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 arterioles

vena cava veins venules

capillaries Heart

Blood Distribution Continued

Pulmonary - 9%

Heart - 7%

Arterial System

Arteries - 13%

Arterioles and

Capillaries - 7%

Venous System

Veins, Venules, and

Venous Sinuses - 64%

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