Download Notes to Heart 3

2402 : Anatomy/Physiology
Dr. Chris Doumen
Lecture 1
Hemo Dynamics and Blood Vessels
Introduction
TextBook Readings
♦ Pages 721 through 734.
♦ Make use of the figures
in your textbook ; a
picture is worth a
thousand words !
♦ Work the Problems and
Questions at the end of
the Chapter
Hemodynamics is the study of the forces that are involved in the circulation of blood
throughout the body
•
Arteries = vessels that carry blood away from the heart
•
Veins = vessels that carry blood to the heart
The complete systemic cardiovascular circulation can thus for example be summarized
as follows
Left Ventricle ---> Aorta ---> arteries ---> arterioles ---> Capillaries ---> venules --> veins ---> vena cava ---> Right atrium
Blood Vessel Anatomy
Ou te r l aye r : Tu nic a e x te rna or A dventiti a
•
elastic fibers and collagen fibers
•
in large arteries, they contain small blood vessels that feed this layer =
vas o vas orum
Mi ddle l aye r : Tu nic a Medi a
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elastic fibers and smooth muscle
Inner l ayer : Tu nica inte rna
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single endothelial cell layer and a basement membrane
Termi nolog y
Vasoconstriction
The decrease in lumen of a blood vessel. It is due to due to contraction of
smooth muscle layerand is mostly a sympathetic impulse
Vasodilation
The increase in lumen and due to relaxation of the smooth muscle
Collin County
Community
College District
2402 : Anatomy/Physiology
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Types of Blood Vessels
ARTE RIES
Large arte ries
• Tunica media contains more elastic fibers, less muscle
•
•
Thus called elastic arteries or conducting arteries due to their wide lumen
Function as a pressure reservoir; blood ejected from the heart stretches the arteries
after which they recoil
•
They help to push blood through the body when heart is in diastole phase ( heart is at
total rest 0.4 sec of each beat)
•
Reduced elasticity reduces efficient blood flow
• Ex : aorta, carotid, subclavian
• Large walls help to withstand the continuous pressure on these vessels
Me diu m A rte ries
• Contain more smooth muscle in T. media. Called the muscular arteries
•
Function to distribute the blood as they are capable of vasodilation and constriction,
thus regulating blood flow
Arte riol es
• small arteries : they resemble muscular artery at beginning but end up into a capillary
Capill ari es
• Composed of a single layer of cells (endothelium layer) and a basement membrane
•
•
No T. media or T. externa
Function to permit exchange of nutrients and waste products between the blood and
the tissues they serve . The more metabolic active a tissue is, the more extensive the
capillary network
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2401 : Anatomy/Physiology
Types of Capillaries
1.Continuous
•
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endothelium cells provide an un-interrupted lining
adjacent cells joined by tight junctions (like zippers)
•
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occasional gaps in the tight junctions allow for fluid and small solutes to pass
common in skin, muscles
brain has very snug fitting tight junctions with no gaps at all; forms the blood-brain
barrier
2. Fe nes t rate d
•
•
same as above but endothelial cells have oval pores lined with thin membrane and allows
for more permeability
3. Sinu soi dal
• many fenestrations, fewer tight junctions which allows for passage of larger molecules
Capillary beds are the interweaving networks formed by the capillaries. They consist of
• an incoming terminal arteriole and branches into a meta-arteriole
•
•
meta-arteriole branches into a network of true capillaries
true capillaries unite again with an extension of the meta-arteriole ( = thoroughfare
channel )
•
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thoroughfare channels ends up in the postcapillary venule
precapillary sphincters between meta-arteriole and true capillaries can direct blood into
the capillary bed or shunt it directly into the venule (= vascular shunt)
VEINS
Venule s
• drain blood from the capillaries and guide the blood towards the veins
• initial venules are mostly T. interna, eventually developing a thin T. media and T. externa
Veins
•
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Have all 3 Tunica present but very poorly develop;ed
T. externa usually several times thicker than T. media
•
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Lumens are larger than corresponding arteries
Many veins have valves that prevent backflow (low BP in veins)
•
collapse of these valves results in varicose veins
•
obesity, standing, pregnancy can cause such results
2401 : Anatomy/Physiology
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Blood distribution
Veins, venules
Systemic arteries , arterioles
Pulmonary vessels
Heart
Syst. capillaries
65 %
15
12
8
5
Veins and venules are thus a blood reservoir (esp. liver and spleen). Protects against massive
blood loss .
HEMODYNAMICS
Blood fl ow
= volume of blood that flows per unit of time (ml/min)
Blood vel ocit y
= distance blood travels per unit of time (cm/min)
= blood flow per cross sectional area
= ml/min/cm2 (and since 1 ml = 1 cm3 , this becomes cm/min)
The translation of this thus means that
blood flows slower in a vessel with
greater lumen
Does this mean blood flows faster in a
capillary versus an artery ?
In theory yes. However, each time an
artery branches, the blood will flow in
all these branches and one needs to
take into account the t ot al cross
sectional area of all the branches.
Although each capillary is extremely
small, the total cross sectional area of
all capillaries is much larger than the
diamater of the original vessel.
The C.S. area of th e aort a is 3-5
cm 2 ; th at of the c apill aries is
4500-600 0 cm 2 . Blood veloci ty
thus drops from an origi nal 40
cm/min t o 0.1 cm/mi n. Thi s sl ow
blood
flow
is
of
ex treme
import ance i n th at i t allows f or
adequ ate
time
f or
e fficie nt
exch ange of g ase s and nu t rients
be twe e n blood and su rrou ndi ng
tiss ues
2401 : Anatomy/Physiology
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Blood Pressure
Blood flow in the complete circulatory system is equal to cardiac output (CO)
CO = H e art rat e x St rok e volum e
Besides HR and SV, two other factors influence CO : Blood pressure and Resistance
It is a straightforward to see that the higher the pressure difference between two points, the
higher the blood flow. Also, the more resistance the "pipes" offer, the lower the flow will be. Or
putting it in mathematical form
CO = Bl ood p ress ure
Re sist ance
or
Blood p res su re = CO x R
Let us examine how each component comes into effect
1. Blood pressure
•
is the pressure exerted by the blood on the systemic artery walls
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increase (decrease) in CO results in increase (decrease) in BP
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since blood volume determines veous return and thus CO, it also determines BP
2. Resistance
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is the opposition to blood flow (friction)
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several factors determine resistance
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increased blood viscosity (ratio of RBC to plasma) increases Resistance,
increases BP at constant CO
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Resistance increases with vessel length (obese person needs more vessels to
serve additional fat layers)
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Resistance is inversely proportional to the fourth power of the radius. So the
smaller the radius , the higher the resistance. (If radius decreases by a
factor 2, resistance goes up by a factor 16 )
•
Simply put, blood flow is directly proportional to the fourth power of the
radius of a vessel. ( see relationship between CO and R ). Thus, by relaxing a
vessel such that the rdius increases two fold, blood flow will increase 16 fold.
It would appear from the other equation ( BP = CO x R ) and that due to the fact that the
capillaries have an extremely small diameter, that BP would be high in the capillaries. However
resistance of tubes placed in parallel add up reciprocally. The total resistance (Rt) of tubes with
resistance R1, R2, R3,... put in parallel is such that
1 =
Rt
1 + 1 + 1 + ....
R1 R2 R3
Whe n m ore th an 16 of such tu be s are arrang ed, the t ot al R esis t anc e (Rt ) bec ome s
much sm all er th at the resis t anc e of one tu be of simil ar t otal c ros s s ecti onal are a.
That's wh y the B P i n t he c apill aries i s dras t ically sm alle r t han for ex am ple aortic
BP and t he re as on wh y the re i s a drop i n BP goi ng f rom aorta t o c apillarie s.
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The resistance of the total vascular system is called the s ys temic vasc ul ar resis t anc e (SVR).
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Most resistance occurs in the arterioles, capillaries and venules.
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The veins and arteries are too big to offer any major contribution to SVR.
Arterial Blood Pr essure
Pressure in a container , such as a ventricle or blood vessel, is determined by
• the volume in the container
• the “stretchability” of the container
The easier the container can be stretched, the more volume can be added without increasing
pressure.
In a similar way, adding a small amount of water to a balloon that does not stretch well will
increase the pressure more rapidly.
This is expressed as Compliance.
Compliance = Δ Volume / Δ pressure
A high compliance indicates that a large increase in volume is correlated with small increase in
pressure, or that small pressure increases will result in large volume expansions.
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2401 : Anatomy/Physiology
When the heart ejects its blood, it creates a flow and a pressure.
If the arteries were rigid and not expandable, pressure in the arteries would be
• high when the ventricles eject blood
• practically zero when the ventricles relaxed
This would make the blood flow in spurts in our circuits and not a desired condition.
The elastic properties and high compliancy of the elastic arteries results that, each time blood is
ejected into the arteries, they will expand and stretch, and thus hold more blood.
About 30 % of the heart stroke volume, exits the elastic arteries at each systole. The rest expands
the arteries and builds up pressure.
When the ventricles go into diastole, the elastic arteries recoil and expel the rest of the arterial
blood into the circuits.
The Blood pressure in the aorta and arteries is thus pulsating due to the elastic arteries that expand
and recoil with each heart beat.
General aspec ts of Arte rial Bl ood P re ss ures
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The peak arterial pressure is called systolic pressure and is on average about 120 mm Hg
The minimal arterial pressure (after recoil) is the diastolic pressure is about 80 mm Hg
The difference between systolic and diastolic pressure is the pul se p res su re that is being
felt when palpitating for your heat rate. (also very visible when an aorta is cut; blood comes
out in spurts).
Reduced elasticity in the aorta and arteries increases the diastolic pressure ( less relaxing of
arteries, pressure stays higher)
2401 : Anatomy/Physiology
MAP
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= Diastolic pressure + {Pulse pressure/3}
Look at the figure above. You can see that Pulse pressure is highest in the aorta and elastic arteries
and gradually decreases. It eventually disappears when reaching the muscular arteries and blood
flow becomes steady.
Blood pressure drops as well due to the accumulating effect of resistance. By the time blood reaches
the capillaries, BP ~ = 40 mm Hg and at the venous end of the capillary bed BP ~= 20 mm Hg.
The pressure difference between venules and atria is about 15
mm Hg. This and is too low to promote adequate venous return to the heart by simple pressure
difference. Remember that blood flow is dependant on a Pressure differential.
The pressure difference between venules and atria is about 15 mm Hg, and is too low to promote
adequate venous return to the heart by simple pressure difference.
Several factors help to bring the blood that resides in the veins side of the systemic circuit back to
the heart ( = ve nous ret u rn ).
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Low resistance in the veins ; the low resistance conduit system allows low pressure
differentials to provide adequate flow
Veins run between muscles; contraction of muscles has a milking effect, squeezing blood
upwards. Also, veins contain one-way valves that operate like elevator stages
Respiratory pump action of inspiration causes a decrease in pressure in the chest cavity while
an increase in the abdominal cavity by the downward movement of the diaphragm. Helps to
promote blood flow into the thoracic area
Sympathetic stimulation of smooth muscles in veins results in vasoconstriction and increases
venous pressure, driving more blood back to the heart.
Weak valves in the veins causes varicose veins ( due to too much pressure on them; obesity,
pregnancy, standing). Also long immobilization with no leg movement may prevent adequate venous
return as well and causes dizzy spells when standing up or when standing still for too long.