Download Total Cross Sectional Diameter and Pressures

Vascular Functional Organization
Arterial Resistance &
Venous Capacitance
Richard P. Wyeth
Gentleman’s Review
Daniel Martingano OMS I
Total Cross Sectional Diameter and
Pressures
•Total cross sectional areas are
greatest within capillary beds
circuits
• systemic (~3000 cm2)
• pulmonary (~4000 cm2)
•Pressure varies inversely with
total cross sectional area
•Pressure is low within systemic
and pulmonary capillary beds
Cardiovascular Parameters
• At each branching point the combined cross sectional area of
daughter vessels is greater than parent vessel through systemic
(peripheral) and pulmonary circuits.
• The radius for each subsequent generation is not ½ of previous.
• Anastomses are exceptional.
Composition and Size
• Vascular:
• Resistance is principally a
function of vascular smooth
muscle (VSM >>EF = CF)
• Compliance is principally a
function of elastic fibers (EF >>
VSM)
• Capacitance is principally a
function of elastic fibers
(EF>VSM)
Arteries– Functional Summary
• Physical Properties
– Arteries have greater tunica media
– Arteries are more elastic
• Arteries carry blood away from the
heart
• Arteries maintain vascular resistance
– A function primarily of small arteries
and arterioles
Veins – Functional Summary
• Venules have small effect on vascular
resistance
• Veins carry blood to heart
– Veins mediate venous return
• Veins provide a reservoir and are
capacitive.
– A function of primarily of small veins
and venules
Arteries v. Veins
Total Area and
Pressures
~XS
area
(cm2)
~Mean P
(mmHg)
aorta
2.5
97 to 90
Small arteries
20
90 to 80
arterioles
40
80 to 40
capillaries
2500
35 to 15
venules
250
15 to 7
small veins
80
7 to 4
venae cavae
8
Vessel
proximal - distal
<4
Pressure in Blood Vessels
• P effecting blood flow within vasculature is of three types:
– Driving Pressure along the long axis of the vessel:
• Dx (x1 – x2)
• work of the heart
– Transmural Pressure:
• Dr (r1 – r2)• function of the vessel wall and interstium
– Hydrostatic Pressure:
• Dh (h1 – h2)
• function of gravity
Mean Circulatory Filling
Pressure
• If the heart stops (no pump)
• Pressures equilibrate
throughout the body =
mean circulatory filling
pressure
Mean Circulatory Filling Pressure Approximates
Mean Circulatory Filling Pressure
• MCFP represents both the pulmonary and systemic pressures
v.
• MSFP represents the pressure equilibrated only in the systemic
circulation.
• Pulmonary circulation has little capacitance
– Little resistance (~ 12% max)
– Small reserve volume(~10% max)
• MCFP ~ MSVP
Mean Circulatory Filling Pressure and
Right Atrial Pressure
• MSFP is a force pushing peripheral
blood to right heart
• Greater positive difference between
mean systemic filling pressure and
right atrial pressure (RAP) = greater
venous return
• Difference between these two
pressures is called pressure gradient
for venous return
The Converse – Resistance to
Venous Return
• Principally due to venous resistance
• RVR is important because high venous capacitance and great
compliance allows distension with little increase in pressure
• As mean RA pressure (rAP) increases VR decreases
• Conversely low capacitance of arteries and arterioles (~30% of the
capacitance of veins)  small increases in volume raise pressure
greatly (~30X) in arteries and thus overcome resistance
Factors Affecting Mean Circulatory
Filling Pressure
• Factors Affecting MSFP
– Change in circulating volume
– Change in vascular compliace
• Arterial
• Venous
– Changes in systemic vascular resistance does
not alter MSFP
How RVR Effects Venous Return and
Cardiac Output
• VR is inversely related to RVR
– As RVR decreases VR increases
– As RVR increases VR decreases
MSFP Effects Venous Return
• Factors may change resistance to venous
return
• Constriction or dilation of arteries and veins
• Constriction or dilation of arterioles
• Vasoconstrictors like epinephrine, norepinephrine, angiotensin
increase MSFP and resistance to VR
• Vasodilators act conversely (decrease RVR)
Energetics of Blood Flow (CO)
• Blood within the circulation has a fixed content of energy (Et):
– Potential Energy (Blood Pressure; Ep)
– Kinetic Energy (Blood Flow; Ek)
•
Et = Total E of system
•
Ek = kinetic energy (mass velocity)
•
Ep = potential energy (pressure)
•
Conservation of energy. (Et) is constant.
Et  E k  E p
Summary
What is circulatory filling pressure (MCFP)?
If the heart stops (no pump) Pressures equilibrate throughout the
body = MCFP it is approximately equal to MSFP.
Why approximately?
MSFP excludes the lung capillary filling pressure. The lungs add
little to MSFP- Normally.
Summary
What about pulmonary hypertension?
Increased RAP, CVP. MSFP may be increased. Depends on nature of the
pulmonary vasculature and how much compliance lost.
Why do peripheral veins of the leg have a high incidence of
varicosity?
The central venous circulation is surrounded by tissue that exerts pressure
inward on the vessel; Peripheral veins do not - venous stasis can generate
thrombosis (blood clots if it doesn’t move in circulation
Summary
• Arteries and veins are unique in their compositions
– a reflection of their specific roles within the circulatory system as
they deliver blood to and receive blood from capillaries
• As blood flows from higher to lower resistance,
arteries transport blood under high pressure.
– Arteries require strong vascular walls with well developed
musculature and significant elastic tissue to maintain resistance.
– Blood flows rapidly in arteries which assists in maintaining
resistance and thus blood pressure.
Summary
• In the absence of shunts (anastomoses), arteries
arborize into arterioles.
– Arterioles are the direct control conduits for blood into capillaries
and the primary determinates for vascular resistance.
• Capillaries are not muscular and one cell layer in
thickness.
– They provide the exchange surface for gasses, H2O, nutrients,
electrolytes, and hormones between blood and interstitium.
Summary
• Venules are found on the distal end of capillaries with which
they are contiguous and collect blood from these capillaries.
– They are weakly muscular and may thus influence capillary
pressure.
• Venules dearborize to veins.
– Veins are somewhat muscular, and quite elastic.
– Veins transport blood at low velocity and pressure providing a
blood reservoir as they are capacitance vessels
– able to increase significantly in volume with small increases in
pressure.
Summary
• As such resistance to venous return, set by right
atrial pressure, decreases venous return as it
increases.
• Conversely Mean systemic filling pressure
increases (if right atrial pressure are not
increased) results in an increased venous return.
Summary
•
Pressures effecting blood flow within vasculature are of three types
– driving pressure - the direct work of the heart
– transmural pressure - a function of the vessel wall and interstium
– hydrostatic pressure - gravitational action on the blood mass.
•
Blood vessels increase and decrease progressively in cross sectional
geometry from arteries to veins affecting vascular resistance and
consequently pressure.
•
Local capillary vascular pressure is proportionate to the resistance that
precedes and follows a particular capillary bed.