Blood flow and pressure exchange
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Blood Flow and Pressure Exchange. Outline. Overview of circulation Components of the Vascular system Medical physics of blood flow Vascular distensibility and compliance Arterial damping of pressure pulses Veins as reservoirs of blood Capillary exchange. Learning Objectives.

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  • Overview of circulation

  • Components of the Vascular system

  • Medical physics of blood flow

  • Vascular distensibility and compliance

  • Arterial damping of pressure pulses

  • Veins as reservoirs of blood

  • Capillary exchange

Learning objectives
Learning Objectives

  • Know each component of the vascular system.

  • Understand blood flow using Ohm’s and Poiseuille’s laws.

  • Know how vascular distensibility allows arteries to dampen pressure pulses and veins to act as reservoirs.

  • Know how hydrostatic and colloid osmotic forces determine the flow of fluid in the capillaries.

Components of the vascular system blood vessels

Closed circulatory system






3 tunics


Tunica interna


Connective tissue

Tunica media

Smooth muscle




Tunica externa

Collagen fibers

Nerve fibers

Lymphatic vessels

Elastin fibers

Components of the Vascular System - Blood Vessels


Away from the heart

Thick, muscular walls

Very elastic


Diameter varies in response to neural stimuli and local chemical influences.


Consist of a single tunica interna

Gas, nutrient, and waste exchange

Brain capillaries

Blood-brain barrier

Capillary beds

Precapillary sphincter

Shunting of blood



Venous system

Toward the heart

Venules—porous—free movement of fluids and white blood cells.


3 tunics—but thin

Venous valves

Varicose veins

Incompetent valves


Maintenance of Blood Pressure

Neural control

Shunting and vasoconstriction.

Vasomotor center


Carotid and aorta


Higher brain centers



Atrialnatrietic peptide



Histamine—other vasodilators

Venous System


30% of people over 50

Damages arteries

Causes heart failure, vascular disease, renal failure, stroke, and blindness.

Enlargement falled by hypertrophy of the myocardium

Contributing factors:

Diet (sodium, saturated fat, cholesterol)






Smoking—nicotine is a vasoconstrictor.



Damage to the tunica interna






LDLs—”bad cholesterol”

Foam cells

Fatty streak stage




Heart attack

Coronary bypass


tPA—tissue plasminogen activator

Clot buster

HDL—removes cholesterol from vessel walls.



  • Aorta—largest artery

    • Ascending

    • Descending

    • Right and left coronary arteries

    • Common carotid arteries—branch to form internal and external carotids

      • External—supply tissues of the head except the brain and orbits.

      • Internal—supply the orbits and most of the cerebrum.

    • Vertebral arteries—branch to the cervical spinal cord, neck, cerebellum, pons, and inner ear.

Arteries to know
Arteries to Know

  • Know the arteries on the proceeding chart plus:

    • Arteries of the arm—brachial, radial, ulnar

    • Arteries of the leg—femoral, popliteal, anterior tibial, posterior tibial

    • Be able to identify these arteries on a diagram. Also know the locations served by these arteries.


  • Dural sinuses—veins of the brain drain into these enlarged chambers and drain to the internal jugular veins.

  • External jugular veins—superficial head structures.

  • Vertebral veins—cervical vertebrae and neck muscles.

  • Brachiocephalic—mammary glands and first 2 or 3 intercostal spaces.

Veins to know
Veins to Know

  • Know the veins on the preceding chart plus:

    • The veins of the arms—cephalic, axillary, brachial, radial, ulnar.

    • The veins of the legs—external iliac, femoral, popliteal, anterior tibial, posterior tibial, great saphenous vein, hepatic portal vein.

    • The great saphenous vein is a superficial vein. Connect with many of the deep veins of the legs and thighs.

    • Be able to identify these veins on a diagram. Also know the locations served by these arteries.

Overview of circulatory system arteries veins and everything in between
Overview of Circulatory System: Arteries + Veins and Everything in Between

Function of Circulatory System: To carry nutrients and hormones to tissues and wastes products away from tissues.

Basic circulatory function
Basic Circulatory Function Everything in Between

  • Rate of blood flow to tissues changes based on need.

    - e.g., during exercise, blood flow to skeletal muscle increases.

    - In most tissues, blood flow increases in proportion to the metabolism of that tissue.

  • Cardiac output is mainly controlled by venous return.

  • Generally, arterial pressure is controlled independently of local blood flow or cardiac output control.

Parts of the vasculature
Parts of the Vasculature Everything in Between

Aorta receives blood from left ventricle.

Arteries transport under high pressure, strong vascular walls.

Arterioles control conduits, last branch of arterial system, strong muscular walls that can strongly constrict or dilate.

Capillaries exchange substances through pores.

Venules collect blood from capillaries.

Veins low pressure, transport blood back to the heart, controllable reservoir of extra blood.

Blood volume and vasculature cross sectional area
Blood Volume and Vasculature Cross-Sectional Area Everything in Between

Cross-sectional area (cm2)

Aorta 2.5

Small Arteries 20

Arterioles 40

Capillaries 2500

Venules 250

Small Veins 80


Ohm s law applied to blood flow
Ohm’s Law Applied to Blood Flow Everything in Between

Blood pressure
Blood Pressure Everything in Between

  • BP is the force exerted by the blood against the vessel wall.

    - Typically measured as mm Hg.

    - E.g., 100 mm Hg is the force needed to push a column of Hg to a level of 100 mm.

Resistance Everything in Between

  • Resistance is the impediment to blood flow.

  • Not measured directly, but determined from pressure and flow measurements.

    - If ΔP = 1 mm Hg and F = 1 ml/sec, then R = 1 PRU (peripheral resistance unit).

    - In the adult systemic circulatory system, ΔP = 100 mm Hg, and F = 100 ml/sec; so R = 1 PRU.

    - In the pulmonary system, ΔP = 14 mm Hg and F = 100 ml/sec; so R = 0.14 PRU.

Conductance Everything in Between

  • Conductance is the opposite of resistance:

    Conductance = 1/resistance

  • Conductance may be easier to conceptualize than resistance and is sometimes easier to use in calculating the total resistance of parallel vessels.

Laminar flow
Laminar Flow Everything in Between

Poiseuille s law
Poiseuille’s Everything in Between Law

Turbulant flow
Turbulant Everything in Between Flow

Summary of blood flow physics
Summary of Blood Flow Physics Everything in Between

Vascular distensibility
Vascular Everything in BetweenDistensibility

  • Vascular  distensibility  is  the  ability  of  the   vascular  system  to  expand  with  increased   pressure,  which

    - Increases  blood  blow  as  pressure  increases.

    - In  arteries,  averages  out  pulses.

    - Allows  veins  to  act  as  reservoirs

Calculate distensibility
Calculate Everything in BetweenDistensibility

  • Fractional increase in volume per rise in pressure:

    Vascular = Increase in Volume

    DistensibilityIncr in P xorigVol

    If  1mm  Hg  increases  a  vessel  from  10mm  to   11mm,  the  distensibility  would  be  0.1  per  mm   Hg  or  10%  per  mm  Hg.

Distensibility of arteries and veins
Distensibility Everything in Between of Arteries and Veins

  • Artery walls are much stronger than those of veins and thus, much less distensible.

  • The larger distensibility of veins allows them to act as blood reservoirs.

Vascular compliance
Vascular Compliance Everything in Between

  • The quantity of blood that can be stored in a particular portion of the vasculature for a rise in pressure:

    Vascular compliance = Increase in volume

    Increase in pressure

  • Compliance =distensibilityxvol

Athersclerosis arteries become less compliant
Athersclerosis Everything in Between – Arteries become less Compliant

Veins Everything in Between

Can distend to hold large amounts of blood.

Contraction of skeletal muscles can constrict the veins and propel blood to the heart and increase cardiac output.

The contraction-induced constriction and the valves prevent the venous pressure from building up on the feet of standing adults.

Veins as blood reservoirs
Veins as Blood Reservoirs Everything in Between

  • > 60% of blood in the circulatory system is in  the veins.

  • When blood is lost, sympathetic stimulation   causes veins to constrict and make up for  the   lost blood.

  • Conversely, veins can distend to hold excess blood if too much is given during a   transfusion.

The distribution of blood
The Distribution of Blood Everything in Between

Blood volume
Blood Volume Everything in Between

  • Distribution of H2O within the body:

  • Intracellular compartment:

    • 2/3 of total body H2O within the cells.

  • Extracellular compartment:

    • 1/3 total body H2O.

      • 80% interstitial fluid.

      • 20% blood plasma.

  • Maintained by constant balance between H2O loss and gain.

Capillaries Everything in Between

  • Exchange  nutrients  and  waste  with  tissues.

  • ~ 10  billion  capillaries  with  500 – 700 m2 total   surface  area  in  whole  body.

Capillaries are porous
Capillaries are Porous Everything in Between

  • The  exchange  of  water-soluble  nutrients  and   waste  between  blood  plasma  and  interstitial   fluid  occurs  by  diffusion  through  pores  in  the   capillary  walls.

  • Lipid-soluble  substances  pass  directly  through   the  capillary  wall  (e.g.,  O2 and  CO2).

Capillary structure
Capillary Structure Everything in Between

Capillary exchange
Capillary Exchange Everything in Between




Capillary exchange1
Capillary Exchange Everything in Between

Colloid osmotic pressure
Colloid Osmotic Pressure Everything in Between

  • Starling force=(Pc + Pif) - (Pif+ Pp)

  • Pc

    • Hydrostatic pressure in the capillary.

  • Pif

    • Colloid osmotic pressure of the interstitial fluid.

  • Pif

    • Hydrostatic pressure in the interstitial fluid.

  • Pp

    • Colloid osmotic pressure of the blood plasma.

Cardiac output co recall the frank starling mechanism
Cardiac Output (CO Everything in Between)Recall the Frank-Starling Mechanism?

  • Volume of blood pumped/min. by each ventricle.

    • Pumping ability of the heart is a function of the beats/ min. and the volume of blood ejected per beat.

  • CO = SV x HR

    • Total blood volume = about 5.5 liters.

  • Each ventricle pumps the equivalent of the total blood volume/ min.

Forces at venous end of capillary
Forces at Venous End of Capillary Everything in Between

Mean capillary forces
Mean Capillary Forces Everything in Between