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Cardiovascular System
Blood Vessels and Hemodynamics
Dr. Michael P. Gillespie
Cardiovascular System
 Transports and delivers blood to the body to
deliver oxygen, nutrients, and hormones as
well as carries away wastes.
 Blood vessels form a closed system of tubes,
which carries blood away from the heart,
transports it to the tissues of the body, and
then returns it to the heart.
Hemodynamics
 Hemo – blood.
 Dynamics – power.
Main Types Of Blood Vessels
 Arteries – carry blood away from the heart.
 Arterioles – very small arteries.
 Capillaries – tiny vessels which allow
exchange of substances between the blood
and body tissues.
 Venules – very small veins.
 Veins – carry blood back to the heart.
Vaso Vasorum
 Larger blood vessels require oxygen and
nutrients just like other tissues of the body.
 Vaso vasorum (vasculature of the vessels)
are located within the walls of larger vessels
and supply them.
Tunics (Coats) Of Arteries
 Tunica interna (intima) – contains a lining of
endothelium which makes contact with the
lumen and blood.
 Tunica media – thickest layer and has high
compliance (stretches).
 Tunica externa – outer coat, elastic and
collagen fibers.
Changes In Vascular Diameter
 Vasoconstriction – a decrease in the diameter
of the lumen of a blood vessel.


Sympathetic stimulation causes the smooth
vessels of the vessels to contract, squeezing the
vessel wall and narrowing the lumen.
Occurs when an artery or an arteriole is
damaged, producing vascular spasm and limiting
the blood flow to reduce blood loss.
Changes In Vascular Diameter
 Vasodilation – an increase in the diameter of
the lumen of a blood vessel.

Occurs when sympathetic stimulation decreases
or when nitric oxide, K+, H+, and lactic acid are
present.
Elastic Arteries
 Elastic arteries propel blood forward while the
ventricles are relaxing.
 Blood is ejected from the heart and stretches the
walls of the elastic arteries.
 The stretch of the arteries stores mechanical energy
and act as a pressure reservoir.
 The vessels recoil and convert stored (potential)
energy in the vessel into kinetic energy of the
blood.
Muscular Arteries
 Medium sized arteries are muscular arteries.
 They contain more smooth muscle and fewer elastic
fibers than elastic arteries.
 They are capable of greater vasoconstriction and
vasodilation.
 They are called distributing arteries because they
distribute blood to various parts of the body.
Arterioles
 A very small (almost microscopic) artery that
delivers blood to capillaries.
 Arterioles regulate resistance.
 Vasoconstriction of arteriole walls increases
resistance to capillaries and vasodilation of arteriole
walls decreases resistance.
 Resistance regulates blood flow to the capillaries.
Capillaries
 Microscopic vessels that connect arterioles to
venules.
 The flow of blood from arterioles to venules is
microcirculation.
 Tissues with high metabolic requirements, such as
muscles, liver, kidneys, and nervous system, have
more capillaries.
 Tissues with lower metabolic requirements, such as
tendons and ligaments, contain fewer capillaries.
Capillaries
 Capillaries are absent in a few tissues, such
as covering and lining epithelia, the cornea of
the lens of the eyes, and cartilage.
 Exchange vessels – exchange nutrients
between blood and tissue cells through the
interstitial fluid.
Capillaries
 Single layer of endothelial cells.
 Branch extensively to increase surface area
for exchange.

Usually only a small part of the capillary
network is active; However, when a tissue is
active (i.e. Contracting muscle) the entire
network fills with blood.
Metarteriole
 A metarteriole (met = beyond) – is a vessel that
emerges from an arteriole and supplies a group of
10 – 100 capillaries (capillary bed).
 The proximal end of the metarteriole is surrounded
by smooth muscle fibers, which regulate blood flow
through the capillary bed.
 The distal end of the metarteriole has a
thoroughfare channel, which bypasses the
capillary bed.
True Capillaries
 True capillaries emerge from arterioles or
metarterioles.
 Precapillary sphincter – ring of smooth muscle
that controls blood flow into a true capillary.
 Vasomotion – intermittent contraction and
relaxation of precapillary sphincters and
metarteriole smooth muscle (5-10 times per
minute).
Types Of Capillaries
 Continuous capillaries 

Continuous tube interrupted only by intercellular
clefts.
Found in smooth muscle, connective tissue, and
lungs.
Types Of Capillaries
 Fenestrated capillaries (fenestr = window) 

The plasma membranes have fenestrations (small
pores).
Located in the kidneys, villi of the SI, choroid
plexus of the ventricles of the brain, cilary
processes of the eyes, and endocrine glands.
Types Of Capillaries
 Sinusioids –
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

Wider and more winding than other capillaries.
Unusually large fenestrations which allow
protein and blood to pass from the tissues into
the bloodstream.
Found in the liver, spleen, anterior pituitary, and
parathyroid glands.
Venules
 Small veins formed when several capillaries
unite.
 The walls of the smallest venules (closest to
the capillaries) are very porous and serve as a
site of emigration for white blood cells.
Veins
 Veins contain the same three coats as
arteries.
 The lumen of a vein is larger than that of a
comparable artery.
Veins
 Many veins also contain valves (especially in
the lower limbs).
 The valves are thin folds of the tunica
interna. The cusps point toward the heart.
 The valves prevent backflow of returning
blood in the lower pressure venous system.
Vascular (Venous) Sinus
 A vascular (venous) sinus is a vein with a
thin endothelial wall that has no smooth
muscle to alter its diameter.
 Examples:


Dural venous sinuses (supported by dura mater).
Coronary sinus of the heart.
Varicose Veins
 Leaky valves can cause veins to become dilated and
twisted in appearance.
 This is most common in the esophagus and veins of
the lower limb, although it can occur in any veins.
 Hemorrhoids are varicose veins in the anal canal.
 Deeper veins are not as susceptible because
surrounding skeletal muscles prevent their walls
from stretching.
Anastomoses
 The union of the branches of two or more arteries
supplying the same body region is called an
anastomosis.
 Anastomoses between arteries provide alternate
routes for blood to reach a tissue or an organ.
 The alternate route of blood flow is known as
collateral circulation.
 Arteries that do not anastomose are known as end
arteries.
Blood Distribution
 The largest portion of your blood volume at rest is
in the veins (60%).
 Systemic capillaries hold about 5%.
 The veins and venules function as a blood reservoir.
 Blood can be diverted quickly if the need arises
through venoconstriction.
 The veins of the abdominal organs and skin serve as
principal blood reservoirs.
Capillary Exchange
 The mission of the cardiovascular system is to keep
blood flowing through the capillaries to allow for
exchange of nutrients and waste products with the
interstitial fluid.
 Substances enter and leaved the capillaries through
three basic mechanisms:



Diffusion.
Transcytosis.
Bulk flow.
Diffusion
 Substances diffuse down their concentration
gradients (from areas of high concentration
to low).
 All plasma solutes except proteins pass
easily across most capillary walls.
 Water soluble substances such as glucose
and amino acids pass easily through either
fenestrations or intercellular clefts.
Diffusion
 Lipid-soluble materials (O2, CO2, & steroid
hormones) pass through the lipid bilayer.
 Liver capillaries have large gaps which do allow
proteins to pass through. Hepatocytes synthesize
proteins such as fibrinogen (clotting) and albumin
(osmotic pressure), which diffuse into the blood.
 Brain capillaries have tight junctions, which allow
only a few substances to enter and leave. This
forms the blood-brain barrier.
Transcytosis
 Substances within the blood plasma are
enclosed in tiny pinocytic vesicles that enter
endothelial cells by endocytosis.
 They move across the membrane and exit the
other side by exocytosis.
Transcytosis
 This method of transport is utilized for large,
lipid-insoluble molecules that cannot cross
the capillary walls in any other way.
 Insulin enters the blood this way and some
maternal antibodies enter the fetal circulation
this way.
Bulk Flow: Filtration &
Reabsorption
 Bulk flow is a process by which large
numbers of ions, molecules, or particles in a
fluid move together in the same direction.
 It occurs from an area of high pressure to an
area of low pressure at a rate faster than
diffusion would produce alone.
 Regulates relative volumes of fluids rather
than concentrations of solutes.
Bulk Flow: Filtration &
Reabsorption
 Continues as long as pressure variances exist.
 Pressure driven movement of fluid and
solutes from blood capillaries to interstitial
fluid is termed filtration.
 Pressure driven movement of fluid and
solutes from interstitial fluid into blood
capillaries is called reabsorption.
Pressures Involved In Filtration
And Absorption
 Blood hydrostatic pressure (BHP) – pressure
from the pumping action of the heart
promotes filtration.
 Interstitial fluid osmotic pressure filters
blood promotes filtration.
Pressures Involved In Filtration
And Absorption
 Blood colloid osmotic pressure (BCOP)
promotes reabsorption.
 Interstitial fluid hydrostatic pressure
promotes reabsorption.
 Net filtration pressure is the balance of these
pressures (NFP).
Starling’s Law Of The
Capillaries
 The volume of fluid and solutes reabsorbed
normally is almost as large as the volume
filtered.
Edema
 If filtration greatly exceeds reabsorption, the
result is edema (swelling), an abnormal
increase in interstitial fluid volume.
Excess Filtration
 Increased capillary blood pressure.
 Increased permeability of capillaries which
allows plasma proteins to escape. Chemical,
bacterial, thermal, or mechanical agents can
damage capillary walls.
Inadequate Reabsorption
 Decreased concentration of plasma proteins
associated with liver disease, burns,
malnutrition, and kidney disease.
Hemodynamics
 Hemodynamics refer to the factors that affect
blood flow.
 Blood flow is the volume of blood that flows
through any tissue in a given period of time.
 Cardiac output (CO) is the total blood flow.
 Cardiac output (CO) – heart rate (HR) *
stroke volume (SV).
Factors That Determine
Distribution Of Cardiac Output
 Pressure difference drives blood flow
through a tissue.

Blood flows from regions of higher to lower
pressure.
 Resistance to blood flow in specific blood
vessels.

The higher the resistance, the smaller the blood
flow.
Blood Pressure
 Contraction of the ventricles generates blood
pressure (BP).
 Systolic blood pressure is the highest
pressure attained in the arteries during
systole.
Blood Pressure
 Diastolic blood pressure is the lowest arterial
pressure during diastole.
 Mean arterial blood pressure (MABP) is the
average pressure in the arteries.

MABP = diastolic BP + 1/3 (systolic BP – diastolic BP).
 Blood pressure also depends on the total volume of
blood in the cardiovascular system.
Resistance
 Vascular resistance is the opposition to blood flow
due to friction between blood and the walls of blood
vessels.
 Vascular resistance depends upon:
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

The size of the blood vessel lumen.
Blood viscosity.
Total blood vessel length.
 Systemic vascular resistance (SVR) is the total
peripheral resistance from all factors combined.
Venous Return
 Venous return to the heart is caused by the
following:
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Pressure generated from contractions of the
heart’s left ventricle.
Skeletal muscle pump.
Respiratory pump.
Velocity Of Blood Flow
 The speed or velocity of blood flow is
inversely related to the cross-sectional area.
 Each time an artery branches, the cross
sectional area increased and the velocity
decreases.
 Each time a venule merges to form a vein,
the cross sectional area decreases and the
velocity increases.
Syncope
 Syncope, or fainting, is a sudden, temporary
loss of consciousness that is not due to head
trauma.
Syncope
 It is commonly due to cerebral ischemia.
 Causes:
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Vasodepressor syncope – sudden emotional stress.
Situational syncope – pressure stress associated with
urination, defecation, or severe coughing.
Drug-induced syncope – antihypertensives, diuretics,
vasodilators, & tranquilizers.
Orthostatic hypotension – an excessive decrease in blood
pressure that occurs upon standing up.
Control Of Blood Pressure &
Blood Flow
 Negative feedback systems control blood
pressure by adjusting the following factors:


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Heart rate.
Stroke volume.
Systemic vascular resistance.
Blood volume.
Cardiovascular Center
 The cardiovascular (CV) center of the
medulla oblongata regulates heart rate and
stroke volume.
 Sympathetic nerves reach the heart via the
cardiac accelerator nerves. Sympathetic
stimulation increases the heart rate and
contractility.
Cardiovascular Center
 Parasympathetic stimulation decreases the
heart rate and is conveyed by the vagus
nerves (cranial nerve X).
 The CV center sends impulses to smooth
muscle in blood vessel walls via vasomotor
nerves. They moderate the rate of
vasoconstriction (vasomotor tone).
Neural Regulation Of Blood
Pressure
 Baroreceptor Reflexes – baroreceptors are pressuresensitive receptors located in the aorta, internal
carotid arteries, and other large arteries of the neck
and chest.
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Carotid sinus reflex – carotid sinuses are small widenings
of the right and left internal carotid arteries. Pressure
stretches the wall of the carotid sinus. Signals are sent to
the CV center via the glossopharyngeal nerves (CN IX).
Aortic reflex – signals are sent to the CV center via the
vagus nerves (CN X).
Carotid Sinus Massage &
Carotid Sinus Syncope
 Carotid sinus massage involves massaging
the neck over the carotid sinus, to slow the
heart rate in a person who has paroxysmal
superventricular tachycardia (originates in
the atria).
 Carotid sinus syncope – fainting due to
excessive pressure on the carotid sinus from
hyperextension of the head or tight collars.
Chemoreceptor Reflexes
 Chemoreceptors monitor the chemical
composition of the blood.
 They are located close to the baroreceptors in
carotid bodies and aortic bodies.
Chemoreceptor Reflexes
 They detect changes in blood level of O2,
CO2, and H+.
 Hypoxia, acidosis, or hypercapnia stimulates
the chemoreceptors to send impulses to the
cv center producing sympathetic stimulation
and vasoconstriction.
Hormonal Regulation Of Blood
Pressure
 Renin-angiotensin-aldosterone (RAA) system raises
blood pressure.

Angiotensin II is a vasoconstrictor and stimulates
aldosterone which increases absorption of Na+ ions by
the kidneys.
 Epinephrine and norepinephrine raise blood
pressure.


Increase cardiac output by increasing heart rate.
Cause vasoconstriction of arterioles in the skin and
abdomen and vasodilatation of arterioles in cardiac and
skeletal muscles.
Hormonal Regulation Of Blood
Pressure
 Antidiuretic hormone (ADH) raises blood
pressure.

Causes vasoconstriction.
 Atrial natriuretic peptide (ANP) lowers blood
pressure.

Causes vasodilatation and promotes loss of salt
and water in the urine.
Autoregulation Of Blood
Pressure
 Physical changes.


Warming promotes vasodilation and cooling
causes vasoconstriction.
Myogenic response – smooth muscle contracts
more forcefully when it is stretched and relaxes
when stretching lessens.
Autoregulation Of Blood
Pressure
 Vasodilating and vasoconstricting chemicals.


Vasodilating chemicals include K+, H+, lactic
acid, ATP, and nitric oxide (NO). Kinins and
histamine are released from tissue trauma and
cause vasodilation.
Vasoconstricting chemicals include thromboxane
A2, superoxide radicals, serotonin (from
platelets), and endothelins.
Checking Circulation