3 Cardiovascular System
The Vascular System
Categories and structures of blood vessels
Arteries
metarterioles
arterioles
capillaries
venules
veins
Principles of fluid behavior #1: there is a universal law that
relates force, friction and movement
P
P
R
F
Flow rate F is proportional to the pressure gradient along the length of
the tube (P – P) and inversely proportional to flow resistance
R.
Flow resistance is directly proportional to vessel length, but to the 4th
power of vessel diameter.
Translating this law to the vascular system as a
whole…
• The driving force for blood flow is the mean
arterial pressure MAP (minus the venous
pressure CVP, but this is so small we can almost
neglect it)
• The resistance is the total peripheral resistance
TPR
• The flow is the cardiac output CO, which is equal
to the venous return VR
• So, CO = (MAP- CVP)/TPR
#2 In most vessels, blood flows fastest along the
core of the vessel
P
P
This is a laminar flow pattern. In turbulent flow, individual
drops of blood swirl around rather than following a linear
path. Turbulent flow can occur only within a few large
vessels, or in special circumstances such as the vicinity of a
cardiac valve.
Where does pressure come from?
• Gravity causes hydrostatic pressure when there is a
difference in height.
• Adding volume to an elastic vessel causes pressure to
increase – this is how the heart generates arterial blood
pressure.
• So, blood pressure measured at any point in the body
contains a combination of the two sources of pressure blood in a vessel above the level of the heart loses some
pressure due to the gravity effect – below the heart, it
gains some pressure.
• However, the gravitational component cannot add to the
driving force for blood flow because it applies equally to
arterial and venous blood.
#3 Blood flows from higher total energy to
lower total energy
• The total energy of flowing blood includes a potential
form (seen as vessel wall pressure) and a kinetic form
(the momentum of the flowing mass of blood). The
partition of the total energy between the two forms is
dependent on linear flow velocity. As blood flows along
the length of a vessel, it spends kinetic energy as heat,
while continuously converting its store of potential
energy into kinetic energy.
• When a vessel narrows, linear flow velocity goes up and
wall pressure goes down. If a vessel is obstructed,
linear flow velocity falls to zero and all of the energy
reverts to wall pressure.
Flow resistance and vessel branching
• When a vessel branches, the flow resistance
may increase or decrease, depending on the
diameter of the branches.
• So, when metarteries branch into arterioles, the
effect of the branching is to greatly add to the
resistance of the pathway, causing arterioles to
be the resistance elements of the vascular
system.
• In contrast, when arterioles branch into
capillaries, the effect is a decrease in resistance.
Arteries
• Wall is thick, tough, and relatively noncompliant (compliance is the change in
volume of a vessel in response to a
change in pressure)
Smooth muscle
• Three ‘coats’:
– tunica externa
– tunica media
– tunica intima - endothelial cells + connective
tissue
Arterioles
• Together with precapillary sphincters,
individual arteries function to regulate
blood flow through individual capillary
beds.
• Arterioles are the resistance elements of
the circulation – so the sum of the
resistances of all of the arterioles of the
systemic loop largely determines the total
peripheral resistance.
Capillaries
• Walls are thin, porous (usually), composed
of endothelial cells without smooth muscle,
except that the head of the capillary may
have a precapillary sphincter.
• Capillaries are the sites of all exchange of
dissolved materials between tissues and
bloodstream.
Capillaries consist of endothelial cells lying on a basement
membrane
Veins
• Walls are thinner and more compliant than those
of arteries.
• Diameters are larger.
• As a result, about 60% of the blood is in veins at
any instant, so they are the capacitance
elements of the circulation.
• Peripheral veins have valves that promote oneway flow
The respiratory and skeletal muscle pumps
Processes that apply a compressive force to
veins (skeletal muscle contraction) or
decrease central venous pressure
(respiratory movements) favor the
movement of blood from the venous
reserve into the heart. These processes
operate as pumps because of the
presence of venous valves
A trip through the systemic
and pulmonary loops
Pressure changes as blood passes through the
systemic loop
Thought
question:
how would
arteriolar
dilation or
constriction
affect this
curve?
Pressure changes in the pulmonary loop are
smaller than in the systemic loop
Compared to
the systemic
loop, the
pulmonary
loop is a lowresistance
loop
Control of the peripheral
circulation
The sympathetic n.s. exerts a major role in
extrinsic control
• Several regions in the medulla constitute the medullary
cardiovascular center
• Ordinarily, blood vessels (except capillaries) get only
sympathetic input.
• Both direct sympathetic innervation and circulating
epinephrine are important in maintaining vascular tone.
• For an individual vessel, whether an increase in
sympathetic input will cause dilation or constriction
depends on whether it is dominated by alpha receptors
(constriction) or beta receptors (dilation).
• For the systemic loop as a whole, alpha-dominated
vessels are more influential than beta-dominated
vessels.
Extrinsic vascular regulation can…
• Shift blood flow between body surface and body
core in thermoregulation
• Reduce blood flow to extrinsically dominated
vascular beds in response to blood loss or
hypotension
• Shift blood flow from splanchnic beds to skeletal
muscle in response to acute stress
• Increase flow into splanchnic beds during sleep
or after eating
• Mediate genital sexual responses
Intrinsic regulation (= autoregulation) allows individual
tissues to match their blood flow to their metabolism
This figure shows that
tissues can control the
flow resistance of their
arterioles to keep flow
constant in the face of
pressure changes.
Since pressure is usually
well-regulated by central
processes, the most
important implication of
this effect is that if tissue
metabolism changes,
intrinsic regulation will act
to change flow rates while
pressure remains
constant.
How does intrinsic regulation work?
• Mediated by local factors and/or a direct
response of arteriolar smooth muscle to stretch
• Local factors include (depending on the tissue
and the circumstances): oxygen, carbon dioxide,
hydrogen ion, potassium ion, nitric oxide,
prostaglandins, bradykinin, angiotensin, VIP,
adenosine.
Which vascular control system dominates depends
on anatomy and function
• Dominated by
external control
– Skin
– Splanchnic beds
– Resting skeletal
muscle
– genitalia
• Dominated by
autoregulation
– Heart
– Brain
– Active skeletal muscle