Download Review: What is the importance of peripheral resistance for the

Review:
We discussed the factors that affect vascular resistance which are the size of the lumen, the viscosity of
the blood, and the length or the total length of the vessels. So the wider the lumen the lower resistance.
And then we discussed the term: total peripheral resistance or systemic vascular resistance. The higher
resistance, the higher mean arterial blood pressure (MAP). The type of vessels that are more responsible
for resistance are actually the smaller vessels which are capillaries, arterioles, and venules (peripheral
resistance). Any changes in the diameter of these vessels will affect largely the total peripheral
resistance and the vascular resistance.
Then we discussed the meaning or the term Venus return: it is the volume of blood that returns to the
heart from the body or from the systemic veins.
We should link her the Venus return with the function of the heart. Venus return determines the filling
of the heart and determines the preload. The more Venus return the more preload and the more end
diastolic volume. The more we have end diastolic volume, we will have more stroke volume. While the
less Venus return the filling will be less and the end diastolic volume will be less also, so the stroke
volume will be significantly reduced. That shows the importance of linking vascular system with
function of the heart (cardio part of the cardiovascular sys.), they both work together in harmony and
so any changes in resistance or in Venus return will affect significantly the function of the heart.
What is the importance of peripheral resistance for the function of the heart ? As the peripheral
resistance increases the afterload will increase. We measure the afterload from aorta which is the
pressure that will face the heart and the heart must exceed it by contraction of the ventricles in order to
pump blood. So as this pressure was high (afterload), the heart should work harder in order to
overcome this increase in the afterload. So this is the relationship between the resistance and afterload.
Venus return is an important volume and we know that the driving pressure for the Venus return mainly
is the difference between the heart (right atrium and ventricle) and the vessels on the venus side which
is from the venus side of the capillary bed 16 mmHg and zero in the heart. So this difference between 16
and zero will drive blood back to the heart.
The new info:
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It’s very small pressure difference to drive venus return and in different situations and different
positions this pressure might not be sufficient to drive blood to the heart. So because it is
important we need other mechanisms to aid the Venus return:
o The presence of valves in the structure of the veins and venules. These valves actually
aid in one direction of blood. So it prevents back flow of blood when the pressure or the
compression of the veins decreases. For example:
 skeletal muscle pump:
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As you see in slide 9 this the proximal and distal valve in
essential vein in lower limp. There are two valves.
Pic.1 at resting position. The blood will move in one
direction.
- Pic. 2 When the muscle contract the vein undergoes
compression. The pressure increased near the
proximal valve so it pushes the blood upward as
if you are milking of a goat or a cow (this process
called milking). While the blood that are
accumulating near the distal valve, it will develop
volume and start to make back flow so the distal
valve will be affected by downward pressure from
the blood and so the valve will close to prevent blood to go back to the other
direction (down ward).
- Pic. 3: resting position again. No pressure to push blood upward, so the blood tends
to go back and the valve will become closed (proximal valve) because the pressure
below it decreases. At the same time there will be a pressure in the foot because
there are more venus return and at contraction it wasn’t able to go upward but
now there is a sufficient pressure to push the valve and open it (distal valve).
So this process proceeds continuously as the person walks more and it will milk the
blood upward aiding the venus return. So if the person was in long travel in the plain,
he will lose the aid of this skeletal pump because the leg will be in resting position all
the time and the blood will accumulate in the foot. As the person become older that
problem will be larger.
 Respiratory pump:
It aids venus return from the abdomen-chest area due to pressure changes in
thoracic and abdominal cavity.
By the movement of the diaphragm downward during inhalation so the
pressure in the thoracic cavity drops down to a great extent while the pressure
in abdomen increases because the diaphragm make compression on the
abdomen. So the compression in the abdominal cavity makes pressure on the
veins there so it pushes the blood upward to the thoracic cavity where the
pressure is less.
The opposite happens during exhalation; the pressure in abdomen will drop
down while in thoracic cavity the pressure will increase so the blood tends to
backflow but the valve closes. So blood movement is only upward and valve
closed at any downward movement aiding the movement of the blood back to
the heart from thoracic and abdominal cavity.
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The network in slide 10:
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Shows the different factors that can increase MAP .
MAP can be increased by 2 ways: either by increasing cardia output or by increasing peripheral
resistance (systemic resistance).
The resistance can be increased by:
o Vasoconstriction: the diameter become smaller.
o Increasing the length of vessels.
o Increasing the viscosity by dehydration, increase red blood cells count.
Factors that can increase the cardiac output:
o Increasing the heart rate using autonomic nervous system by increasing the sympathetic
and decreasing the parasympathetic.( in cardiovascular center in medulla oblongata)
o Increasing the stroke volume by increasing venus return so the end diastolic volume will
increases and so the stroke volume will increase. We can increase venus return either by
vasoconstriction of vessels or by increasing the function of the respiratory pump or
skeletal pump. Also we can increase venus return by increasing the blood volume (give
him more liquid to drink).
When somebody suffer from hypotension, one way to protect him from coma is to give
him more liquids to reduce dehydration or to increase the volume of the blood. We can
decrease the afterload if it caused by vasoconstriction resistance, so we can give him
agent that makes dilation for the vessels and decreases the resistance but if there are
pathology in the vessels we can’t use any agent (pathology as: atherosclerosis) because
it will be difficult to control afterload due to the pathological condition and that cause a
continuous hypertension.
Velocity of blood flow:
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Blood flow: the volume of blood that flows or passes in a tissue or a vessel at a given time
ml/min.
Velocity of blood flow: it’s a speed measured in cm/sec. that means how many cm the blood
passes in one second and that related to the total cross sectional area.
We calculate the cross section of different types of blood
vessels, the total cross section of each type (we sum the
cross section of all the arteries, and all the arterioles. For
capillaries also we sum the cross section of all capillaries and
so on)
The green curve in the figure represents the cross section. As
you note the cross section for aorta alone is very small
compared with arterioles and capillaries. As the capillaries
become smaller and more in number the total of its cross
sections will increase.
Remember that as the capillaries were smaller, their
distribution will be more and their number will increase and
so its total cross sections will be the highest total.
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The relationship between the cross section and the velocity (the velocity represented with the
black line). Whenever the cross section is smaller, the velocity will be higher. So the capillaries
and venules which have the largest total cross section, they have the lowest velocity. The
relevance or the importance of this that: this aids in exchange processes in capillaries and
venules (microcirculation) because the low velocity ease the exchange processes. While the
velocity is high in large vessels because they are conducting and distributing vessels so they
needs high velocity.
Circulation time: is the time that the blood needs it to start from a point (from right atrium as an
example) and get back to the same point after passing in pulmonary and systemic circulation. So
it represent a full cycle in both systemic and pulmonary circulation. The blood needs only 1 min
for this full cycle. We can calculate it by ourselves by knowing that the volume of our blood is 5
liters and these 5 liters represent the cardiac output, that means these five liters cross the
whole body in 1 min. so all the volume of our blood circulate all the body in one min at rest
because at different conditions and different HR, the circulation time will be different.
The control of blood pressure and blood flow:
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Blood pressure, blood flow and resistance undergo control mechanisms because those are very
important factors for our life. If any changes happen in these factors and no control or balance
was done by the body, the person might die.
The control is done by adjustment of several factors such as: heart rate, stroke volume,
resistance, and total blood volume. By playing with these factors we can make compensation for
any change to prevent any risk on the body and to ensure that the blood supply reach all the
body parts.
There are control systems that can acts very fast and others that needs more time but makes
long term correction. Some make acute change and others make long term change.
The control systems:
 Cardiovascular system:
- This is the main control system.
- It is located in medulla oblongata.
- Part of the central nervous system.
- It is responsible in controlling heart rate mainly which will affect stroke volume and
cardiac output.
- It also controls resistance and so affect the venus return and stroke volume.
- It is a group of neurons that are located in part of the central nervous system called
medulla oblongata. There are groups of these neurons share the same function but
all of them cooperate to serve one function. Each group has a distinct name
according to the function or the change that they perform.
For example there are:
o Groups of neurons affects the heart by sending an input directly to the
heart so it might change the heart rate by working on SA node and AV node
(change the heart rate “rhythm”). There are two types according to their
action if it was inhibition or stimulation. We call them Cardiostimulatory
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which makes stimulation and the other group we call it cardioinhibitory
center. These Cardiostimulatory may increase the heart rate or the
contractility of the heart muscle so increasing the efficiency of the heart
function of stroke volume. While the cardioinhibitory center decreases the
heart rate and reduces the contractility so it weakens the function of the
heart.
o Vasomotor center: a third group in cardiovascular center affects the
peripheral resistance. It send input to the vascular system to vessels, to
vasoconstricte it or vasodilate it. So vasomotor center controls the
diameter of vessels.
Medulla oblongata can receive input also form higher brain regions. That’s means
its function may be affected by input from higher brain regions such as cerebral
cortex, limbic system, and hypothalamus according to the condition.
For example when a person wants to run, before the muscular activity starts the
heart rate starts to increase even before starting the race. This is caused by an input
from limbic system on the cardiovascular center (the input is preparation for the
situation) to increase the heart rate because the body will need a large amount of
blood for the skeletal muscles. When the race starts, another input comes from
muscles via sensory fibers to cardiac center to increase more the heart rate.
So it can receive input from several regions from higher brain regions and also from
the effector organs which might be muscles, joints, respiratory center…
In slide 15, the pic. Shows the cardiovascular center in medulla oblongata.
o The blue arrows represents the afferents because they comes from outside
the brain (sensory output). Some of these afferents comes from the higher
brain regions and others come from peripheral regions (effector tissue).
The effector tissues might be sensory receptors, either mechanoreceptors
or chemoreceptors. Mechanoreceptors are two types:
 Proprioceptors: come from joints and skeletal muscles as a result of
movement. So they give input indicating that there are muscular
activity, so we need higher blood flow because there are higher
demand on oxygen.
 Baroreceptors: they sense increased blood pressure. When they
face increased blood pressure the stretch. So the more they
stretch, the more impulses that they send to cardiovascular center.
While the less stretch reduces the impulses.
Chemoreceptors: they sense chemical compounds which are CO2, O2
(hypoxia), and hydrogen (acidity/ protons). The presence of these receptors
indicates the high demands on oxygen (there are fast metabolism, more
acids, or hypoxia) so it sends a message to the cardiovascular center.
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The red arrows represents the output which is the efferent fibers from the
cardiovascular center to the heart or vessels. To the heart either to
decrease rate or to increase rate and contractility. To blood vessels to
induce vasoconstriction or to inhibit vasoconstriction.
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The output usually accomplished through certain types of fibers (nerve). It
might be sympathetic (increase) or parasympathetic (decrease) fibers. The
parasympathetic outputs are carried by vagus nerves (nerve no. 10) while
sympathetic outputs are carried by cardiac accelerator nerve (to increase
the rate of the heart and the heart contractility) and vasomotor nerves (to
vessels to arterioles mainly to make vasoconstriction). Sympathetic output
increases HR and contractility and cause vasoconstriction: to increase
stroke volume, to redistribute blood, and to use reservoirs and direct it to
other organs that requires higher oxygen supply.
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All feedback mechanisms that control either HR, blood pressure, resistance, or
volume are negative feedback mechanisms. That means the stimulus and the
effect go in opposite directions, for example the increase in the pressure causes
several changes that leads to reduce the pressure to back to the normal state.
While in positive feedback the stimulus and effect go in the same direction, so if the
stimulus is the increase in specific thing, then the result will be more increase in
that thing or for example decrease leads to more decrease.
Done by: Maha Alfaloji
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