Overview of Circulation &
Vascular Distensibility and
Functions
Chad Smurthwaite & Alex
Goncharov
Chapter 14 - Overview of the Circulation;
Biophysics of Pressure, Flow and Resistance
Blood Distribution
aorta
arteries
Heart
arterioles
capillaries
vena cava
veins
venules
Blood
Distribution
Continued
Pulmonary - 9%
Heart - 7%
Arterial System
Arteries - 13%
Arterioles and
Capillaries - 7%
Venous System
Veins, Venules, and
Arterial System
Made of two types of vessels
Conductance Vessels: Largest arteries with the thickest lumen near the heart. They contain
much more elastin that allows them to expand and recoil as the heart ejects blood
allowing a constant flow of blood.
Resistance Vessels - Smaller arteries that contain more smooth muscle that constrict and
relax to allow vasoconstriction and vasodilation
Venous System
Capacitance Vessels: Veins
with large lumens and
thin walls that allow
expansion for storage of
blood.
Capillaries
Capillary Beds: Smallest blood vessels that allow for exchange of gases,
hormones and nutrients for most tissues in the body.
Blood Pressure Throughout the Circulatory System
Pressure ranges
between 120 and
80 mmHg in large
arteries.
Pressure then falls
off.
Venous system very
low pressure
system.
Blood Flow
Rate of flow: Ohm’s law
Proportional to the pressure difference
Inversely Proportional to the resistance
Flow =
ΔPressure/
Resistance
Conductance and Poiseuille’s Law
Conductance: A measure of the blood flow
through a vessel for a given pressure
difference.
Conductance = 1/resistance
Conductance ∝ Diameter4
Poiseuille’s Law
Types of Flow
Laminar Flow: When blood flows in streamlines, with each layer remaining the same distance
from the lumen.
Turbulent Flow: Blood is flowing in all directions in the vessel and continually mixing within the
vessel.
Series vs. Parallel Circuits
Series Circuits: When blood vessels are arranged in series
Parallel Circuits: When blood vessels branch and converge
Rtotal = R1 + R2 + R3 + R4 …..
Autoregulation of Tissue Blood Flow (Perfusion)
Autoregulation: The automatic adjustment of blood flow to each tissue in
proportion to the tissue’s requirements at any moment.
Metabolic Theory: When blood flow is too low to meet metabolic needs, oxygen levels
decline and metabolic products accumulate.
Metabolic factors- low oxygen, increases in hydrogen ions, potassium, adenosine, and
prostaglandins.
Myogenic Theory: When vascular smooth muscle responds directly to a passive stretch by
increased tone, which increases blood flow.
Both work together to determine the final autoregulatory response for a given tissue
Chapter 15 - Vascular Distensibility and Functions
of the Arterial and Venous Systems
Vascular Distensibility and Compliance
Vascular Distensibility: The ability of a blood vessel wall to expand and
contract passively with changes in pressure.
All blood vessels are distensible
Allows for non pulsatile flow at the capillaries
Reservoir function of Veins: can expand to store 0.5 to 1.0 L of extra blood
Distensibility
Veins are 8x more distensible than arteries
Pulmonary arteries are 6x more distensible than systemic arteries.
Compliance (Capacitance)
Total quantity of blood that can be stored in a given portion of the circulation
Volume-Pressure Curves
Arterial vs. venous distensibility
Sympathetic stimulation/inhibition
Delayed Compliance - stress relaxation
Works when blood volume
is added or lost
Functions of Arterial vs. Venous Systems
Arterial Pressure Pulsations
The difference between systolic
and diastolic is pulse pressure
Affects Pulse Pressure:
1. stroke volume output of the heart
2. compliance of arterial tree
Abnormal pressure pulse contours
Pulse Pressure
Transmission
Venous Pressure - Central Venous Pressure vs
Peripheral Venous Pressure
1. Right Atrial Pressure (Central Venous Pressure)
Pressure taken in vena cava -- right before right atrial pressure
Values:
Normal = 0.0 mm Hg
Can increase to 20-30 mmHg in heart failure
Lower limit -3 to -5 mm Hg
Venous Resistance
low resistance - compressions
can lead to collapses
Peripheral Venous Pressure
Pressure driving blood back
to heart is about 7 mm Hg.
If there is pressure gradient
from veins to the heart, blood will
flow back to the heart.
Valves/Muscles Pumps