Download Cardiovascular System

Cardiovascular System
The component of the cardiovascular system are blood, the heart, and
the blood vessels. The heart pumps blood through the circulatory
system (an extensive closed system of blood vessels that carry blood
to and from nearly every cell in the body).
The heart is a hollow muscular organ located in the mediastinum. Its
function is to pump blood to all parts of the body.
The layers of the heart wall include the endocardium, the
myocardium, and the epicardium. The heart is surround by the
pericardial sac, which has a fibrous layer and a serous layer continous
with the epicardium. The pericardial cavity (filled with pericardial
fluid) and lies between the heart and the pericardial sac.
The endocardium consist of a signle layer of endothelial cells on the
surface and several underling layers.
Beneath the endocardium, there is a specialized tissue (derived from
muscle tissue) that responsible for initiating and conducting impulse
to cause the heart muscle to contract.
The myocardium is the muscular part of the heart that make up about
three- fourth of the heart s bulk. It is composed of cardiac muscle.
The epicardium (or visceral pericardium) is a serow membrane that
covers the muscular part of the heart.
Cardic muscle has several distinctive
1- Its fibrous undergo extensive branching.
2-Its cells are connected by intercalated disc through wich action
potential easily move.
3-Its fiber are striated.
4-Its control is involuntary.
The general anatomical plan of the heart
The heart consists of a left and a right pump, each of which has an
atrium and ventrical.
The left and right sides of the heart are separated by the interatrial and
interventricular septa. Valves separate the atria from the ventricles and
the ventricles from the arteries leaving the heart:
1-The tricuspid valve separates the right atrium and ventricle.
2-The bicuspid (mitral) valve separates the left atrium and ventricle.
3-Aortic valve separates the left ventricle and aorta.
4-Pulmonary valve separates the right ventricle and pulmonary artery.
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Blood flow through the heart chambers
Blood enters the heart from the arterior and posterior venae cavae and
passes through the following structures
1-right atrium
2-tricuspid valve
3-right ventricle
4-Pulmonary valve to the pulmonary artery and lung tissue
5-from lung tissue and pulmonary veins to the left atrium
6-bicuspid(mitral) valve
7-left ventricle
8-aortic valve to the aorta and different body tissues.
Impulse-Generation Conducting System of the Heart and
Innervation
The impulse-generating and conducting system of the heart consist of
several structures that make it possible for the atria and ventricles to
beat in succession and permit the heart to function as an effective
pump.
1-The sinoatrial node(S-A node)
The pacemaker of the heart, since it has the most rapid rhythmic
activity. It is located close to the entrance of superior vena cave into
the right atrium. The nodal cells are modified cardiac muscle cells,
smaller than atrial muscle cells and with fewer myofibrils. Nodal cells
are concentration arranged around a large nodle artery.
2-Internodle tracts
Specialized cells that conduct the electrical depolarization of the S-A
node to the A-V node.
3-Mass of specialized cardiac muscle cells lies beneath the
endocardium of the septal wall of the right atrium. The nodle cells are
similar to those of the S-A node. In addition, there are large arteriol
present as well as considerable amounts of adipose tissue.
4-Bundle of Hiss (or called atrio ventricular bundle of His) is formed
by purkinje cells that penetrate the fibous skeleton of the heart and
then divide to four the right and left bundle branches. The left bundle
again divides to form 2 fascicles.
These bundle of purkinje cells travel in the subendocardial layer to the
apex of the heart where they reverse their direction and begin giving
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off side branches that make contact with ordinary (working) cardiac
muscle cells via gap junction. Owing to this arrangement the stimulus
for ventricular contraction is rapidly conducted to the apex of the heart
which , must contract first to eject blood from the ventricles. Then, the
wave of contraction sweep toward the base of the heart (pulmonary
valve and aortic valve).
Purkinje cells has a diameter considerably greater than ordinary cardic
muscle cells (30µm versus 15µm) but are somewhat shorter (20-50µm
versus 80µm). It is the greater diameter of purkinje cells that
responsible for the rapid conduction of electrical activity. Purkinje
cells have one or 2 centrally placed nuclei, an abundance of glycogen
and few peripherally disposed myofibrils. These cells are united
longitudinally by intercalated discs.
Innervation of the heart
Both the parasympathetic and sympatheticm division of the ANS
contribute to innervate the heart and from wide spread plexuses at the
base of the heart.
In the regions close to the S-A node and A-V node gauglionic nerve
cells and nerve fibers are present. It is known that these nerves do not
affect generation of the heart beat which is a precess attributed to the
S-A node. These nerves do affect heart rhythm, where by stimulate of
the parasympathetic division (vagus nerve) promotes a slowing of the
heart beat, where as stimulation of the sympathetic nerve accelerates
the rhythm of the pacemaker (S-A node).
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Excitability of Cardic muscle
A-B= resting membrane potential
B-C= refractory period
1-depolarization
2-repolarization
3-plateau( permeability to K+ and both Na+Ca++)
*The heart is absolutely refractory which means during this period
additional action potential can not be produced no matter how strong
the stimulus. For this reason the cardiac muscle can not be tetanized
because of this long refractory period (20 msec), which the skeletal
muscle has very short refractory period (1msec) so it can be tetanized
by repetitive stimuli.
The Conductive System of the Heart
1-S-A node (normal pacemaker)
2-A-V node
3-Bundle of His a- left branch b-right branch
4-Purkinje fibers.
*Other regions of the heart that initiate action potential beat under
abnormal conditions are called ectopic pacemaker. These region like
A-V node, purkinje fiber, atrial myocardium and ventricular
myocardium. The abnormal condition are :
1-very high rhythmicity of these region
2- low rhythmicity ofs-A node
3-This leads to block the conduction system
* The frequency of discharge of S-A node is varied by :
1-the rate of depolarization
2-the threshold potential
3-the resting potential
*The frequency is controlled by the activity of the autonomic nervous
system.
a-Increased
vagal
activity
through
the
release
of
Ach(parasympathetic) will diminish the heart rate by:
1-reducing the slope of action potential
2-by producing a greater resting potential
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b- Increased sympathetic activity through the release of nor
epinephrine will increase the heart rate (principally by increasing the
slope of action potential).
*How the autonomic mediators (Ach and NE) change the slope of
pacemaker action potential?
Ach will decrease the permbility of membrane to K+ while NE will do
the opposite.
*From the S-A node, the cardiac impulse (wave of excitation) spreads
radially throughout the atria (left and right) at a conduction velocity of
1m/sce.
*The conduction from atria to ventricles is through the A-V node and
the bundle of His. The conduction through the A-V node is delayed
for about 100 msec. to allow the atria to contract befor ventricular
contraction (conduction velocity= 0.05m/sec).
*The excitation then spread through purkinje fibers(conduction
velocity= 1-4m/sec. to the left and right ventricles).
* The A-V conduction system is frequently the site of blocks due to :
1-nervous factors 2-inflammatory factor like rheumatic fever 3pharmacological factor like digitalis and its derivtives.
Cardia Cycle
Is one of heart beat or one cycle of contraction and relaxation of the
cardiac muscle.
*The main function of the heart is to generate the pressures which
produce blood flow. The valves serving to direct the flow. At peak of
P-wave of ECG(atrial depolarization) contraction of atria will takes
place which will be rapidly followed by contraction of the ventricles
at the peak of R-wave (ventricular depolarization).
*The cardiac cycle is divided into two phases:
1- Systole (pumping of blood): period of ventricular contraction.
2-Diastole(filling of the heart): period of ventricular relaxation.
During diastole:
1-the atria and ventricles are relaxed so the pressure inside the atria is
higher than in the ventricles because :
a- blood entering to left atrium from pulmonaryVeins.
b-blood entering to right atrium from the venae cavae.
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2-Therefor, the A-V valves are opened and blood is passing from atria
to ventricles, so the ventricles will be filled to about 70-80% during
diastole (before atrial contraction).
3-The aortic and pulm. Valves are closed because the aorta and pulm.
Artery pressures are higher than the ventricular pressure.
4-At the end of diastole, the S-A node discharge so the atria will
depolarization, then contract to push more blood th the ventricles.
*The amount of blood in the ventricles before systole is called end
diastolic volume.
During systole
1-The wave of depolarization passes through the ventricles which will
be followed by ventricular contraction, so the pressure inside the
ventricles will exceed the atrial pressure, so that the A-V valves will
be closed to prevent back flow of the blood into the atria.
2-Then the aortic and pulmonary valves will be opened because the
ventricles pressure is higher than that of aortic and pulm. Pressure. So
ventricular ejection occurs.
*The ventricles dose not empty completely. The amount of blood
remain after ventricular ejection is called end systolic volume.
3-After that the ventricles will relaxe so the ventricular pressure will
falls below the aortic and pulm. Valves will be closed.
Heart Sounds
Are the sounds result from vibrations caused by valval closure which
can be heard by using a stethoscope placeon the chest wall.
1-First sound (lub): of low intensity and associated with the closure of
the A-V valves at the beginning of systole.
2-Second sound (Dup): of high intensity and associated with closure
of pulm. And aortic valves at the begening of diastole.
*These sounds are normal but heart murmurs are assign of heart
disease.
Heart murmurs are noise or turbulent sounds happens in valve stenosis
or valve regulation.
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Heart Sound
The Electrocardiogram(ECG)
Is a record of the electrical activity associated with the contraction of
cardiac muscle. Or it is a tool for evaluating the electrical events
within the heart.
Differences of potential are recorded between two points on the skin.
The action potential of the cardic muscle is like battery which cause
current flow throughout the body fluids. These currents produce
voltage differences at the body surface which can be detected by metal
plate electrodes.
The uses of ECG is valuable for:
1-detect the anatomical orientation of the heart.
2-detect the size of the chambers.
3-detect the disturbances of rhythm and conduction.
4-detect the extent, location and progress of myocardial ischemia.
5-detect the effect of changed electrolyte concentration.
6-detect the influence of certain drugs (like digitalis).
Normal ECG
P-wave= atrial depolarization
Q-wave=depolarization of ventricular septum
R-wave=depolarization of the ventricular free wall from endocardium
to pericardium
S-wave=depolarization of the base of the ventricles
T-wave=repolarization of ventricles
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P-R interval: is the time from the onest of atrial activation to the onest
of ventricular activation. Normally range from 0.12 to 0.20 seconds.
QRS interval: is the time of ventricular activation which is followed
by compete ventricular contraction. Normally range (0.06-0.10 sec.).
Pathological prolongation of this internal indicates a block in the
ventricular conduction (block of left and right purkinje fibers).
S-T interval: is the time of depolarization of the entire ventricular
myocardium. Ay deviation in this period may indicate ischemic
damage of the myocardium.
Standard limb leads
Leads means recording electrodes located on the skin. Lead system
was first devised by William Einthoven(1860-1927), he consider the
heart lies in the center of a triangle formed by the left and right
shoulders and the pubic region (this is called Einthoven triangle). The
standard limb leads (electrod are connected to the left arm, right arm
and left leg.
Lead I: record the potential differences between LA and RA.
Lead II: between RA and LL.
Lead III: between LA and LL.
Arrhythmias
1-Sinus tacchycardia and sinus bradycardia: due to increase or
decrease S-A node discharge leading shorten the P-R interval (in
tacchycardia) or length the P-R interval (in bradycardia).
2-Atrioventricular block (A-V block)
a-first degree:Characterized by prolongation P-R interval (greater than
0.2 sec.)
b-Second degree: not all P-wave followed by QRS
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c-Third degree:is complete heart block(the atrial and ventricular
rhythm are independent) usually it is associated with syneape due to
insufficient cerebral blood flow . In this case artificial pacemaker are
used to insure normal ventricular frequency.
3-Premature systoles: due to ectopic foci.
Ectopic foci
S-A node‫تعني حصول تحفيز من منطقة غير‬
a-Premature atrial systole:characterized by inverted P-wave and there
is normal QRS
b-Premature ventricular systole: the QRS and T waves are different
from normal waves but the natural rhythm is not different.
4-Fibrillation: it is an irregular type of contraction without adequate
blood pushing.
a-atrial fibrillation:in ECG there is no P-wave but replaced by
irregular fluctuation of potentials called f-waves. The pulse is
irregular. It happen in chronic heart diseases.
b-ventricular fibrillation(lead to loss of consciousness within a few
seconds).
There is irregular continous uncoordinated twitching of the ventricular
muscle result in no output of blood, death will occur. It happen when
the coronary blood supply to the myocardium is stoped (infraction)
also occure in response to certain drugs and anesthesia.
Cardic Output
Is the volume of blood pumped by each ventricle per minute.
Cardic output= Heart rate x Stroke volume
C.O= HR x SV (beat/minute) (Liter/beat).
Stoke volume:is the volume of blood ejected by each ventricle during
each beat.
E.g.: HR= 72,
SV=70ml/beat.
C.O.=72 x 0.07 = 5L/min (in resting adult).
During exersice the C.O. may reach 30-35L/min.
Factors control the C.O.
1-Control of HR
A- Autonomic control
1-Carotid sinus reflex.
Pressure in carotid sinus
glossopharyngeal nerve(IX)
cardio
inhibitory center (medulla) vagus nerve(X) S-A discharge
AV discharge HR C.O
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2-Aortic reflex.
Pressure in aortic (baroreceptors)
medulla (cardiol inhibitory
center) (parasympathetic)
vagus S.A and AV
HR
C.O
3-Atrial reflex (Bainbridge reflex)
Atrial pressure
stimulate baroreceptors
HR and force of
contraction
C.O
B-Chemical
1-Epinephrin from adrnal medulla
Sympathetic activity
epinephrine
S-A node and myocardium
contraction
HR and contraction
C.O
+
+
+
+
2-K and Na
K and Na
HR and contractility
C.O.
++
++
3- Ca
Ca
HR and contractility
C.O.
C-Temperature, emosions, sex, and age.
1-Temperature .
Tempreture (fever, exercise) HR. and contractility C.O
2-Emosions
Fear, angry, and anxiety
sympathetic discharge
HR C.O
Depression
stimulate cardioinhibitory center
HR
C.O
3- Sex and age
In female HR is faster the male.
At birt the HRis faster and will be decreased as the age is progressed.
2-Control of SV
A-End-diastolic volume (DDV)
1-length of ventricular diastole
Ventricular diastole
ventricular filling
vent.volum
SV
C.O
2-venous pressure
Venous pressure
venous return
vent.filling SV CO
B-End-systolic volume(ESV)
1-Arterial pressure
Art.press.
ESV
SV
C.O
Art.press.
ESV
SV
C.O
SV=EDV-ESV ( ESV SV and vice versa).
2-Force of ventricular contraction
a- sympathetic stimulation
SA node and AV node ventricular
muscle contraction SV C.O
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b-changes in the degree of stretching of ventricular muscle secondary
to changes in ventricular volume.
This is the intrinsic factor relaxationship between end-diastolic
pressure volume and stoke volume which called starling law of the
heart which mean that the more distended ventricle responds with
more forceful contraction.
The mechanism for this is that cardic muscle like skeletal muscle
increases its strength of contraction when it is stretched.
C-circulating EP. And NE. also produce increase of myocardial force
of contractiuon .
D- chemoreceptors: is a receptors in the hypothalamus sensitive to
decrease of O2 level and increase of CO2 level in the blood. This
receptors will stimulate the cardioacceletory center in the medulla in
response to
O2 and CO2 to stimulate SA-node, A-V node and
ventricular myocardium which will increase HR and force of
contraction to increase SV which will increase cardiac output.
CO2 and O2 St.cardioacceletory center HR and SV C.O
Cardiovascular Physiology
Function of cardiovascular system:
1-Transport and distribute essential substance to the tissues.
2-Remove by-product of metabolism.
3-Regulate body temperature.
4-Humoral communication throughout the body.
5-Adjustment of O2 and nutrient supply in different physiology states.
The CVS consists of:
1-Pump(heart)
pulmonary circulation , systemic circulation
2-Distributing and collecting tubes (arteries and veins).
3-Extensive system of thin vessels which permit rapid exchange
between tissues and vasculas channels (capillaries).
Hemodynamics
The arterial blood flow is pulsatile in character (due to cardiac
ejection) and become steady in the capillaries because of large surface
area so the exchaenge of diffusible substance occure between blood
and tissue.
*The heart is an extremely complication pump.
*The blood vessels are multibranched elastic tubes of varying
diameters.
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*The blood is not a simple fluid, it is a suspension of cells
(RBCs,WBCs, platelets), lipid globules, proteins.
Velocity of Blood Stream
Vlocity The rate of displaceme with respect to time (cm/sec.) or
(distance per unit time ) .
Flow: volume per unit time (cm3/sec.)
V=velocity
Q= flow
A=cross-sectional area
Steady flow: is the absence of variation of flow in time (non pulsation
flow).
Laminar flow: is a gradient of flow rate between the stationary
peripheral layer of fluid and the rapidly flowing central layers of
fluide.
Viscosity: is the friction exerted by the movement of laminae of blood
relative to each other.
Resistance: hydraulic resistance is the ratio of pressure drop to flow.
The Vascular System
1-Blood flow: the amount of blood that passes through a blood vessel
in a given time. Factors which control blood flow are:
a- differences in pressure between the two ends of the tube.
b- resistance of flow which means friction between tube wall and fluid
and between the molecules of the fluid. So resistance depend on the
nature (property) of the fluid (viscosity) and the geometry of the tube
(length and radius).
*The flow is directly proportional to the pressure difference and
inversely proportional to the resistance.
Fraction
viscosity
resistance
flow
Length
flow (but it is constant in blood vessels).
*radius is markedly determine the resistance while the length of blood
vessels and viscosity of blood is constant.
II=constant , r=radius , n= viscocity, L= length of tube
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This formula is called poiseuiilles formula
*The radius of blood vessel can be change significantly and constitute
the most impotant factor in the control of resistance to blood flow.
2-Arterial Blood pressure.
The contration of the ventricles ejects blood into the systemic and
pulmonary arteries during systole.
This will distends the arteries by raising arterial blood pressure.
The systolic pressure: is the maximum pressure reachen during peak
ventricular ejection.
The diastolic pressure: is the minimal pressure occure just befor
ventricular contraction.
The blood pressure :is generally recorded as
Systolic
Diastolic
Plus pressure: can be felt in an artery is due to the difference between
systolic and diastolic pressure.
Plus pressure= systolic pressure-diastolic pressure
= 120-80=40mmHg.
Factors which change pulse pressure are:
1- an increased stroke volume will systolic pressure.
2-decreased arterial distensibility (as in arteriosclerosis) will systolic
pressure.
The bood pressure is regulated by:
1-Cardic output (SV and HR) C.O BP.
2-Peripheral resistance
R
BP.
3-Blood viscosity B.vis.
BP
Polycythemia and plasma proteins
viscosity.
1-Cardic output
2-Viscosity
3-Peripheral resistance which determined primarily by the caliber of
the arterioles. The caliber of the arterioles under autonomic control by
the control of the vasomotor center in the medulla.
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Mean arterial pressure:
Is the most important pressure because it is the average pressure
driving blood into the tissues throughout the cardiac cycle.
Mean arterial pressure is not the value half way between systolic and
diastolic pressure because diastolic usually lasts longer than systole.
Mean arterial pressure
Hypertension : increased arterial blood pressure than normal.
Hypotension: decreased arterial blood pressure than normal.
Baroreceptors: are pressure or stretch receptors located in the carotid
sinus and aortic arch. They respond to the changes in the arteries
induced by changes in arterial blood pressure through either inhibition
or stimulation of the vasomotor center in the medulla oblongata.
Mareys Law: is the relationship between increased blood pressure and
decreasd heart rate during regulation of blood pressure.
Chemoreceptors : are receptors found in the vasomotor center that
sensetive to CO2 and in the aortic are carotid bodies that sensitive to
O2.
Cerebral Circulation
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R. external carotid artery supplies right side of thyroid gland , tongue,
throat, face , ear , scalp and dura matar.
R. internal carotid artery supplies brain , right eye, and right sides of
forehead and nose.
L. external and internal carotid arteries are similar to that of right .
* Inside the cranium , anastomoses of left and right internal carotids
along with basilar artery from a hexagonal arrangement of blood
vessels at base of brain near sella turcica called cerebral arterial circle
(circle of Willis). From this circle arise arteries supplying most of the
brain .
The function of the cerebral arterial circle are:
1-to equalize blood pressure to brain
2-provide alternate routes for blood to brain in case of arterial damage.
Exchange of gases and material occur in the sinuses of the brain.
Right and left internal jugular veins receive blood from face and neck.
They arise as continuation of sigmoid sinuses at base of skull.
Intracranial vascular sinuses are located between layers of duramater
and receive blood from brain, Other sinuses that drain into internal
jugular include superior sagital sinus, inferior sagital sinus, straight
sinus and transverse (lateral) sinuses.
Internal jugulars descend on either side of neck and join with
subclavian veins in the same side to form right and left
brachiocephalic veins. From here blood flows into superior vena cava.
Right and left external jugulars. They drian blood from parotide
(salivary) glands, facial muscle, scalp and other superficial structures
into subclavian veins, then into brachiocephalic veins and then into
superior vena cava.
Fetal circulation :
It is not necessary for the fetal heart to pump much blood through
either the lungs or the liver because :
1-the lungs are nonfunctioning during fetal life.
2-the liver is only partially functional.
There is a specific anatomic structure of fetal circulation:
1-Blood returning from the placenta through the umbilical vein passes
through the ducts venosus by-passing the liver into inferior vena cava.
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2-Then most of the blood (oxygenated) entering the right atrium from
the inferior vena cava is directed in a straight pathway through the
foramen ovale directly into the left atrium and then into the left
ventricle.
3-Other blood (deoxygenated) entering the right atrium from superior
vena cava is directed into the right ventricle then into pulmonary
artery then into the descending aorta through the ducts arteriosus.
4-Blood is returned through the two umbilical arteries into the
placenta thus the deoxygenated blood becomes oxygenated again.
*At birth, when lungs, renal, digestive and liver functions are
established, the followings changes occure:
1-The umbilical arteries vasoconstrict shut and atrophy to become the
medial umbilical ligaments.
2-The umbilical vein vasoconstricts shut and become the round
ligament of the liver.
3-The placenta is delivered by the mother as the after birth.
4-The ducts venosus vasoconstricts shut and becomes the ligamentum
venosum (a fibrous cord in the liver).
5-the foramen ovale normally closes shortly after birth to become the
fossa ovalis (a depression in the internal septum).
6-The ductus arterious closes by vasoconstriction and atrophies and
become the ligamentum arteriosus.
Pulmonary Circulation :
Pulmonary circulation defined as the flow of deoxygenated blood
from the ventricle to the air sacs of the lungs and the return of
oxygenated blood from the air sacs of the lungs to the left atrium.
Pulmonary trunk emerge from the right ventricle and passes upward,
then divide into right and left pulmonary arteries to the right and left
lungs respectively. On entering the lungs, these arteries divide and
subdivide until they from capillaries around the alveoli (air sacs) in
the lungs. CO2 is passed from the blood into the alveoli and O2 is
passed from the alveoli into the blood. The capillaries unite, venules
and veins are formed and eventually two pulmonary veins exit from
each lung and transport the oxygenated blood to the left atrium.
Blood flow through the lungs differs from systemic circulation in
several ways:
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1-The pulmonary arteries have lager diameter, thinner wall and less
elastic tissue than their counterpart systemic arteries. As a result there
is little resistance to blood flow. This means that less pressure is
required to move blood through the lungs.
2-There is an autoregulation mechanism in response to levels of
oxygen. In systemic circulation, blood vessels dilate in response to
low oxygen concentration. In pulmonary circulation, blood vessels
constricts in response to low oxygen concentration. This mechanism is
very important in the distribution of blood in the lungs who it is
needed most.
3-There is a different between pulmonary and systemic circulation
relates to capillary hydrostatic pressure. In systemic, the capillary
hydrostatic pressure=25 mmHg but in pulmonary, the capillary
hydrostatic pressure= 15mmHg because the colloid osmotic pressure
in the interstitial fluid of the lungs is high. This difference is to
prevent pulmonary oedema.
*Pulmonary oedema may develop from:
a-increased capillary blood pressure due to increased left atrial
pressure (e.g. in case of mitral valve stenosis).
b-increased capillary permeability due to some bacterial toxin.
*Pulmonary oedema lead to decrease diffusion rate of O2 and CO2
which lead to inhibits the exchange of gases in the lungs.
Hepato-Portal Circulation:
Define as the flow of venous blood from the gastrointestinal organs
and spleen to the liver before returning to the heart. (which can be
summarized as following:
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Coronary Circulation:
The left and right coronary arteries and their branches supply blood to
the heart muscle. These arteries branch from the base of the ascending
aorta as it leaves the heart. They traverse the anterior surface of the
heart in the grooves and pass to the posterior surface, where their
branches anastomose or join together. (This anastomoses is for
maintaining a blood supply to the cells of the heart when a vessel
becomes blocked). Numerous branches from the main arteries carry
blood to the capillary networks among the muscle cells.
After passing through the capillaries, blood enters the cardiac veins.
Many of these veins unit to form a large vessel called the coronary
sinus which carries blood into the right atrium.
Other veins(from the wall of the right ventricle) return blood directly
to the right ventricle. Sinusoids ‫ جيبانيااا‬return blood from the
myocardium directly into the cavities of the ventricles. Blood flow in
heart muscle vessels only when the heart is relaxed (contraction
compresses the vessels and prevents the flow).
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