Download Physiology Lec.(1) Dr.Rafah Sami

Physiology
Dr.Rafah Sami
Lec.(1)
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CARDIO VASCULAR SYSTEM
The vascular system is responsible for circulation of blood and consists of heart and
peripheral vessels.
The human heart is four chambered structure consisting of two atria and two ventricles.
the blood from left ventricle is pumped to all parts of the body through arterial circulation.
the venous channels originating from the tissue capillaries return it to the right atrium by
superior and inferior vena cava.
circulation of blood from left ventricle to the right atrium constitutes the systemic
circulation the right ventricle pumps blood into the lungs through the pulmonary artery.
after gaseous exchange, oxygenated blood returns to the left atrium by pulmonary veins.
This is termed as the pulmonary or lesser circulation.
The blood within the cavities of heart and from the ventricles into peripheral circulation is
regulated by valves. They allow the blood to flow in one direction only.
The circulation, shown is divided into the systemic circulation and the pulmonary
circulation. Because the systemic circulation supplies blood flow to all the tissues of the
body except the lungs, it is also called the greater circulation or peripheral circulation.
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Functional Parts of the Circulation:The function of the arteries is to transport blood under high pressure to the tissues. For
this reason,
the arteries have strong vascular walls, and blood flows at a high velocity in the arteries
The arterioles are the last small branches of the arterial system; they act as control
conduits through which blood is released into the capillaries.
The function of the capillaries is to exchange fluid, nutrients, electrolytes, hormones, and
other substances between the blood and the interstitial fluid.
To serve this role, the capillary walls are very thin and have numerous minute capillary
pores permeable to water and other small molecular substances.
The venules collect blood from the capillaries, and they gradually coalesce into
progressively larger veins.
The veins function as conduits for transport of blood from the venules back to the heart; ,
they serve as a major reservoir of extra blood. Because the pressure in the venous system
is very low, the venous walls are thin.
Volumes of Blood in the Different Parts of the Circulation:about 84 per cent of the entire blood volume of the body is in the systemic circulation,
and 16 per cent in heart and lungs. Of the 84 per cent in the systemic circulation, 64 per
cent is in the veins, 13 per cent in the arteries, and 7 per cent in the systemic arterioles
and capillaries. The heart contains 7 per cent of the blood, and the pulmonary vessels, 9
per cent. Most surprising is the low blood volume in the capillaries.
Cross-Sectional Areas and Velocities of Blood Flow:-
If all the systemic vessels of each type were put side by side, their approximate total
cross-sectional areas for the average human being would be as follows:
Aorta
2.5cm2
Small arteries 20cm2
Arterioles
40cm2
Capillaries
2500cm2
Vena cava
8cm2
Pressures in the Various Portions of the Circulation. Because the heart pumps
blood continually into the aorta, the mean pressure in the aorta is high, averaging about
100 mm Hg. Also, because heart pumping is pulsate, the arterial pressure alternates
between a systolic pressure level of 120 mm Hg and a diastolic pressure level of 80 mm
Hg, as shown on the left side of Figure . As the blood flows through the systemic
circulation, its mean pressure falls progressively to about 0 mm Hg by the time it reaches
the termination of the vena cava where they empty into the right atrium of the heart. The
pressure in the systemic capillaries varies from as high as 35 mm Hg near the arteriolar
ends to as low as 10 mm Hg near the venous ends, but their average functional” pressure
in most vascular beds is about 17 mm Hg
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pressures in the different parts of the pulmonary circulation. In the pulmonary arteries,
the pressure is pulsatile, just as in the aorta, but the pressure level is far less: pulmonary
artery systolic pressure averages about 25 mm Hg and diastolic pressure 8 mm Hg, with a
mean pulmonary arterial pressure of only 16 mm Hg.
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Cardiac muscle:The wall of the heart is made up of three distinct layers from inside outwards.
Endocardium It is the smooth inner lining of the heart. Myocardium The cardiac muscle
constitutes this layer.
Pericardium It is a double layered membrane and a small quantity of fluid is present
between the visceral and parietal layers.
Structure of Myocardium:-
The cardiac muscle is contractile in nature like other muscles and exhibits certain
structural peculiarities, .
Muscle fibers are separated by the intercalated discs. They are areas of low electrical
resistance and allow the cardiac impulse to spread over the entire myocardium.
Properties of Myocardium
The cardiac muscle generates its own impulse (auto rhythmicity) and it is conducted to
the entire myocardium. The following electrical and mechanical properties have been
observed:- 1.Excitability 2.Conductivity 3.Contracyility 4.Rhythmicity
All or None Law:-
When a threshold stimulus is given, the entire myocardium contracts maximally and if the
stimulus is subminimal, the response is not observed. This is due to the structural
peculiarity of myocardium, as the muscle fibers are joined by area of low electrical
resistance, which permit easy spread of the impulse. The cardiac muscle acts as a
functional syncytium.
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Refractory Period
The cardiac muscle has a longer refractory period as compared to the skeletal muscle.
During this period it does not respond to a second stimulus. This is known as the absolute
refractory period and lasts throughout the ventricular contraction. It prevents the
myocardium from getting tetanised by repeated stimulation. A strong stimulus may evoke
a response during the early relaxation period, which is termed as the relative refractory
period. Stimuli falling during this period produce an extra systole, which is followed by a
compensatory pause
Specialized Excitatory and Conductive System
of the Heart
Figure
shows the specialized excitatory and conductive system of the heart that
controls cardiac contractions. The figure shows the sinus node (also called sinoatrial or SA node), in which the normal rhythmical impulse is generated;
the internodal pathways that conduct the impulse from the sinus node to the
atrioventricular (A-V) node; the A-V node, in which the impulse from the atria is delayed
before passing into the ventricles; the A-V bundle, which conducts
the impulse from the atria into the ventricles; and the left and right bundle branches of
Purkinje fibers, which conduct the cardiac impulse to all parts of the ventricles.
Sinus (Sinoatrial) Node It is located in the superior posterolateral wall of the right atrium
immediately below and slightly lateral to the opening of the superior vena cava.
Some cardiac fibers have the capability of self-excitation, a process that can cause
automatic rhythmical discharge. Note that the
"resting membrane potential” of the sinus nodal fiber between discharges has a negativity
of about -55 to -60 mill volts, in comparison with -85 to -90 mill volts for the ventricular
muscle fiber. The cause of this lesser negativity is that the cell membranes of the sinus
fibers are naturally leaky to sodium and calcium ions, and positive charges of the entering
sodium and calcium ions neutralize much of the intracellular negativity.
JUNCTIONAL TISSUES OF HEART
The excitation of pacemaker is transmitted to other regions of the heart by a specialized
conducting system, which constitutes the junction tissue of heart
Sinoatrial Node(pace maker)
it is made up of thin tapering fibers with longitudinal striations. Excitability of this region is
high and impulses of auto-rhythmicity originate from the SA node (sinus rhythm). Three
internodal pathways i.e., anterior, middle and posterior, Conduct impulses from the
pacemaker to the atrioventricular node.
A V Node
It takes about 0.1 sec for the wave of excitation to depolarize the atrioventricular node.
This a-v nodal delay is to ensure the completion of arterial systole before the onset of
ventricular contraction.
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Bundle of His
It consists of parallel fibers with a connective tissue covering. It divides into right and left
branches, former being the direct continuation of the original bundle. They are subendocardially situated.
Purkinje Fibers
They are large cells and are rich in glycogen. Striations are seen at the periphery of each
cell. The conduction of impulses is much faster in the purkinje fibers.
A defect in the transmission of impulse through the conducting system may lead to heart
block. This could be of various types.
• Sinoatrial Block Sometimes SA node does not generate the impulse leading to this
condition.
Atrioventricular Block
Impulses generated from the SA node get partially blocked at the AV node. If this block is
complete, it leads to the generation of nodal rhythm
Bundle branch block
Due to a block in either of branches of bundle of His. This lead to abnormal sequence of
ventricular excitation
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