Download Regulation of blood flow

REGULATION OF BLOOD FLOW.
Common characteristic of blood flow
The direct effect of heart contraction is creation of certain
level of blood pressure, which is allows blood circulation.
Blood flow is continuous, although heart pumps the blood by
separate portions. It caused by functioning of all components
of cardio-vascular system: heart, arteries, arterioles,
capillaries, venuls and veins. Besides that continuous blood
flow is caused by extracardial factors as skeletal muscle
contraction and pressure gradient between abdominal and
thoracic cavities. Cardiac output depends on high, mass and
area of human body surface. Cardiac output is regulated by
contractive activity of cardiac muscle; valve function of full
value; blood volume, vascular tonus, blood flow in capillaries;
value of blood returning to the heart. In general distribution of
cardiac output between different organs corresponds to its
functional activity. Part of common blood supply, which every
organ gets, depends on necessity in O2 and substrates of
energy exchange. Average time of blood circulation
measures 20-23 s.
Powers, which causes blood flow
Blood flows in vessels from high pressure
to low. Heart pumping causes initial
pressure. The highest pressure is large
arteries ascending from the heart. Pressure
in aorta at the end of systole is 110-125
mm Hg, at the end of diastole - 70-80 mm
Hg. In pulmonary trunk during systole the
blood pressure is 20-25 mm Hg, in diastole
- 10-15 mm Hg. In large arteries blood flow
velocity is 0.1-0.2 m/s. In large veins
returning blood flow to the heart is caused
by lowest blood pressure - 0 mm Hg.
Functional importance of blood
circulation system.
Both pulmonary and systemic circulation,
compose entire system of blood circulation and
function in correlation. The right ventricle is
responsible for blood pumping into pulmonary
circulation. Here blood is oxygenated and CO2 is
taken out. The left ventricle pumps blood into
systemic circulation. Blood flow in this part of
vascular system provides performing of all other
blood functions as regulatory, protective, excretory
and others. Both right and left parts of heart pump
equal portions of blood into corresponding vessels
and function in interconnection to each other. The
minute blood volume in pulmonary and systemic
circulation is the same.
Circulating blood volume
Blood flowing in vessels is similar to stream of fluid in the
pipe, but has a lot of specificities. Fluid stream in the pipe is
described by formula:
Q= (P1-P2)/R, where
Q - fluid volume,
P1 - pressure in the beginning of the pipe,
P2 - pressure in the end of the pipe,
R - peripheral resistance of the pipe.
So fluid volume, which flows through the pipe is directly
proportional to pressure difference from the end to beginning
of pipe; and inversely proportional to peripheral resistance of
pipe. As vessels have elastic walls, the blood flow in it, is
differ from the same in pipe. Vessel cross-section may
change due to neural and endocrine influences according to
necessity.
Blood volume flowing through every part of vascular system
per time unit is the same. It means that through aorta or
cross-section of all arteries, capillaries or veins flows equal
volume of blood. This volume per minute is called minute
blood volume and measures in adults in rest 4.0 - 6.5 l/min.
Peripheral resistance of vessels
Peripheral resistance in vessels according
to Poiseuille's formula depends on length
of vessels (l), viscosity of blood (η) and
cross-section of vessel (r):
R= 8lŋ/πr.
In accordance to this formula the highest
peripheral resistance might be in the
smallest vessels. In reality the highest
resistance is observed in arterioles.
Average blood flow resistance in adults is
equal to 900-2500 din·s/sm5
Paradoxes of blood flow.
In capillaries blood flow resistance is a bit lower
because of such mechanism. In capillaries blood
cells move one after another, dividing only by
plasma, which decreases friction between blood
cells and capillary wall. On other side, capillaries
are shorter, than arterioles, which caused lower
blood flow resistance too.
Viscosity of blood is also important for resistance
of vessels. It depends on quantity of blood cells,
protein rate in plasma, especially globulins and
fibrinogen. Considerable increase of blood
viscosity may cause lower blood returning to the
heart and than disorders of blood circulation.
In large arteries centralization of blood flow is
observed. Blood cells moves in the central part of
blood stream, and plasma is peripheral. Instead
increase of blood viscosity in arterioles is caused
by higher friction between cells and vessels wall.
Linear velocity of blood flow
Blood flow also is characterized by linear velocity
of blood circulation:
V=Q/πr2, where
V - linear velocity,
Q - blood volume,
r - radius of vessel.
So it is clear the wider cross-section of vessel the
slower linear velocity of blood stream. In large
arteries linear velocity is highest (0.1-0.2 m/s). In
arterioles it measures 0.002 - 0.003 m/s, in
capillaries - near 0.0003 m/s. In veins crosssection decreases and linear velocity increases to
0.001 - 0.05 m/s in large veins and to 0.1 - 0.15 in
vena cava.
Blood pressure
Transversal pressure - is difference between
pressure inside the vessel and squeeze of it from
the tissues. When increasing the tissue pressure
to vessel wall, it closes. Hydrostatic pressure is
corresponding to weight of all blood in vessel
when it has vertical position. For vessels of head
and neck this pressure decreases towards the
heart. For vessels of limbs it has outward
direction. That is why hydrodynamic pressure in
vessels over heart is decreased due to
hydrostatical pressure. Below heart hydrodynamic
pressure is increased, because it is summarized
with hydrodynamic pressure.
Role of changing body position
In horizontal position of the body hydrostatical
pressure is equal in every part of the body and
hydrodynamic pressure doesn't depend on it. In
vertical position transversal pressure in vessels of
limbs creates tension of vessels walls (Laplas
low):
Pt=F/r, where
Pt – transversal pressure,
F - vessel tension,
r - radius of vessel.
So it is shown the smaller radius of vessel, the
lower tension in vessels walls. Due to this
capillaries with thinnest wall don't crush because
of its smallest diameter. Existence of precapillary
sphincters permits proper direction of blood
pressure so that capillaries may close (plasmatic
capillaries).
Local regulatory mechanisms
Collagen fibers of vessels walls form net, which
prevent its tension or decrease tone. Smooth
muscle cells combine with elastic and collagen
fibers in vessels walls. Contracting and stretching
these fibers smooth muscle cells produce active
tension of vessel wall - tonus of vessels.
There are some mechanisms in regulation of
vessel tonus by smooth muscles. When rapid
increasing of blood pressure, smooth muscles
contract and decrease tension by decreasing
vessel diameter. In slow rising of pressure tension
decrease by dilation of smooth muscles and
increase vessel diameter. These mechanisms
occur more often in veins than in arteries. Veins
have equal elasticity in systemic and pulmonary
circulation, but arteries are more extensible in
pulmonary circulation. Arteries in pulmonary
circulation contain a lot of elastic and smooth
muscle fibers.
Functional types of vessels
- Elastic (damping) vessels. Large arteries belong
to this group. The main function of these vessels is
to turn ejection of blood into continuous blood flow.
It is possible due to elastic properties of its wall;
- Resistive vessels are arterioles, precapillary
sphincters and venuls. These vessels may
regulate the blood flow in capillaries by changing
their tonus;
- Exchange vessels are capillaries. Their walls due
to the special structure permit exchange of
materials between blood and tissues;
-Capacitive vessels are veins. To sure one-way
direction of blood flow veins have valves if lying
below the heart. Veins contain 75-80 % of
circulating blood. Veins of skin and abdominal
cavity may function as depot of blood.
Local regulation of blood flow
Role of metabolic factors: Greater rate of
metabolism or less blood flow causes decreasing
O2 supply and other nutrients. Therefore rate of
formation vasodilator substances (CO2, lactic
acid, adenosine, histamine, K+ and H+) rises.
When decreasing both blood flow and oxygen
supply smooth muscle in precapillary sphincter
dilate, and blood flow increases. In is a vasodilator
substance as nitric oxide released from endothelial
cells released from endothelial cells. It causes
secondary dilation of large arteries when micro
vascular blood flow increases. Cardiac muscle
utilizes fatty acids for energy. Cardiac muscle
utilizes glucose through glycolisis that results in
formation of lactic acid.
Basal tone of vessels.
When arterial pressure suddenly increases local
blood flow tends to increase. It leads to sudden
stretch of arterioles cause smooth muscles in their
wall to contract. Than local blood flow decreases
to normal level. Vessel walls are capable to
prolonged tonic contraction without tiredness even
at rest. Such a condition is supported by
spontaneous myogenic activity of smooth muscles
and efferent impulsation from autonomic nerve
centers, which control arterial pressure. Partial
state of contraction in blood vessels caused by
continual slow firing of vasoconstrictor area is
called vasculomotor tone. Due to regulatory nerve
and humoral influences this basal ton changes
according to functional needs of curtain organ.
Neuro-humoral regulation of
systemic circulation
a) Afferent link. Nerve receptors, which are
capable react to changing blood pressure,
lays in heart cameras, aorta arc, bifurcation
of large vessels as carotid sinus and other
parts of vascular system. Irritation of these
mechanical receptors produce nerve
impulses, which pass to higher nerve
centers for processing sensory information
from visceral organs.
b) Central link. Vasoconstrictor area of vasculomotor center
is located bilaterally in dorsolateral portion of reticular
substance in upper medulla oblongata and lower pons. Its
neurons secrete norepinephrine, excite vasoconstrictor
nerves and increase blood pressure. It transmits also
excitatory signals through sympathetic fibers to heart to
increase its rate and contractility.
Vasodilator area is located bilaterally in ventromedial of
reticular substance in upper medulla oblongata and lower
pons. Its neurons inhibit dorsolateral portion and decrease
blood pressure. It transmits also inhibitory signals through
parasympathetic vagal fibers to heart to decrease its rate
and contractility.
Posterolateral portions of hypothalamus cause excitation of
vasomotor center. Anterior part of hypothalamus can cause
mild inhibition of one. Motor cortex excites vasomotor center.
Anterior temporal lobe, orbital areas of frontal cortex,
cingulated gyrus, amygdale, septum and hippocampus can
also control vasomotor center.
c) Efferent link. Stimulation of sympathetic
vasoconstrictor fibers through alfaadrenoreceptor causes constriction of
blood vessels. Stimulation of sympathetic
vasodilator fibers through betaadrenoreceptors as in skeletal muscles
causes dilation of vessels.
Parasympathetic nervous system has
minor role and gives peripheral
innervations for vessels of tong, salivatory
glands and sexual organs.
Mechanical receptors reflexes.
These are spray-type nerve endings, which are
stimulated by stretch. In increasing blood pressure,
from the wall of carotid sinus impulses pass through
Hering's nerve to glossopharyngeal nerve to solitary
tract in medulla. Secondary signals inhibit
vasoconstrictor center and excite vagal center. It
results in peripheral vasodilatation and decreasing
heartbeat. When arterial pressure decreases whole
processes lead to exciting dorsolateral portion of
vasomotor center and increasing blood pressure and
heartbeat. Similar reflex mechanism starts from
receptors of aortic arc.
Bainbridge reflex is observed when arterial pressure
increases due to increasing blood volume and blood
return. Atria and SA node are stretched and send
nerve signals to vasomotor center. Increasing heart
rate and heart contractility prevent damming up of
blood in pulmonary circulation.
Reflexes from proprio-, termo- and
interoreceptors
Contraction of skeletal muscle during
exercise compress blood vessels,
translocate blood from peripheral vessels
into heart, increase cardiac output and
increase arterial pressure. Stimulation of
termoreceptors cause spreading impulses
from somatic sensory neurons to
autonomic nerve centers and so leads to
changing tissue blood supply. Irritation of
visceroreceptors results in stimulation of
vagal nuclei, which cause decreasing blood
pressure and heartbeat.
Haemodinamic in special body
conditions
Changing body position.
Change body position from vertical to horizontal
and vice versa is followed by redistribution of
blood. Under the influence of gravity veins in lower
half of the body are dilated and may contain
additional near 0,5 l of blood. After this
impulsations from baroreceptors is activated and
resistive vessels are contracted, mainly in skin and
muscles. At the same time rate of heartbeat
increases, which permit make up for cardiac
output. In insufficient reflex regulation orthostatic
unconsciousness may occur.
Regulation of blood flow in physical
exercises
In physical exercises impulses from pyramidal
neurons of motor zone in cerebral cortex passes
both to skeletal muscles and vasomotor center.
Than through sympathetic influences heart activity
and vasoconstriction are promoted. Adrenal
glands also produce adrenalin and release it to the
blood flow. Proprioreceptor activation spread
impulses through interneurons to sympathetic
nerve centers. So, contraction of skeletal muscle
during exercise compress blood vessels,
translocate blood from peripheral vessels into
heart, increase cardiac output and increase
arterial pressure.
Changing blood volume after
bleeding.
In changing blood volume volumic
receptors in vena cava or atria are
activated. These impulses spread to
both medulla oblongata and
osmolarity regulating neurons in
hypothalamus. In consequence
decreasing blood volume heart
activity rises through sympathetic
activation and vasopressin in
released from hypophisis.
Cerebral circulation
Intensity of blood flow is 750 mL/min; 54 mL/100
g/min.
Except for a small contribution from the anterior
spinal artery to the medulla, the entire arterial
inflow to the brain in humans is via 4 arteries: 2
internal carotids and 2 vertebrals. The vertebral
arteries unite to form the basilar artery; and the
circle of Willis, formed by the carotids and the
basilar artery, is the origin of the 6 large vessels
supplying the cerebral cortex. Venous drainage
from the brain by way of the deep veins and dural
sinuses empties principally into the internal jugular
veins in humans, although a small amount of
venous blood drains through the ophthalmic and
pterygoid venous plexuses, through emissary
veins to the scalp, and down the system of
paravertebral vein in the spinal canal.
Regulatory influences
The sympathetic fibers on the pial arteries
and arterioles come from the cervical
ganglia of the sympathetic ganglion chain.
The parasympathetic fibers pass to the
cerebral vessels from the facial nerve via
the greater superficial petrosal nerve.
Cerebral has autonomic circulation.
Adenosine, histamine, serotonine,
prostaglandines caused vasodilatation.
Coronary circulation
2 coronary arteries that supply the
myocardium arise from the sinuses behind
the cusps of the aortic wave at the root of
the aorta. The right coronary artery has a
greater flow in 50 % of individuals, the left
has a greater flow in 20 %, and the flow is
equal in 30 %. There are 2 venous
drainage systems: a superficial system,
ending in the coronary sinus and anterior
cardiac veins, that drains the left ventricle;
and a deep system that drains the rest of
the heart.
Compressive influences role
The heart is a muscle that compresses its
blood vessels when it contracts. The
pressure inside the left ventricle is slightly
greater than in the aorta during systole.
Consequently, flow occurs in the arteries
supplying the subendocardial portion of the
left ventricle only during diastole, although
the force is sufficiently dissipated in the
more superficial portions of the left
ventricular myocardium to permit some
flow in this region throughout the cardiac
cycle.
Metabolic and chemical factors
significance
Factors that cause coronary
vasodilatation: O2, CO2, H+, K+,
lactic acid, prostaglandines, adenine
nucleotides, adenosine. Asphyxia,
hypoxia, intracoronary injections of
cyanide all increase coronary blood
flow 200-300 % in denervated as well
as intact hearts.
Neural regulation
The coronary arterioles contain alphaadrenergic receptors, which mediate
vasoconstriction, and beta-adrenergic
receptors, which mediate
vasodilatation. Activity in the
noradrenergic nerves cause coronary
vasodilatation.
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