?
bind calcium in the T-tubules to keep it available for contractions
The strength of contraction of cardiac muscle
depends to a great extent on the concentration of
calcium ions in the:
extracellular fluid
Each cycle is initiated by spontaneous generation of an action potential in the:
sinus node
P wave
atrial depolarization
QRS complex
ventricular depolarization
T wave
ventricular repolarization
If the atria fail to function, why might the person not notice?
the heart can continue to operate under most conditions even without this extra 20 per cent effectiveness because it normally has the capability of pumping 300-400% more blood than is required by the resting body
a wave
atrial contraction (right and left atrial pressure increases)
c wave
ventricles begin to contract (increasing pressure in ventricles)
v wave
end of ventricular contraction
The period of rapid filling lasts for about the first
_____ of diastole.
third
What causes the A-V valves to open?
pressure increase in the atria during ventricular systole
What is isovolumic contraction of the ventricles?
contraction is occurring in the ventricles but no emptying is occurring because the pressure is not significant enough to push the semilunar valves open
During the period of rapid ejection, first third, how much blood is ejected?
70% (remaining 30% during the last two thirds - period of slow ejection)
What is the end-diastolic volume?
filling of the ventricles during diastole (110-120mL)
What is the stroke volume output?
ventricles empty during systole, volume decreases about 70 mL
What is the end-systolic volume?
remaining volume in each ventricle, about 40-50 mL
Fraction of the end-diastolic volume that is ejected is called the ejection fraction—usually equal to about ___%.
60
Increasing the end-diastolic volume and decreasing the end-systolic volume have what effect on stroke volume?
increases
prevent backflow of blood from the ventricles to the atria during systole
A-V valves (mitral and tricuspid)
prevent backflow from the aorta and pulmonary arteries into the ventricles during diastole
semilunar valves (aortic and pulmonary artery valves)
What is cardiac output?
the amount of blood pumped every minute
What are two structural differences between the A-V and semilunar valves?
A-V valves: thin, large diameter so soft closure and slow velocity; semilunar valves: fast closure and high velocity
What helps prevent the A-V valves bulging too far backward toward the atria during ventricular contraction?
papillary muscles by the chordae tendineae
When listening to the heart with a stethoscope, do you hear the valves opening or closing?
closing
first heart sound
closure of A-V valves
second heart sound
semilunar valves close at the end of systole
Work Output of the Heart
Phase I period of filling, Phase II period of isovolumic contraction, Phase III period of ejection, Phase IV period of isovolumic relaxation
preload
degree of tension on the muscle when it begins to contract (end-diastolic pressure)
afterload
load against which the muscle exerts its contractile force (aortic pressure)
When a person is at rest, the heart pumps only ___
liters of blood each minute.
4-6
The basic means by which the volume pumped by the heart is regulated:
1) intrinsic cardiac regulation of pumping in response to changes in volume of blood flowing into the heart and 2) control of heart rate and strength of heart pumping by the autonomic nervous system
Within physiologic limits, the heart pumps
all the blood that returns to it by the way of the veins. What mechanism is this?
Frank-Starling mechanism
As the ventricles fill in response to _____ atrial
pressures, each ventricular volume and strength of
cardiac muscle contraction increase, causing the
heart to pump increased quantities of blood into the arteries.
higher
The cardiac output increases during increased sympathetic stimulation and decreases during increased parasympathetic stimulation. These changes in output caused by nerve stimulation
result both from:
changes in HR and changes in contractile strength of the heart
How does excess potassium decrease the strength of heart contraction?
decreases the resting membrane potential in the cardiac muscle fibers (calcium would be opposite)
