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Chapter 21
Blood Vessels
and
Hemodynamics
Copyright © John Wiley & Sons, Inc. All rights reserved.
Vessel Structure and Function

The blood vessels of the body should not be thought of as
mere “pipes” carrying blood –
they are dynamic,
interactive, essential
components of the
cardiovascular
system.
Basic components of the CV organ system
Copyright © John Wiley & Sons, Inc. All rights reserved.
Vessel Structure and Function

Blood Vessel Types
• Arteries – carry blood away from the heart

Large elastic arteries; medium muscular arteries;
arterioles
• Capillaries – site of nutrient and
gas exchange
• Venules are small veins
• Veins – carry blood towards
the heart
Copyright © John Wiley & Sons, Inc. All rights reserved.
Vessel Structure and Function

Blood vessels in the body share components of 3 basic
layers or “tunics” which comprise the vessel wall:
• Tunica interna
(intima)
• Tunica media
• Tunica externa
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Vessel Structure and Function

The tunica intima (interna) is the inner lining in direct
contact with blood. It consists of:
• The epithelium or the endothelium with underlying
basement membrane
• Internal elastic lamina

The tunica media is chiefly composed of smooth muscle that
regulates the diameter of the vessel lumen, by
vasoconstriction ( VC) & vasodilation ( VD)

It is seperated from the tunica externa by external elastic
lamina.

The tunica externa is connective tissue- contains vasa vasorum
, nerves & anchors vessel to surrounding tissue
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Elastic arteries

The largest arteries are the elastic arteries, the aorta and
its major branches

Also called conducting arteries
• Their tunica media is mainly made up of elastic fibers.
• Elastic arteries are stretched as blood is ejected from
the heart during systole accomodating the surge of
blood (pressure reservoirs)
• They recoil during diastole – help propel the blood
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Pressure reservoir function of elastic arteries
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Muscular arteries

Medium sized muscular (distributing) arteries have more
smooth muscle in their tunica media.

Capable of VC and VD
• Muscle layer remains in a state of
partial contraction- vasomotor tone
to ensure efficient blood flow
• Examples- axillary artery, brachial artery in
the arm and radial artery in the forearm.
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Anastomosis

An anastomosis is a union of vessels supplying blood to
the same body tissue. Should a blood vessel become
occluded, a vascular anastomosis provides
collateral circulation (an alternative
route) for blood to reach a tissue.
• The shaded area here
shows overlapping blood
supply to the ascending colon.
Arteries that do not anastomose are end arteries
Copyright © John Wiley & Sons, Inc. All rights reserved.
Arterioles

Arterioles deliver blood to capillaries and have the
greatest influence on local blood flow and blood pressure.

Sympathetic nerve supply & chemicals can alter the
diameter of the arterioles & change the resistance to flow
• VC- decreases the vessel diameter increases the
resistance & decreases blood flow to capillaries
• VD- increases the vessel diameter decreases the
resistance & increases blood flow
•
Called resistance vessels
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Arterioles

The terminal end of an arteriole
tapers toward the capillary
junction to form a metarteriole.
• At the metarteriole-capillary
junction, muscle cells forms the
precapillary sphincter which
monitors and regulates blood
flow into the capillary bed.
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Capillaries

Capillaries are the only sites in the entire vasculature
where gases, water, nutrients & wastes are exchanged
(exchange vessels)
Capillaries function as capillary beds ( of 10-100
capillaries)

Postcapillary venules are formed
when capillaries unite
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The Microcirculation: flow of blood from a metarteriole, through
capillaries & into post capillary venules
•Pre-capillary sphincters
control blood flow
through capillaries
•When arterioles dilate
and sphincters are openblood flows in entire
capillary bed
•Distal part of metarteriole
is a thoroughfare channeldirect route of blood from
an arteriole to venule
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Capillaries

Capillaries are different from other vascular structures in
that they are made of endothelial lining & a basement
membrane – they lack a tunica media & externa
• This allows capillaries to be permeable to
many substances (gases,
fluids, and small ionic
molecules).
Copyright © John Wiley & Sons, Inc. All rights reserved.
Capillaries

The body contains three types of capillaries:
• Continuous capillaries are the most common with
endothelial cells forming a continuous tube, interrupted
only by small intercellular clefts. E.g. in skin, blood brain
barrier of nervous system
• Fenestrated capillaries (fenestra = windows), found in the
kidneys, villi of small intestines, and endocrine glands are
much more porous.
• Sinusoids form very porous channels (large intercellular
clefts) through which even blood cells can pass, e.g., in
liver, spleen, bone marrow.
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3 Types of capillaries
in the body
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Veins

Veins have thinner walls, and larger lumen compared to
arteries

The largest part of blood volume ( 64%) is in the systemic
veins- and are called blood reservoirs
• Because intravenous pressures are low, veins have
valves to keep blood flowing in only one direction.

When exposed to higher than normal pressures,
veins can become incompetent (varicose veins).
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Veins
Copyright © John Wiley & Sons, Inc. All rights reserved.
Venous Reserve

Because systemic veins and venules contain a large
percentage of the blood volume (about 64% at rest), they
function as blood reservoirs from which blood can be
diverted quickly if needed.
• With a drop in BP,
stimulation of the sympathetic
NS will cause venoconstriction,
allowing a greater volume of
blood to flow to tissues where it is needed more.
Copyright © John Wiley & Sons, Inc. All rights reserved.
Artery
Vein
http://student.britannica.com/eb
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Capillary Exchange

Diffusion:

Of gases, nutrients, wastes, occurs
down the concentration gradient

Water-soluble substances such as
glucose and amino acids diffuse
across capillaries through
intercellular clefts or fenestrations

Lipid-soluble materials, such as
O2, CO2, and steroid hormones,
may diffuse across endothelial cell
plasma membrane
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Capillary Exchange

Transcytosis:

Substances in blood plasma become enclosed within tiny
pinocytic vesicles

enter endothelial cells by endocytosis

move across the cell and exit on the other side of cell by
exocytosis.

Important for large, lipid-insoluble molecules for
example, the hormone insulin ( a protein)
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Capillary Exchange

Bulk flow

Bulk flow is important for regulation of blood & interstitial fluid
volume

Bulk flow is the fluid exchange with a large number of ions and
particles from an area of high to low pressure
• Filtration is the pressure driven movement of fluid through the
walls of the capillary into the interstitial fluid
• Reabsorption is the pressure driven movement of fluid from the
interstitial fluid back into the capillary

As blood flows to the tissues of the body, hydrostatic and osmotic
forces at the capillaries determine how much fluid leaves the
arterial end of the capillary and how much is then reabsorbed at the
venous end. These are called Starling Forces.
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Fluid Exchange & Starling Forces

Two pressures promote filtration:
• Blood hydrostatic pressure (BHP) the capillary blood
pressure - decreases from 35 to 16 from the arterial to the
venous end of the capillary- the pushing force
• Interstitial fluid osmotic pressure (IFOP), which is about 1
mmHg ( due to tiny amount of proteins in IF)
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Fluid Exchange - Starling Forces

Two pressures promote reabsorption:
• Blood colloid osmotic pressure (BCOP) is due to the
presence of plasma proteins too large to cross the
capillary – averages 26 mmHg on both ends- the
pulling force
• Interstitial fluid hydrostatic pressure (IFHP) is
normally zero
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Fluid Exchange - Starling Forces
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Fluid Exchange - Starling Forces

Normally there is nearly as much fluid reabsorbed as
there is filtered.
• At the arterial end, net filtration pressure is outward at
10 mmHg and fluid leaves the capillary (filtration).
• At the venous end, net filtration pressure is inward at –
9 mmHg (reabsorption).


On average, about 85% of fluid filtered is reabsorbed
Fluid that is not reabsorbed (about 3L/ day for the entire
body) enters the lymphatic vessels to be eventually
returned to the blood.
Copyright © John Wiley & Sons, Inc. All rights reserved.
Clinical connection- Edema

Edema is an abnormal increase in interstitial fluid, due to either excess
filtration or inadequate reabasorption

Causes of edema:

Increased capillary hydrostatic pressure causes more fluid to be filtered
from capillaries. e.g. in congestive heart failure

Increased permeability of capillaries – due to chemical released in
inflammation; more fluid to be filtered


Blocked lymphatics- tissue fluid not drained away
Decreased blood colloid osmotic pressure due to reduced concentration of
plasma proteins
• Inadequate dietary intake in malnutrition
• decreased plasma protein synthesis with liver disease
• Loss of plasma proteins in burns
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Hemodynamics: Factors Affecting Blood
Flow

Blood flow is the volume of blood that flows through any
tissue/ organ in a given time period in ml/min

Total blood flow is cardiac output (CO), the volume of blood
that circulates through systemic (or pulmonary) blood vessels





each minute
Blood Pressure (BP)
Force exerted on the vessel wall by the contained bloodmmHg
Blood moves from higher to lower pressure areas
Resistance
Resistance is opposition to flow- the friction blood encounters
as it passes through BVs
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Pressure, Flow, and Resistance

Blood flow (F) is directly proportional to the difference in
blood pressure
• The greater the difference in blood pressure between two
points, the more the blood flow

Blood flow is inversely proportional to resistance (R)
• If resistance increases, blood flow decreases

In an effort to meet physiological demands, we can increase
blood flow by:
• Increasing BP
• Decreasing systemic vascular resistance in the blood vessels
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Vascular Resistance

Peripheral resistance (PR) most resistance is encountered
in the peripheral systemic circulation well away from the
heart

Vascular Resistance is influenced by:
• Blood viscosity ( due to RBCs)- constant
• Vessel length (body size)- constant
• Blood vessel diameter

Changes in vessel diameter alter peripheral resistance

Mainly the arterioles determine peripheral resistancedilate & constrict in response to neural & chemical
stimuli
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Vascular Resistance

Resistance is inversely proportional to the diameter of the blood
vessel's lumen ( R= 1/d4)

The smaller the diameter of the blood vessel, the greater the
resistance to blood flow

Vasoconstriction narrows the lumen, and vasodilation widens it

Moment-to-moment fluctuations in blood flow through a tissue
are due VC and VD of the tissue's arterioles

As arterioles dilate, resistance decreases, and blood pressure
falls

As arterioles constrict, resistance increases, and blood pressure
rises.
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Factors Affecting Vascular Resistance
Copyright © John Wiley & Sons, Inc. All rights reserved.
Blood Pressure

BP= CO X PR

BP is determined by 3 factors :

cardiac output, vascular resistance, blood volume
• Increase in any of these factors increases BP

BP is: highest in the aorta, declines throughout the length of
the pathway
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Blood Pressure




Arterial Blood Pressure:
Systolic pressure – pressure exerted on arterial walls during
ventricular systole- 110mmHg
Diastolic pressure – lowest level of arterial pressure during
ventricular diastole- 70mmHg
Pulse pressure is the difference between systolic & diastolic
pressure- felt as a pulse

Mean arterial pressure (MAP) – pressure that propels the
blood to the tissues
MAP = diastolic pressure + 1/3 pulse pressure

MAP & PP decrease with distance from the heart
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BP in various parts of CVS
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Blood Pressure

Capillary BP ranges from 35mmHg at the arterial end
of capillaries & 16 mm Hg at the venous ends of the
capillaries

BP continues to drop in the venules & then veins

BP is 0 mm Hg in the right atrium
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Venous Return

The volume of blood returning back to the heart
through the systemic veins is called the venous return.
• Because venous BP is low,
venous return is aided by the
presence of venous valves,
a skeletal muscle pump,
and the action of breathing.
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Venous Return

The skeletal muscle pump :
• uses the action of muscles to milk
blood in one direction (valves
prevent backflow).

The respiratory pump:
• intra abdominal pressure increases
during inspiration- squeezing local
veins forcing blood toward the
heart
• Pressure in chest falls- veins in
chest expand to pull venous blood
towards the heart.
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Venous Return

Although the venous
circulation flows under
much lower pressures than
the arterial side, usually the
small pressure differences
(venule 16 mmHg to
right atrium 0 mmHg),
plus the aid of muscle
and respiratory pumps
is sufficient.
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Velocity of Blood Flow

Blood velocity (in cm/sec):
• Changes as blood travels through the systemic
circulation
• Is inversely proportional to the cross-sectional area
• Flow fastest in aorta & slow in capillaries- (allows time
for exchange between blood and tissues), speeds up
again increases in veins
• (total cross sectional area of all the branches of a vessel is
more than the cross sectional area of the original vessel)
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Control of Blood Pressure & Blood Flow

Several negative feedback loops control BP by
changing:

CO ( by changes in HR & SV)

PR

Blood volume

Short-term controls correct moment-to-moment
fluctuations in BP by altering CO & peripheral
resistance

Long-term controls mainly regulate blood volume
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Cardiovascular center

The Cardicovascular center not only regulates HR & SV, but also
control blood flow and BP

The cardiac center ( cardioinhibitory & the cardiostimulatory
center )- regulate HR & SV

The vasomotor center ( VMC)- controls blood vessel diameter via
sympathetic nerves
• Maintains vasomotor tone
• Increasing sympathetic stimulation causing vasoconstriction
• Decreasing sympathetic stimulation causing vasodilation


The CV center receives input most importantly from baroreceptors
Output from CV center stimulates sympathetic & parasympathetic
nerves
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Function of Cardiovascular center in the
Medulla oblongata
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Neural Regulation of Blood Pressure

Baroreceptor Reflexes

Correct moment-to-moment fluctuations in BP as in moving from prone
to erect position

Baroreceptors are located in the arch of the aorta and the carotid sinus.
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Neural Regulation of Blood Pressure

Baroreceptor Reflexes :

The carotid sinus reflex helps regulate blood pressure in the brain


The aortic sinus reflex regulates systemic blood pressure
Example: BP falls- the baroreceptors are stretched less, they send
nerve impulses at a slower rate to the cardiovascular center
• CV center decreases parasympathetic stimulation of the heart via
vagus N & increases sympathetic stimulation to the heart
• CV center increases sympathetic stimulation to blood vessels
(skin, GI tract, and kidneys )- causing VC & increased PR

As cardiac output and peripheral resistance rise- BP increases
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Neural Regulation of Blood Pressure

Baroreceptor Reflexes :

Example: BP rises:

the baroreceptors send impulses at a faster rate to CV center

The CV center responds by increasing parasympathetic
stimulation and decreasing sympathetic stimulation

HR and force of contraction decrease- cardiac output
decreases

The cardiovascular center also decreases sympathetic
stimulation to cause vasodilation- lowers vascular resistance.

As cardiac output and peripheral resistance fall- BP falls
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Regulation of BP
via baroreceptor
reflexes
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Neural Regulation of Blood Pressure

Chemoreceptor Reflexes

Chemoreceptors are found in the carotid bodies (located
close to baroreceptors of carotid sinus) and aortic bodies
(located in the aortic arch).

Low oxygen & pH, or raised CO2 of blood stimulates
chemoreceptors
• Impulses to cardiac center- increases CO
• Impulses to VMC- increase PR
• BP rises, speeding return of blood to heart & lungs
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Hormonal Regulation of Blood Pressure

The Renin-angiotensin-aldosterone (RAA) system

Long term regulation of BP
• Renin is released by kidneys when blood volume falls or blood flow
decreases.
• It converts a protein in blood angiotensinogen into angiotensin I
• Angiotensin I is converted to the active hormone angiotensin II
• Angiotensin II raises BP by:
• causing vasoconstriction – increasing PR
• and by stimulating secretion of aldosterone from the
adrenal glands.
• Aldosterone causes Na & water retention in blood , increases blood
volume and BP
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Copyright © John Wiley & Sons, Inc. All rights reserved.
Hormonal Regulation of Blood Pressure

Epinephrine and norepinephrine are also released from the
adrenal medulla in response to sympathetic nerve stimulation.
• They increase cardiac output by increasing rate and force of
heart contractions.
• Cause VC of vessels in skin and digestive organs

Antidiuretic hormone (ADH) is released from the posterior
pituitary in response to dehydration or decreased blood
volume

Causes VC ( also called vasopressin for this action)

Promotes water retension & increases blood
volume & BP
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Hormonal Regulation of Blood Pressure

Atrial Naturetic Peptide (ANP) is a natural diuretic
polypeptide hormone released by cells of the cardiac atria
in response to high blood volume or high BP
• ANP participates in autoregulation by:

Lowering blood pressure (it causes a direct
vasodilation)

Reducing blood volume (by promoting loss of salt
and water in urine)
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Autoregulation of Blood Pressure

Autoregulation – automatic adjustment of blood flow
within the organ/tissue in proportion to its requirements

Blood flow through an organ is controlled by modifying
the diameter of local arterioles feeding its capillaries

Two types if stimuli cause autoregulatory changes in
blood flow:

Myogenic

Vasodilating & vasoconstricting chemicals
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Autoregulation of Blood Pressure

Myogenic response: responsesof vascular smooth
muscle to stretch of vessel wall

Vascular smooth muscle responds to increased blood
pressure (increased stretch) by VC

It responds to decreased blood flow & decreased
blood pressure ( decreased stretch) by VD, which
promotes increased blood flow to the tissue
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Autoregulation of Blood Pressure
Vasodilating chemicals
 In metabolically active tissues VD of arterioles serving
the capillary beds leads to increased blood flow, in
response to:
 Declining oxygen levels (hypoxia)
 Nitric oxide (NO)
 Other substances released by metabolically active
tissues- H+, lactic acid (acidosis).



Vasoconstrictors:
Serotonin
endothelin
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Pulse and pulse points

A measure of peripheral
circulation can be done by
checking the pulse.

The pulse is a result of the
alternate expansion and recoil
of elastic arteries after each
systole.
• Normally the pulse
is the same as
the heart rate.
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Alterations Of Blood Pressure

Hypertension (HTN)

It is condition of sustained elevated arterial pressure of
140/90 or higher.
• It is the major cause of
atherosclerotic vascular
disease, heart failure,
kidney disease and stroke.
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Alterations Of Blood Pressure

Hypotension is defined as low blood pressure

Hypotension leading to hypo-perfusion of critical organs
can result in shock

Circulatory shock – any condition in which blood vessels
are inadequately filled and blood cannot circulate
normally

Results in inadequate blood flow to meet tissue needs
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Shock And Homeostasis

The 4 basic types of shock are:
• Hypovolemic shock, due to decreased blood volume
• Cardiogenic shock, due to poor heart function
• Obstructive shock, due to obstruction of blood flow
• Vascular shock, due to excess vasodilation - as seen in
cases of a massive allergy (anaphylaxis) or sepsis
•
In the U.S., septic shock is an important cause of
death in hospital critical care units.
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Shock and Homeostasis
 Sympathetic
stimulation:
• Heart rate & contractility
increases
• Selective vasoconstriction to
shunt blood flow to vital
organs
 ADH released  conserve
water
 Renin released  Angiotensin
II---Aldosterone released 
conserves Na+ and water
 ANP inhibited
The body responds via negative feedbacks to restore homeostasis
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