Chapter 21
Peripheral Circulation
& Regulation
Vessels carrying blood away from
the heart
Arteries – carry blood away from the heart and toward
the body organs/tissues
Most arteries carry blood rich in oxygen
exception – Pulmonary Aorta & arteries (heart  lungs)
Arterioles – smaller branches of the arteries
Capillaries – smallest vessels – this is the exchange point
between the circulatory system and the tissues.
Nutrients & O2 pass to tissues from capillaries
Waste products from tissues into capillaries
Capillaries – Several Types
Capillaries – first stage on return to heart, typically only
slightly larger than the diameter of a single RBC. Capillaries
are composed primarily of a layer of simple squamous
epithelium, external to this is a basement membrane then
a delicate layer of connective tissue
Continuous Capillaries– have no areas where substances can
freely flow across the epithelial cells. No pores. Substances
must be VERY small to go through endothelial cells or must
be actively transported. O2 can flow across due to small size.
Continuous capillaries are an effective part of the blood brain
barrier.
Several more types… (next slide)
Capillary types cont.
Fenestrated Capillaries – have fenestrae (“windows”) areas
70-100 m in diameter where the cells have no cytoplasm,
simply a porous diaphragm. Highly permeable areas. Found
in intestinal villi, ciliary process of eye, kidneys, endocrine
glands, & choroid plexus of brain. – “leaky capillaries”
Sinusoidal Capillaries – relatively large lumen (inner diameter)
occur in endocrine glands allowing for greater blood flow and
thus greater absorption of hormones produced in these areas
Sinusoids – still larger diameter sinusoidal capillaries (may
lack a basement membrane). Found in liver & bone marrow,
often have macrophages associated with their interior
More Capillaries
Venous Sinuses – also larger than sinusoidal capillaries,
occur in spleen. These have large gaps between their
endothelial cells (actual gaps in wall of capillary)
As you can see, capillaries range from being almost
impermeable to relatively leaky. This changes with capillary
type, and with location within the body
Networking – Capillaries are often arranged in branching
networks. Blood flows into these networks through small
branches off the arterioles know as metarterioles. Branches
of the metarterioles form the arterial half of the capillary bed
Flow is controlled by precapillary sphincter muscles
Return to Heart
While flow into a capillary bed is often controlled by
precapillary sphincter muscles. The return half usually
lacks these ‘valves’ – free flow
Venules – vessels slightly larger than capillaries which drain
the capillary beds
Veins – larger still… these vessels merge into gradually
larger veins which proceed back to the heart. Largest
diameter veins are the superior & inferior vena cava which
drain the upper & lower portions of the body into the heart
Structural information about arteries & veins follows…
Wall of “Typical” Blood Vessel
3 distinct layers – it’s not really THIS easy. but you’ll see that in a moment…
Tunica intima (inside layer)
Tunica media (… middle layer)
Tunica externa (you guessed it! – external layer)
sometimes called
tunica adventitia
Fig 21-3 – ‘typical’ vessel - this is similar to a muscular
artery which we will discuss in a moment
Tunica Intima
Proceeding to describe  inside to outside
Tunica intima
Endothelium layer of simple squamous called
Basement membrane a delicate layer of connective tissue
Internal Elastic Membrane – elastic fibers (fenestrated)
Tunica Media
Smooth muscle cells
Allows for control of blood flow by contraction or
relaxation  Vasoconstriction or Vasodilation
Some elastic and collagen fibers may be present (varies
with size of vessel)
some longitudinally oriented smooth muscle in some arteries
near the border of the tunica intima.
External elastic membrane occurs at the outer edge of
tunica media in elastic arteries (see figure later)
Tunica Adventitia (externa)
Connective tissue – eventually merges with the connective
tissue surrounding the blood vessel
Composition varies from dense connective tissue near
T. media, to loose connective tissue near its outer edge
Elastic Arteries
Largest arteries – sometimes called Conducting Arteries
Larger amount of elastic tissue, smaller amount of smooth
muscle. Much elastic fiber in the tunica media.
Blood pressure higher in these vessels – they are nearer to
the heart. Their elastic nature lets them tolerate fluctuations
between systole and diastole
Tunica intima – relatively thick
Tunica adventitia relatively thin
Muscular Arteries
Midsize arteries – large enough to have distinct names
More muscle in the tunica media than seen in elastic arteries
25 – 40 layers of smooth muscle – circular arrangement
Yes, I know when you look at the figures, it looks like
less muscle. Remember these are smaller in diameter
than the elastic arteries…
Tunica intima has well developed internal elastic membrane
In general – arteries tend to be muscular and elastic
elastic artery
muscular artery
Arterioles
Tunica intima has no obvious internal elastic membrane
Tunica media has only one or two layers of smooth muscle
cells which wrap circularly (capable of constricting vessel)
Capable of vasoconstriction & vasodilation
Venules
Diameter of 40-50 microns
Endothelium plus a basement membrane
Walls thin enough for substantial nutrient exchange,
similar to capillaries
Except for larger size these are structurally very similar
to capillaries
Small veins
Larger than veinules
Have an outer layer (tunica adventitia) of collagenous
connective tissue
Walls too thick to allow much nutrient exchange to take place
Medium & Large Veins
Thin tunica intima (mostly epithelium)
Thin tunica media – a sparse layering of smooth muscle,
some collagen and a very few elastic fibers
Tunica adventitia – most prominent layer, mostly
collagenous connective tissue
Valves in Veins
Veins larger than 2mm have one-way valves
These valves prevent backflow, permit blood to flow toward
the heart, but not away from the heart
Structure – two ‘leaflets’ or flaps overlapping in the vein
when blood flows backward it presses the flaps closed
More valves present in lower extremities than in other parts
of the body – Naturally! They prevent flow against gravity
Varicose veins due to failed valves allowing blood to pool
Blood Supply to Vessels!
Yes, they need blood too, and don’t get it from ‘inside’
Vasa vasorum – small blood vessels providing nutrients to
all blood vessels greater than 1mm in diameter
Vessels smaller than 1mm can absorb nutrients from the
blood they carry
Laminar Flow
Blood does not flow in ‘one bulk unit’ through a blood vessel
Flow is much like the currents in a river
This type of flow helps reduce turbulence
The End Day 1 Ch 20.
Day 2 Chapter 21
Continues….
Peripheral Circulation
&
Regulation
Blood Flow
Flow – how much blood passes between two points
in a given amount of time
Flow is affected by :
Pressure exerted on the blood by the heart
Resistance (friction) from the walls of the blood vessel
Two types of flow
Laminar
Turbulent
fig 21.32 p 674
Turbulence
Any constriction of a blood vessel decreases rate of flow
& creates turbulence
Turbulence creates resistance, usually by narrowing the vessel
Cross Sectional Diameter
(related to blood flow)
Greater CSD = less resistance and  blood flow
This differs from Total Cross Sectional Area
Total Cross Sectional Area determined by the diameter of
the vessel and # of vessels
For example, the aorta is relatively large in diameter but
there is only one aorta
An individual capillary is quite small in diameter but there
are millions of capillaries  greater TCSA in capillaries
Blood flows faster in aorta even though capillaries have
a greater TOTAL area
velocity of flow
fig 21.34 p 677
More factors affecting flow
Viscosity – the ‘thickness’ of blood or its resistance of a
liquid to flowing
In blood this is, in large part due to the Hematocrit
(packed cell volume)
If blood viscosity increases, more force must be applied to get
it to move, producing more friction against the vessel wall
Blood Pressure – Pressure exerted against the walls of a blood
vessel by the blood
Blood Pressure
affects flow also…
Blood Pressure – Pressure exerted against the walls of a blood
vessel by the blood
Needed to keep blood vessels open
If BP decreases below a Critical Closing Pressure vessels
will collapse Force = Diameter x Pressure (see LaPlace’s Law pg 676)
CCP important in cardiovascular shock:
BP decreases  vessels collapse vessels downstream
loose pressure and collapse (heart attack or electrocution)
Blood Pressure as a Clinical Factor
Force exerted on a vessel keeps it moving BUT some pressure
is expended against the wall of the vessel
If this pressure is too great the wall of the vessel can be damaged
Damage to vessel wall increases the rte at which plaque is
deposited, increasing chance of stroke & heart attack
Systolic Pressure – exerted when heart is contracting (maximal)
Diastolic (residual BP) – exerted when heart is relaxing, this is
the least amount of pressure a vessel is under at any given time
DIASTOLIC is more important
Blood Pressure…
Normal BP  120/80 (systolic/diastolic)
Clinically you are hypertensive (high blood pressure) if:
Systolic > 165
OR
Diastolic >90
Measuring BP clinically
Sphygmomanometer – “that cuff thingy” 
Sphygmomanometer – includes an inflatable cuff and a
method of measuring (column of mercury or digital readout)
Stethoscope placed over brachial artery
Pump inflates cuff over brachial artery (constricting flow)
until no sound is heard
Pressure slowly released until first sound heard = Systolic
Dramatic change in sound (no sound) = Diastolic
Vascular Compliance
Compliance =
increase in volume (mL)
--------------------------------increase in pressure (mm Hg)
Tendency for vessel volume to increase as pressure increases
Greater elasticity  greater compliance
Lower elasticity  lower compliance
Veins are much more compliant than are arteries
about 24:1 - walls of arteries too muscular to be very compliant
So… veins act as a ‘storage’ area for blood volume
Pulse Pressure
Serves as an index of whether BP is  or 
Systolic – Diastolic = PP
Vascular Compliance affects pulse pressure since it reflects
the vessels ability to increase in volume as BP increases
Compliance decreases with age (less elastic):
decreased compliance causes  pulse pressure
Stroke Volume of the heart also affects PP
as SV  - PP
Stroke volume – volume pumped from one ventricle with
each heartbeat
Pulse Pressure – fig 21.35 p 679
Starling’s Law of Capillaries
Equal volumes of fluid move out of the capillaries at the
arteriole end and at the venous end
Factors affecting Starling’s Law:
At arteriole end Hydrostatic Pressure dominates:
Result - pushes fluids into tissues
At venous end Osmotic Pressure (pull) dominates:
pulls fluids from tissues back into blood vessel
15% of fluid remains in tissues and is ultimately
removed by the lymphatic system
Regulation of BP – Short Term
Baroreceptor Reflexes
Adrenal Medullary Mechanism
Chemoreceptor Reflexes
CNS Ischemic Response
Vasomotor Center
Blood vessel diameter is controlled by this (VM) center in
the medulla oblongata
Stimulation from the VMC increases muscle tone in blood
vessels (especially arteries) which causes vasoconstriction
If blood pressure coming from heart increases this is detected
by baroreceptors (pressure receptors) in the aortic arch and
in the carotid arteries
These receptors send an afferent signal via glossopharyngeal
and vagus nerves which inhibits VMC
With VMC inhibited vessels relax
 vasodilation and BP decreases
decreased BP
 vasoconstriction
Adrenal Medulla
Activated when stimulated by sympathetic nervous system
increases secretion of epinephrine and smaller amounts
of norepinephrine
These hormones increase heart rate & stroke volume
Also cause vasoconstriction in skin and gut
Additionally cause vasodilation in heart
Rapid onset, short duration
Cardiac Regulatory Center
in Medulla oblongata
Chemoreceptors located in Aorta and in Carotid artery
Detect levels of CO2, O2, and blood pH
If pH is low (acidic) and CO2 is high these signal the CRC
which stimulates the sympathetic nervous system
Sympathetic increases heart rate and causes vasoconstriction
Decreases in CO2 , and increased pH result in decreased
heart rate and in vasodilation
CNS Ischemic Response
RARE – Is important only when the blood supply to the
Vasomotor Center decreases dramatically (less O2)
VMC stimulates vasoconstriction causing BP
to rise dramatically
Occurs only when BP falls below 50 mm Hg
This is used as a message to say:
Get blood to the brain NOW!
Long Term BP regulation
Renin-Angiotensin-Aldosterone Mechanism
Vasopressin Mechanism
Atrial Natriuretic Mechanism
Renin-Angiotensin-Aldosterone
Mechanism
Kidneys release an enzyme - Renin
Renin acts on angiotensinogen (a blood protein)
Causes angiotensinogen to release a fragment called angiotensin
Angiotensin is then activated by another enzyme called
angiotensin-converting enzyme  Angiotensin II (active form)
Angiotensin II causes vasoconstriction & it stimulates the
adrenal cortex to release aldosterone
Aldosterone causes kidneys to retain Na, Cl & water
Angiotensin II also causes thirst and salt craving
and ADH secretion – all cause increase blood volume
Renin-Angiotensin-Aldosterone
Vasopressin
When dissolved solids in blood increase or when BP decreases
the hypothalamus increases secretion of vasopressin (ADH)
from the posterior pituitary
Vasopressin (ADH) – causes kidneys to reduce production
of urine – increasing the amount of fluids in blood
Vasopressin also acts as a mild vasoconstrictor
Vasopressin Mechanism
Atrial Natriuretic Mechanism
Atrial Natriuretic Hormone – released from cells in atrium
Release of ANH stimulated by increased venous input to atrium
ANH causes kidneys to increase urine production and
excretion of sodium – this decreases blood volume
Chapter 21
That’s it for now…