Transcript Chapter 20

Chapter 20
Cardiovascular System
The Heart
THE HEART
As the nervous system connects entire body with its network for the
purpose of communication, the circulatory system and its circulating
matter, blood, connect the entire body and the heart must pump the
blood.
In a simpler organism, diffusion may have sufficed to transport
nutrients and oxygen throughout the organisms, but in a more complex
organism, the gases and nutrients need to be transported through a
specialized system and disposed.
In fact, in the early stage of embryonic development of humans, and in
many other higher organisms, the beating of the heart is the first
observable organ activity.
The heart that continuously beats about 70 times a
minutes throughout your life-time and performs a
tremendous amount of work.
It beats 100,000 times a day. (How many times
in your life-time?)
Pumps about 8,000 liter of blood a day.
Can you change the rate of heart beat?
Cardiovascular overview
Functions of the Heart
• Generating blood pressure
• Routing blood
– Heart separates pulmonary and systemic
circulations
• Ensuring one-way blood flow
– Heart valves ensure one-way flow
• Regulating blood supply
– Changes in contraction rate and force match
blood delivery to changing metabolic needs
Anatomy and histology
Heart Skeleton
• Consists of plate of
fibrous connective tissue
between atria and
ventricles
• Fibrous rings around
valves to support
• Serves as electrical
insulation between atria
and ventricles
• Provides site for muscle
attachment
Blood Flow Through Heart
2.
Route of blood flow through the heart
Study Seeley’s Fig. 20.10 flow of blood through the heart
chambers.
The heart is divided into the left and right chambers.
The deoxygenated blood from tissues pass through the right
chambers and oxygenated blood from the lungs pass through the
left chambers.
In order for the blood to pass through these chambers, both arteria
contract at about the same time and both ventricles contract at
about the same time.
A sequence of events to flow blood through the heart depends on
the change in pressure inside the chambers and two sets of valves
which prevent the back flow of blood.
Phases of cardiac cycles
For example from the perspectives of the blood flow: Fig. 20-16.
(1)
Blood enters the right atrium because pressure in the
superior/inferior vena cava is higher than that in the right atrium.
The pulmonary semilunar valve stay closed.
(2)
The blood flow into the right atrium, which is in a relaxed
state after contraction.
(3)
The right atrium contacts (atrial systole) and pushes
through remaining blood into the right ventricle. Tricuspid valve
open.
(4)
Upon contraction of the right ventricle, (systole) the
pressure inside forces the tricuspid valve to close, but the
pulmonary semilunar valve to open, allowing the blood to flow out
into the pulmonary trunk then to pulmonary arteries.
(5)
Blood in the lung capillaries releases carbon dioxide and
picks up oxygen.
(6)
Blood returning from the lungs through the pulmonary veins
(note that these are veins, but carry oxygenated blood), and fill up
the left atrium. The bicuspid valve is opened.
(7)
The contraction of left atrium (while aortic semilunar valve
closed) completes the transfer of blood to the left ventricle.
Contraction of the left ventricle forces the bicuspid valve to close
and blood gushes out through the now opened aortic semilunar
valve.. This is a powerful push.
Note that contraction of the both ventricles take place at the same
time, as do the atria.
Also note the thickness of ventricular walls.
Cardiac muscles
(1)
Cardiac muscle cells are less ordered than skeletal muscle
cells.
(2)
They are branched and each cell has one or two nuclei.
Sarcomeres are still evident i.e. some striation, but T tubules are not
extennsive.
(3)
Cells are surrounded with connective tissue and intercalated
disks connect cardiac muscle cells.
(4)
Slow and prolonged contraction of cardiac muscle cells allow
Ca++ to move in and out the myofibrils.
(5)
Desmosomes hold the cells together and gap junctions connect
them for easy transmission of action potentials.
(6)
Well vasculated for aerobic respiration for steady and enough
oxygen supply for mitochondrial activities.
(7)
Cardiac muscle cells, which are under continuous steady
contraction/relaxation cycles, glucose, fatty a acids and lactic acid to
produce ATP.
Conducting system
Conducting System of Heart
b.
The conducting system (Fig. 20-12,13)
Unlike skeletal muscle, cardiac muscle can contract
spontaneously -automaticity.
The sequence of rhythmic contraction, the atria to the
ventricles,
is regulated by nodal cells, that determines the rate
of contraction,
and conducting cells, which distribute the
contractile stimuli.
i.Nodal cells: The pace maker nodal cells spontaneously
depolarize at 70 - 80 times a minute. The pace maker cells
in the right atrium near at the entrance of the superior vena
cava is the sinoatrial node (SA node) and regulates the
heart beat.
ii.
node.
Conducting cells: The signal for contraction starts with the SA
(1)The action potential from the SA node spreads along the
wall of
the atrium to the atrioventricular (AV) node.
(2)The action potential from the AV node spreads along the
atrioventricular (AV) bundle into the interventricular septum.
(3)The AV bundles are divided into right and left
branches.
(4)These bundles are connected to the Purkinje fibers of the
ventricular wall and stimulate both
right and left
entricular muscle
cells
Although the cardiac muscles are capable of
contracting spontaneously, the frequency of
contraction is regulated by the generation of
action potentials by the pace maker SA node.
While the action potential from the SA node
induce contraction of the atrium, its spread to the
AV node takes 0.04 second. Further, the spread
of the action potential within the AV node is slow.
Thus all together 0.11 seconds of delay is caused
before ventricular contraction takes place.
That makes total of 0.15 seconds.
Electrical properties
i.
Action potentials
The electrical properties, thus ionic distributions, of
cardiac muscles are similar to those of the neurons
and skeletal muscle cells.
The difference of action potentials between skeletal
muscle or neuron and cardiac muscle lies in the
special role of voltage-gated Ca++ slow channels in
cardiac muscle. (Fig. 20-15)
Also note the roles of voltage gated Na+ and
K+(small contribution) channels.
The initial fast depolarization by the voltage
gated Na+ channels (opened) is followed by
the opening of voltage-gated Ca++ slow
channels.
Presence of the opened voltage-gated Ca++
slow channels extends the partial
repolarization phase (plateau phase) thus
prolong duration of the action potential up to
500 msec until the Ca++ channels begin to
close.
Thus there will be a long contraction
period, hence
a long refractory period.
Autorhythmicity of cardiac muscle
(Fig. 20-12)
SA Node Action Potential
ii.
Autorhythmicity of cardiac muscle (Fig. 20-12)
The SA node has a significantly large number of voltageregulated Ca++ slow channels (slow channels). Gated K+
channels are present and behave opposite to Ca++ channels
with relatively small contribution.
The slow channels spontaneously open and let Ca++ into the cell
to generate a local membrane potential.
When more slow channels are opened, there will be a complete
depolarization - the action potential.
The resting potential is reestablished by closing the Ca++ slow
channels and opening the K-channels.
The process will repeat, thus autorhythmic generation of action
potentials.
The action potential spreads out through the local
cardiac muscle fibers and reaches the AV node to
depolarize it.
The SA node appears to have the largest number
of Ca++ slow channels, thus more frequent cycles
(70 - 80 bpm).
While the AV node can beat at the rate of 40 - 60
bpm and usually let the SA node to dictate the
frequency.
iii.
Refractory period
Although contraction of cardiac muscle is initiated similar to
that of skeletal muscle by depolarization and subsequent distribution of
action potentials in the sarcolemma, there are two important differences
between the two types of muscles.
(1)
In cardiac muscles the entire cycle of an action potential in
sacrolemma takes up to 250 -300 msec and is about 30 times longer
than that in a sacrolemma of skeletal muscle. Adding to this is a
relatively long refractory period of cardiac muscle cell, thus making only
up to 200 contractions per minute is possible.
(2)
We have seen that summation of contraction is possible in
skeletal muscle fibers, but it is not possible in cardiac muscle cells.
Thus no summation or tetanic contraction can take place. This is an
important protective mechanism for the heart.
iv.
Electrocardiogram
Electrocardiogram
• Action potentials through
myocardium during
cardiac cycle produces
electric currents than can
be measured
• Pattern
– P wave
• Atria depolarization
– QRS complex
• Ventricle depolarization
• Atria repolarization
– T wave:
• Ventricle repolarization
The sequential depolarization of muscle cells may be
recorded and the result is called an electrocardiogram
(ECG or EKG). Fig. 20-14. (Three electrode system,
left and right wrists and left lower leg.)
The P wave: The depolarization of the atria. After 100
msec the atria begin to contract.
The QRS complex: The depolarization of the ventricles.
Contraction is after the R wave.
The T wave: Ventricular repolarization.
Cardiac arrhythmia etc. Seeley’s Fig. 20.17.
Cardiac Arrhythmias
• Tachycardia: Heart rate in excess of 100bpm
• Bradycardia: Heart rate less than 60 bpm
• Sinus arrhythmia: Heart rate varies 5%
during respiratory cycle and up to 30%
during deep respiration
• Premature atrial contractions: Occasional
shortened intervals between one contraction
and succeeding, frequently occurs in healthy
people
Alterations in Electrocardiogram
Cardiac Cycle
• Heart is two pumps that work together, right
and left half
• Repetitive contraction (systole) and
relaxation (diastole) of heart chambers
• Blood moves through circulatory system
from areas of higher to lower pressure.
– Contraction of heart produces the pressure
Cardiac Cycle
Heart Sounds
• First heart sound or “lubb”
– Atrioventricular valves and surrounding fluid vibrations
as valves close at beginning of ventricular systole
• Second heart sound or “dupp”
– Results from closure of aortic and pulmonary semilunar
valves at beginning of ventricular diastole, lasts longer
• Third heart sound (occasional)
– Caused by turbulent blood flow into ventricles and
detected near end of first one-third of diastole
Location of Heart Valves
4.
Heart dynamics
The amount of blood ejected from a ventricle during a
single hear beat is the stroke volume (SV).
Instead of measuring the blood volume after a
single stroke, the amounts of blood pumped in
one minute is more often used as the cardiac
output (CO).
Cardiac output(CO) = Stroke volume(SV) X Heart
rate(HR)
Fig. 20-17, 18 summarize all the events. Use them to
review the events.
a.
Factors controlling cardiac
output (Fig. 20-20, 21)
Baroreceptor and Chemoreceptor
Reflexes
a.
Factors controlling cardiac output
Both intrinsic and extrinsic factors regulate the heart rate.
i.
Intrinsic regulation
The amount of blood flow affect the cardiac muscles and stretches
the SA node.
ii.
Extrinsic regulation
Both parasympathetic and sympathetic nerves can regulate heart
beat.
Parasympathetic stimulation decreases heart rate.
Sympathetic stimulation increases heart rate.
Epinephrine and norepinephrine released by sympathetic
stimulation increase heart rate.
Heart Homeostasis
• Effect of blood pressure
– Baroreceptors monitor blood pressure
• Effect of pH, carbon dioxide, oxygen
– Chemoreceptors monitor
• Effect of extracellular ion concentration
– Increase or decrease in extracellular K+ decreases heart
rate
• Effect of body temperature
– Heart rate increases when body temperature increases,
heart rate decreases when body temperature decreases
Baroreceptor Reflex
Chemoreceptor Reflex-pH
Effects of Aging on the Heart
• Gradual changes in heart function, minor
under resting condition, more significant
during exercise
• Hypertrophy of left ventricle
• Maximum heart rate decreases
• Increased tendency for valves to function
abnormally and arrhythmias to occur
• Increased oxygen consumption required to
pump same amount of blood